Vegetation modulates the impact of climate extremes on gross primary production Journal article

Authors: M Flach and A Brenning and F Gans and M Reichstein and S Sippel and M D Mahecha
Journal: Biogeosciences
Drought and heat events affect the uptake and sequestration of carbon in terrestrial ecosystems. Factors such as the duration, timing, and intensity of extreme events influence the magnitude of impacts on ecosystem processes such as gross primary production (GPP), i.e., the ecosystem uptake of CO2. Preceding soil moisture depletion may exacerbate these impacts. However, some vegetation types may be more resilient to climate extremes than others. This effect is insufficiently understood at the global scale and is the focus of this study. Using a global upscaled product of GPP that scales up in situ land CO2 flux observations with global satellite remote sensing, we study the impact of climate extremes at the global scale. We find that GPP in grasslands and agricultural areas is generally reduced during heat and drought events. However, we also find that forests, if considered globally, appear in general to not be particularly sensitive to droughts and heat events that occurred during the analyzed period or even show increased GPP values during these events. On the one hand, normal-to-increased GPP values are in many cases plausible, e.g., when conditions prior to the event have been particularly positive. On the other hand, however, normal-to-increased GPP values in forests may also reflect a lack of sensitivity in current remote-sensing-derived GPP products to the effects of droughts and heatwaves. The overall picture calls for a differentiated consideration of different land cover types in the assessments of risks of climate extremes for ecosystem functioning.

Event‐based storylines to address climate risk Journal article

Authors: Jana Sillmann and Theodore G Shepherd and Bart van den Hurk and Wilco Hazeleger and Olivia Martius and Julia Slingo and Jakob Zscheischler
Journal: Earth's Future

Identifying meteorological drivers of extreme impacts: an application to simulated crop yields Journal article

Authors: Johannes Vogel and Pauline Rivoire and Cristina Deidda and Leila Rahimi and Christoph Alexander Sauter and Elisabeth Tschumi and Karin van der Wiel and Tianyi Zhang and Jakob Zscheischler
Journal: Earth System Dynamics
Compound weather events may lead to extreme impacts that can affect many aspects of society including agriculture. Identifying the underlying mechanisms that cause extreme impacts, such as crop failure, is of crucial importance to improve their understanding and forecasting. In this study, we investigate whether key meteorological drivers of extreme impacts can be identified using the least absolute shrinkage and selection operator (LASSO) in a model environment, a method that allows for automated variable selection and is able to handle collinearity between variables. As an example of an extreme impact, we investigate crop failure using annual wheat yield as simulated by the Agricultural Production Systems sIMulator (APSIM) crop model driven by 1600 years of daily weather data from a global climate model (EC-Earth) under present-day conditions for the Northern Hemisphere. We then apply LASSO logistic regression to determine which weather conditions during the growing season lead to crop failure. We obtain good model performance in central Europe and the eastern half of the United States, while crop failure years in regions in Asia and the western half of the United States are less accurately predicted. Model performance correlates strongly with annual mean and variability of crop yields; that is, model performance is highest in regions with relatively large annual crop yield mean and variability. Overall, for nearly all grid points, the inclusion of temperature, precipitation and vapour pressure deficit is key to predict crop failure. In addition, meteorological predictors during all seasons are required for a good prediction. These results illustrate the omnipresence of compounding effects of both meteorological drivers and different periods of the growing season for creating crop failure events. Especially vapour pressure deficit and climate extreme indicators such as diurnal temperature range and the number of frost days are selected by the statistical model as relevant predictors for crop failure at most grid points, underlining their overarching relevance. We conclude that the LASSO regression model is a useful tool to automatically detect compound drivers of extreme impacts and could be applied to other weather impacts such as wildfires or floods. As the detected relationships are of purely correlative nature, more detailed analyses are required to establish the causal structure between drivers and impacts.

Evaluating the dependence structure of compound precipitation and wind speed extremes Journal article

Authors: Jakob Zscheischler and Philippe Naveau and Olivia Martius and Sebastian Engelke and Christoph C Raible
Journal: Earth System Dynamics

Simulating compound weather extremes responsible for critical crop failure with stochastic weather generators Journal article

Authors: Peter Pfleiderer and Aglaé Jézéquel and Juliette Legrand and Natacha Legrix and Iason Markantonis and Edoardo Vignotto and Pascal Yiou
Journal: Earth System Dynamics

Novel multivariate quantile mapping methods for ensemble post-processing of medium-range forecasts Journal article

Authors: Kirien Whan and Jakob Zscheischler and Alexander I Jordan and Johanna F Ziegel
Journal: Weather and Climate Extremes
Statistical post-processing is an indispensable tool for providing accurate weather forecasts and early warnings for weather extremes. Most statistical post-processing is univariate, with dependencies introduced via use of an empirical copula. Standard empirical methods take a dependence template from either the raw ensemble output (ensemble copula coupling, ECC) or the observations (Schaake Shuffle, SSh). There are drawbacks to both methods. In ECC it is assumed that the raw ensemble simulates the dependence well, which is not always the case (e.g. 2-meter temperature in The Netherlands). The Schaake Shuffle is not able to capture flow dependent changes to the dependence and the choice of observations is key. Here we compare a re-shuffled standard ensemble model output statistics (EMOS) approach with two multivariate bias adjustment approaches that have not been used before in a post-processing context: 1) the multivariate bias correction with N-dimensional probability density function transform (MBCn) and 2) multivariate ranks that are defined with optimal assignment (OA). These methods have the advantage that they are able to explicitly capture the dependence structure that is present in the observations. We apply ECC, the Schaake Shuffle, MBCn and OA to 2-meter and dew point temperature forecasts at seven stations in The Netherlands. Forecasts are verified with both univariate and multivariate methods, and using a heat index derived from both variables, the wet-bulb globe temperature (WBGT). Our results demonstrate that the spatial and inter-variable dependence structure is more realistic in MBCn and OA compared to ECC or the Schaake Shuffle. The variogram score shows that while ECC is most skilful for the first two days, at moderate lead times MBCn is most skilful and at the longest lead times OA is more skilful than both ECC and MBCn. Overall, we highlight the importance of considering the dependence between variables and locations in the statistical post-processing of weather forecasts.

Increasing compound warm spells and droughts in the Mediterranean Basin Journal article

Authors: Johannes Vogel and Eva Paton and Valentin Aich and Axel Bronstert
Journal: Weather and Climate Extremes
The co-occurrence of warm spells and droughts can lead to detrimental socio-economic and ecological impacts, largely surpassing the impacts of either warm spells or droughts alone. We quantify changes in the number of compound warm spells and droughts from 1979 - 2018 in the Mediterranean Basin using the ERA5 data set. We analyse two types of compound events: 1) warm season compound events, which are extreme in absolute terms in the warm season from May to October and 2) year-round deseasonalised compound events, which are extreme in relative terms respective to the time of the year. The number of compound events increases significantly and especially warm spells are increasing strongly – with an annual growth rates of 3.9 (3.5) % for warm season (deseasonalised) compound events and 4.6 (4.4) % for warm spells –, whereas for droughts the change is more ambiguous depending on the applied definition. Therefore, the rise in the number of compound events is primarily driven by temperature changes and not the lack of precipitation. The months July and August show the highest increases in warm season compound events, whereas the highest increases of deseasonalised compound events occur in spring and early summer. This increase in deseasonalised compound events can potentially have a significant impact on the functioning of Mediterranean ecosystems as this is the peak phase of ecosystem productivity and a vital phenophase.

A counterfactual perspective on compound weather risk Journal article

Authors: Gordon Woo
Journal: Weather and Climate Extremes
ABSTRACT Extreme weather outcomes are a multi-dimensional function of interacting physical processes. Actual compound events correspond to particular specific historical realisations of these coupled processes. But due to their intrinsic stochastic nature, they might have led to different outcomes. Historical meteorological studies tend to focus on explaining what actually happened, rather than on considering the phase space of other possibilities. In contrast with extreme event catalogues, information about near misses and proximity to tipping points is not systematically collated. Consequently, stakeholder awareness of such high risk system states is limited. The exploration of alternative realisations provides a counterfactual perspective on compound weather risk, which broadens understanding of extreme weather events, especially in respect of severe impact consequences. This perspective would be an insightful supplement to statistical studies of extreme compound events.

Clustering bivariate dependencies of compound precipitation and wind extremes over Great Britain and Ireland Journal article

Authors: Edoardo Vignotto and Sebastian Engelke and Jakob Zscheischler
Journal: Weather and Climate Extremes
Identifying hidden spatial patterns that define sub-regions characterized by a similar behaviour is a central topic in statistical climatology. This task, often called regionalization, is helpful for recognizing areas in which the variables under consideration have a similar stochastic distribution and thus, potentially, for reducing the dimensionality of the data. Many examples for regionalization are available, spanning from hydrology to weather and climate science. However, the majority of regionalization techniques focuses on the spatial clustering of a single variable of interest and is often not tailored to extremes. Extreme events often have severe impacts, which can be amplified when co-occurring with extremes in other variables. Given the importance of characterizing compound extreme events at the regional scale, here we develop an algorithm that identifies homogeneous spatial sub-regions that are characterized by a common bivariate dependence structure in the tails of a bivariate distribution. In particular, we use a novel non-parametric divergence able to capture the similarities and differences in the tail behaviour of bivariate distributions as the core of our clustering procedure. We apply the approach to identify homogeneous regions that exhibit similar likelihood of compound precipitation and wind extremes in Great Britain and Ireland.

Recent increasing frequency of compound summer drought and heatwaves in Southeast Brazil Journal article

Authors: João Geirinhas and Ana Russo and Renata Libonati and Pedro M Sousa and Diego G Miralles and Ricardo M Trigo
Journal: Environmental Research Letters
An increase in the frequency of extremely hot and dry events has been experienced over the past few decades in South America, and particularly in Brazil. Regional climate change projections indicate a future aggravation of this trend. However, a comprehensive characterization of drought and heatwave compound events, as well as of the main land–atmosphere mechanisms involved, is still lacking for most of South America. This study aims to fill this gap, assessing for the first time the historical evolution of compound summer drought and heatwave events for the heavily populated region of Southeast Brazil and for the period of 1980–2018. The main goal is to undertake a detailed analysis of the surface and synoptic conditions, as well as of the land–atmosphere coupling processes that led to the occurrence of individual and compound dry and hot extremes. Our results confirm that the São Paulo, Rio de Janeiro and Minas Gerais states have recorded pronounced and statistically significant increases in the number of compound summer drought and heatwave episodes. In particular, the last decade was characterized by two austral summer seasons (2013/14 and 2014/15) with outstanding concurrent drought and heatwave conditions stemmed by severe precipitation deficits and a higher-than-average occurrence of blocking patterns. As result of these land and atmosphere conditions, a high coupling (water-limited) regime was imposed, promoting the re-amplification of hot spells that resulted in mega heatwave episodes. Our findings reveal a substantial contribution of persistent dry conditions to heatwave episodes, highlighting the vulnerability of the region to climate change.

A standardized index for assessing sub-monthly compound dry and hot conditions with application in China Journal article

Authors: J Li and Z Wang and X Wu and J Zscheischler and S Guo and X Chen
Journal: Hydrology and Earth System Sciences

Statistical modelling and climate variability of compound surge and precipitation events in a managed water system: a case study in the Netherlands Journal article

Authors: V M Santos and M Casas-Prat and B Poschlod and E Ragno and B van den Hurk and Z Hao and T Kalmár and L Zhu and H Najafi
Journal: Hydrology and Earth System Sciences

Compound high-temperature and low-chlorophyll extremes in the ocean over the satellite period Journal article

Authors: N Le Grix and J Zscheischler and C Laufkötter and C S Rousseaux and T L Frölicher
Journal: Biogeosciences

Canopy temperature and heat stress are increased by compound high air temperature and water stress and reduced by irrigation - a modeling analysis Journal article

Authors: X Luan and G Vico
Journal: Hydrology and Earth System Sciences

Towards a compound-event-oriented climate model evaluation: a decomposition of the underlying biases in multivariate fire and heat stress hazards Journal article

Authors: R Villalobos-Herrera and E Bevacqua and A F S Ribeiro and G Auld and L Crocetti and B Mircheva and M Ha and J Zscheischler and C De Michele
Journal: Natural Hazards and Earth System Sciences

Classifying compound coastal storm and heavy rainfall events in the north-western Spanish Mediterranean Journal article

Authors: M Sanuy and T Rigo and J A Jiménez and M C Llasat
Journal: Hydrology and Earth System Sciences

Assessing internal changes in the future structure of dry-hot compound events: the case of the Pyrenees Journal article

Authors: M Lemus-Canovas and J A Lopez-Bustins
Journal: Natural Hazards and Earth System Sciences

Space-time dependence of compound hot-dry events in the United States: assessment using a multi-site multi-variable weather generator Journal article

Authors: M I Brunner and E Gilleland and A W Wood
Journal: Earth System Dynamics

Regional analysis of multivariate compound coastal flooding potential around Europe and environs: sensitivity analysis and spatial patterns Journal article

Authors: P Camus and I D Haigh and A A Nasr and T Wahl and S E Darby and R J Nicholls
Journal: Natural Hazards and Earth System Sciences

Global analysis of cyclone-induced compound precipitation and wind extreme events Journal article

Authors: Martina Messmer and Ian Simmonds
Journal: Weather and Climate Extremes

Understanding compound hazards from a weather system perspective Journal article

Authors: Jennifer L Catto and Andrew Dowdy
Journal: Weather and Climate Extremes

A global perspective on the sub-seasonal clustering of precipitation extremes Journal article

Authors: Alexandre Tuel and Olivia Martius
Journal: Weather and Climate Extremes
The occurrence of several precipitation extremes over sub-seasonal time windows can have major impacts on human societies, leading for instance to floods. Here, we apply a simple statistical framework based on Ripley’s K function, at a global scale and for each season separately, to identify regions where precipitation extremes tend to cluster in time over timescales of a few days to a few weeks. We analyze several observational and reanalysis datasets, as well as output from CMIP6 Global Climate Models (GCMs). Good agreement is found on the spatio-temporal clustering patterns across datasets. Sub-seasonal temporal clustering is largely concentrated over the tropical oceans, where it can be detected year-round. It is also significant over certain tropical lands, like Eastern Africa, and seasonally outside the tropics in several regions, most notably around the eastern subtropical oceans (Iberian Peninsula and Western North America during the DJF and MAM seasons) Southwest Asia (especially during JJA and SON) and Australia (in SON). We also find that CMIP6 models generally correctly reproduce clustering patterns, paving the way for an assessment of trends in sub-seasonal clustering under climate change. Clustering of present-day extremes increases in many areas under climate change. Changes diagnosed by comparing present day and future extreme percentiles are positive and negative and strongest in the tropical areas.

Physical storylines of future European drought events like 2018 based on ensemble climate modelling Journal article

Authors: Karin van der Wiel and Geert Lenderink and Hylke de Vries
Journal: Weather and Climate Extremes
In the aftermath of observed extreme weather events, questions arise on the role of climate change in such events and what future events might look like. We present a method for the development of physical storylines of future events comparable to a chosen observed event, to answer some of these questions. A storyline approach, focusing on physical processes and plausibility rather than probability, improves risk awareness through its relation with our memory of the observed event and contributes to decision making processes through their user focus. The method is showcased by means of a proof-of-concept for the 2018 drought in western Europe. We create analogues of the observed event based on large ensemble climate model simulations representing 2 °C and 3 °C global warming scenarios, and discuss how event severity, event drivers and physical processes are influenced by climate change. We show that future Rhine basin meteorological summer droughts like 2018 will be more severe. Decreased precipitation and increased potential evapotranspiration, caused by higher temperatures and increased incoming solar radiation, lead to higher precipitation deficits and lower plant available soil moisture. Possibly, changes in atmospheric circulation contribute to increased spring drought, amplifying the most severe summer drought events. The spatial extent of the most severe drought impacts increases substantially. The noted changes can partly be explained by changes in mean climate, but for many variables, changes in the relative event severity on top of these mean changes contribute as well.

Compound precipitation and wind extremes over Europe and their relationship to extratropical cyclones Journal article

Authors: Laura E Owen and Jennifer L Catto and David B Stephenson and Nick J Dunstone
Journal: Weather and Climate Extremes
Extratropical cyclones and their associated extreme precipitation and winds can have a severe impact on society and the co-occurrence between the two extremes is important when assessing risk. In this study the extremal dependency measure, χ, is used to quantify the co-occurrence of extreme precipitation and wind gusts, and is investigated at individual grid points and spatially over Europe. Results using three observational datasets and a higher spatial and temporal resolution version of ERA5 than previously used confirm previous studies. Over Europe high co-occurrence is found over western coasts and low co-occurrence is found over eastern coasts. All datasets have qualitatively similar spatial patterns over most regions of Europe excluding some regions of high topography where ERA5 χ values are much larger. ERA5 represents the timings of daily extreme co-occurring events well, compared to observations. The differences in precipitation accumulation timescales are also accounted for by considering hourly, 6, 24 and 48 hourly co-occurrence. In a few regions co-occurrence changes with longer accumulations, indicating the different speeds and sizes of weather systems affecting these regions. χ in most regions has little increase by allowing a 24 h lag and lead between the precipitation and wind, with a few exceptions where χ is increased by up to 24%. Regions with the larger of these increases are on or around elevated topography. Using an objective feature tracking method, insight into the spatial pattern of extreme precipitation and wind within cyclones over Europe is given. As well as suggesting how many hours apart the extremes occur from one another in a particular location. Extreme co-occurring events are associated with cyclones far more of the time than non extreme events. Given an extreme co-occurring event the chance of a cyclone being within 1110 km is more than 70% for much of Europe. Regions with low co-occurrence have extremes caused by different weather systems and regions with large co-occurrence have both extremes caused by the same weather system. Cyclones linked to extreme events, particularly co-occurring and extreme wind, have larger intensity than those not and for most of Europe these cyclones also have faster mean speed.

How well can a seasonal forecast system represent three hourly compound wind and precipitation extremes over Europe? Journal article

Authors: Laura E Owen and Jennifer L Catto and Nick J Dunstone and David B Stephenson
Journal: Environmental Research Letters
Extreme precipitation and winds can have a severe impact on society, particularly when they occur at the same place and time. In this study the Met Office’s Global Seasonal forecast system version 5 (GloSea5) model ensembles are evaluated against the reanalysis dataset ERA5, to find out how well they represent three hourly extreme precipitation, extreme wind and extreme co-occurring events over Europe. Although substantial differences in magnitude are found between precipitation and wind extremes between the datasets, the conditional probability of exceedance above the 99th percentile, which measures the co-occurrence between the two extremes, compares well spatially over Europe. However, significant differences in frequency are found around and over some areas of high topography. Generally GloSea5 underestimates this co-occurrence over sea. The model’s co-occurring events at individual locations investigated occur with very similar synoptic patterns to ERA5, indicating that the compound extremes are produced for the correct reasons.

Larger Spatial Footprint of Wintertime Total Precipitation Extremes in a Warmer Climate Journal article

Authors: Emanuele Bevacqua and Theodore G Shepherd and Peter A G Watson and Sarah Sparrow and David Wallom and Dann Mitchell
Journal: Geophysical Research Letters
Abstract The simultaneous occurrence of extremely wet winters at multiple locations in the same region can contribute to widespread flooding and associated socio-economic losses. However, the spatial extent of precipitation extremes (i.e., the area in which nearby locations experience precipitation extremes simultaneously) and its future changes are largely overlooked in climate assessments. Employing new multi-thousand-year climate model simulations, we show that under both 2.0 °C and 1.5 °C warming scenarios, wintertime total precipitation extreme extents would increase over about 80%–90% of the Northern Hemisphere extratropics (i.e., of the latitude band 28°–78°N). Stabilizing at 1.5 °C rather than 2.0 °C would reduce the average magnitude of the increase by 1.7–2 times. According to the climate model, the increased extents are caused by increases in precipitation intensity rather than changes in the spatial organization of the events. Relatively small percentage increases in precipitation intensities (e.g., by 4%) can drive disproportionately larger, by 1–2 orders of magnitude, growth in the spatial extents (by 93%).

Compound Climate Events and Extremes in the Midlatitudes: Dynamics, Simulation, and Statistical Characterization Journal article

Authors: Gabriele Messori and Emanuele Bevacqua and Rodrigo Caballero and Dim Coumou and Paolo De Luca and Davide Faranda and Kai Kornhuber and Olivia Martius and Flavio Pons and Colin Raymond and Kunhui Ye Ye and Pascal Yiou Yiou and Jakob Zscheischler
Journal: Bulletin of the American Meteorological Society

The compound nature of soil temperature anomalies at various depths in the Czech Republic Journal article

Authors: Vera Potopová and Luboš Tűrkott and Marie Musiolková and Martin Možny and Ondřej Lhotka
Journal: Theoretical and Applied Climatology

Guidelines for studying diverse types of compound weather and climate events Journal article

Authors: Emanuele Bevacqua and Carlo De Michele and Colin Manning and Anaïs Couasnon and Andreia F S Ribeiro and Alexandre M Ramos and Edoardo Vignotto and Ana Bastos and Suzana Blesić and Fabrizio Durante and John Hillier and Sérgio C Oliveira and Joaquim G Pinto and Elisa Ragno and Pauline Rivoire and Kate Saunders and Karin van der Wiel and Wenyan Wu and Tianyi Zhang and Jakob Zscheischler
Journal: Earth's Future
Abstract Compound weather and climate events are combinations of climate drivers and/or hazards that contribute to societal or environmental risk. Studying compound events often requires a multidisciplinary approach combining domain knowledge of the underlying processes with, for example, statistical methods and climate model outputs. Recently, to aid the development of research on compound events, four compound event types were introduced, namely (1) preconditioned, (2) multivariate, (3) temporally compounding, and (4) spatially compounding events. However, guidelines on how to study these types of events are still lacking. Here, we consider four case studies, each associated with a specific event type and a research question, to illustrate how the key elements of compound events (e.g., analytical tools and relevant physical effects) can be identified. These case studies show that (1) impacts on crops from hot and dry summers can be exacerbated by preconditioning effects of dry and bright springs. (2) Assessing compound coastal flooding in Perth (Australia) requires considering the dynamics of a non-stationary multivariate process. For instance, future mean sea-level rise will lead to the emergence of concurrent coastal and fluvial extremes, enhancing compound flooding risk. (3) In Portugal, deep-landslides are often caused by temporal clusters of moderate precipitation events. Finally, (4) crop yield failures in France and Germany are strongly correlated, threatening European food security through spatially compounding effects. These analyses allow for identifying general recommendations for studying compound events. Overall, our insights can serve as a blueprint for compound event analysis across disciplines and sectors.

Attributing compound events to anthropogenic climate change Journal article

Authors: Jakob Zscheischler and Flavio Lehner
Journal: Bulletin of the American Meteorological Society

Depth-Duration-Frequency Relationship Model of Extreme Precipitation in Flood Risk Assessment in the Upper Vistula Basin Journal article

Authors: Iwona Markiewicz
Journal: Water
The Upper Vistula Basin is a flood-prone region in the summer season (May–October) due to intensive rainfall. From the point of view of water management, it is particularly important to assess the variability in this main factor of flood risk, as well as to establish the depth-duration-frequency (DDF) relationship for maximum precipitation, this having not yet been derived for the region. The analysis of a 68-year (1951-2018) data series of summer maximum precipitation collected by 11 meteorological stations showed the series’ stationarity, which supports the conclusion that there is no increase in the risk of rainfall floods due to the intensification of extreme precipitation. A new approach is proposed for the determination of the DDF relationship, where the best-fitted distribution for each station is selected from among the set of candidate distributions, instead of adopting one fixed distribution for all stations. This approach increases the accuracy of the DDF relationships for individual stations as compared to the commonly used approach. In particular, the traditionally used Gumbel distribution turns out to be not well fitted to the investigated data series, and the advantage of the recently popular GEV distribution is not significant.

Storm-Surge Induced Water Level Changes in the Odra River Mouth Area (Southern Baltic Coast) Journal article

Authors: Halina Kowalewska-Kalkowska
Journal: Atmosphere
The Odra River mouth area is a region of the Southern Baltic coastal zone especially prone to the influence of storm surges. In the present study, the height and extent of the Baltic storm surges, and temporal offsets of the respective maximum water level occurrences in the Odra River mouth area were explored using cross-correlation, cluster analysis and principal component analysis. The analyses were based on hourly water level readings retrieved from water gauging stations located along the lower Odra reaches and at the coasts of the Szczecin Lagoon and the Pomeranian Bay during storm surge years 2008/2009–2019/2020. The analysis of mutual relationships between water levels during storm surges indicated that the extent of marine influence on the lower Odra River and within the Szczecin Lagoon was variable during the studied surge events, and dependent on meteorological conditions (the strongest during the sustained occurrence of wind blowing from the northern sector), discharge from the Odra River catchment (the strongest at low discharge), ice conditions on the lower Odra (suppressing the storm surge propagation upstream), and general sea level in the Pomeranian Bay (stronger at high sea levels). The strongest correlation between sea levels at Świnoujście and water levels in the Szczecin Lagoon and the lower Odra was found at a 6–7 h offset. The extent of storm surges usually reached 100 km up the lower Odra channels, less frequently reaching 130 km away from the sea.


Statistical modelling of drought-related yield losses using soil moisture-vegetation remote sensing and multiscalar indices in the south-eastern Europe Journal article

Authors: Vera Potopová and Miroslav Trnka and Pavel Hamouz and Josef Soukup and Tudor Castraveț
Journal: Agricultural Water Management
Meteorological and agricultural information coupled with remote sensing observations has been used to assess the effectiveness of satellite-derived indices in yield estimations. The estimate yield models generated by both the regression (MLR) and Bayesian network (BBN) algorithms and their levels of predictive skill were assessed. The enhanced vegetation index (EVI2), soil water index (SWI), standardized precipitation evaporation index (SPEI) have been considered predictors for three rainfed crops (maize, sunflower and grapevine) grown in 37 districts in the Republic of Moldova (RM). We used the weekly EVI2, which was collected by MODIS instruments aboard the Terra satellite with a 250m × 250m spatial resolution and aggregated for each district during the 2000–2018 period. We also used the weekly SWI, which was collected from the ASCAT instruments with a 12 km x 12 km spatial resolution and aggregated for each district at the topsoil (0–40 cm; SWI-12) and the root-zone layer (0–100 cm; SWI-14) during 2000–2018. The multiscalar SPEI during 1951–2018 farming years proved to be a significant addition to the remote sensing indices and led to the development of a model that improved the yield assessment. The study also summarized (i) the optimal time window of satellite-derived SWIi and EVI2i for yield estimation, and (ii) the capability of remotely sensed indices for representing the spatio–temporal variations of agricultural droughts. We developed statistical soil-vegetation-atmosphere models to explore drought-related yield losses. The skill scores of the sunflower MLR and BBN models were higher than those for the maize and grape models and were able to estimate yields with reasonable accuracy and predictive power. The accurate estimation of maize, sunflower and grapevine yields was observed two months before the harvest (RMSE of ∼1.2 tha-1). Despite the fact that summer crops (maize, sunflower) are able to develop a root system that uses the entire root zone depth, however, the SWI-12 had the stronger correlation with crop yield, then SWI-14. This explains much better the fit between yields of the crops and SWI-12, which represents soil moisture anomaly in the key rooting layer of soil. In any case, all summer crops showed negative correlations with each of the remote sensing soil moisture indices in the early and middle of the growing season, with SWI-12 performing better than SWI-14. Based on the crop-specific soil moisture model, we found that topsoil moisture declines in the most drought-susceptible crop growth stages, which indicates that RM is a good candidate for studying drought persists as main driver of rainfed yield losses in the south-eastern Europe.

Climate change effects on hydrometeorological compound events over southern Norway Journal article

Authors: Benjamin Poschlod and Jakob Zscheischler and Jana Sillmann and Raul R Wood and Ralf Ludwig
Journal: Weather and Climate Extremes
Hydrometeorological compound events cause severe economical, societal and environmental damage, but their investigation is difficult as they occur rarely and are multivariate. Here we use 50 high-resolution climate simulations from the single model initial condition large ensemble CRCM5-LE to examine two such compound event types in southern Norway: (1) Heavy rainfall on saturated soil during the summer months (June, July, August, September; SES) and (2) Concurrent heavy rainfall and snowmelt (rain-on-snow; ROS). We compare present-day conditions (1980–2009) with future conditions under a high-emission scenario (2070–2099) and investigate the impact of climate change on the frequency and spatial distribution of SES and ROS events. We find that the probability of occurrence of SES events during the summer increases by 38% until 2070–2099 over the whole study area. The areas with the highest occurrence probability extend from the west coast into the interior. In contrast, the frequency of ROS is projected to decrease by 48% on average, largely driven by decreases in snowfall. Moreover, the spatial pattern of ROS are projected to change, with the most frequently affected areas shifting from the west coast towards the inner country. Our study highlights the benefits of single model large ensemble simulations for the analysis of compound events.

A typology of compound weather and climate events Journal article

Authors: Jakob Zscheischler and Olivia Martius and Seth Westra and Emanuele Bevacqua and Colin Raymond and Radley M Horton and Bart van den Hurk and Amir AghaKouchak and Aglaé Jézéquel and Miguel D Mahecha and Douglas Maraun and Alexandre M Ramos and Nina N Ridder and Wim Thiery and Edoardo Vignotto
Journal: Nature Reviews Earth & Environment
Compound weather and climate events describe combinations of multiple climate drivers and/or hazards that contribute to societal or environmental risk. Although many climate-related disasters are caused by compound events, the understanding, analysis, quantification and prediction of such events is still in its infancy. In this Review, we propose a typology of compound events and suggest analytical and modelling approaches to aid in their investigation. We organize the highly diverse compound event types according to four themes: preconditioned, where a weather- driven or climate- driven precondition aggravates the impacts of a hazard; multivariate, where multiple drivers and/or hazards lead to an impact; temporally compounding, where a succession of hazards leads to an impact; and spatially compounding, where hazards in multiple connected locations cause an aggregated impact. Through structuring compound events and their respective analysis tools, the typology offers an opportunity for deeper insight into their mechanisms and impacts, benefiting the development of effective adaptation strategies. However, the complex nature of compound events results in some cases inevitably fitting into more than one class, necessitating soft boundaries within the typology. Future work must homogenize the available analytical approaches into a robust toolset for compound-event analysis under present and future climate conditions.

Understanding and managing connected extreme events Journal article

Authors: Colin Raymond and Radley M Horton and Jakob Zscheischler and Olivia Martius and Amir AghaKouchak and Jennifer Balch and Steven G Bowen and Suzana J Camargo and Jeremy Hess and Kai Kornhuber and Michael Oppenheimer and Alex C Ruane and Thomas Wahl and Kathleen White
Journal: Nature Climate Change
Extreme weather and climate events and their impacts can occur in complex combinations, an interaction shaped by physical drivers and societal forces. In these situations, governance, markets and other decision-making structures—together with population exposure and vulnerability—create nonphysical interconnections among events by linking their impacts, to positive or negative effect. Various anthropogenic actions can also directly affect the severity of events, further complicating these feedback loops. Such relationships are rarely characterized or considered in physical-sciences-based research contexts. Here, we present a multidisciplinary argument for the concept of connected extreme events, and we suggest vantage points and approaches for producing climate information useful in guiding decisions about them.

Direct and seasonal legacy effects of the 2018 heat wave and drought on European ecosystem productivity Journal article

Authors: A Bastos and P Ciais and P Friedlingstein and S Sitch and J Pongratz and L Fan and J P Wigneron and U Weber and M Reichstein and Z Fu and P Anthoni and A Arneth and V Haverd and A K Jain and E Joetzjer and J Knauer and S Lienert and T Loughran and P C McGuire and H Tian and N Viovy and S Zaehle
Journal: Science Advances
In summer 2018, central and northern Europe were stricken by extreme drought and heat (DH2018). The DH2018 differed from previous events in being preceded by extreme spring warming and brightening, but moderate rainfall deficits, yet registering the fastest transition between wet winter conditions and extreme summer drought. Using 11 vegetation models, we show that spring conditions promoted increased vegetation growth, which, in turn, contributed to fast soil moisture depletion, amplifying the summer drought. We find regional asymmetries in summer ecosystem carbon fluxes: increased (reduced) sink in the northern (southern) areas affected by drought. These asymmetries can be explained by distinct legacy effects of spring growth and of water-use efficiency dynamics mediated by vegetation composition, rather than by distinct ecosystem responses to summer heat/drought. The asymmetries in carbon and water exchanges during spring and summer 2018 suggest that future land-management strategies could influence patterns of summer heat waves and droughts under long-term warming.

Investigation of Rain-On-Snow Floods under Climate Change Journal article

Authors: Cenk Sezen and Mojca Šraj and Anže Medved and Nejc Bezak
Journal: Applied Sciences
Rain-on-snow (ROS) floods can cause economic damage and endanger human lives due to the compound effect of rainfall and snowmelt, especially under climate change. In this study, possible future changes of seasonality, magnitude and frequency characteristics of ROS floods were investigated for the selected catchments in Slovenia, Europe. For this purpose, five global/regional climate models (GCM/RCM) combinations were applied using the RCP4.5 climate scenario for the period 1981–2100. To determine ROS floods’ characteristics in the future, a lumped conceptual hydrological model Génie Rural à 6 paramètres Journalier (GR6J) with snow module CemaNeige was applied. The results indicate that the number of ROS floods could increase in the future. Moreover, also the magnitudes of extreme ROS floods could increase, while a slight decrease in the median values of ROS flood magnitudes was observed. The strength of seasonality for a high-altitude catchment could decrease in the future. A slight shift in the average ROS floods’ timing could be expected. Furthermore, a catchment located in a temperate continental climate could have a different response to the climate change impact in comparison to a catchment located in a mountain climate with alpine characteristics. Additionally, differences among investigated climate models show a large variability.

Measuring compound flood potential from river discharge and storm surge extremes at the global scale Journal article

Authors: Anaïs Couasnon and Dirk Eilander and Sanne Muis and Ted I E Veldkamp and Ivan D Haigh and Thomas Wahl and Hessel C Winsemius and Philip J Ward
Journal: Natural Hazards and Earth System Sciences
The interaction between physical drivers from oceanographic, hydrological, and meteorological processes in coastal areas can result in compound flooding. Compound flood events, like Cyclone Idai and Hurricane Harvey, have revealed the devastating consequences of the co-occurrence of coastal and river floods. A number of studies have recently investigated the likelihood of compound flooding at the continental scale based on simulated variables of flood drivers, such as storm surge, precipitation, and river discharges. At the global scale, this has only been performed based on observations, thereby excluding a large extent of the global coastline. The purpose of this study is to fill this gap and identify regions with a high compound flooding po- tential from river discharge and storm surge extremes in river mouths globally. To do so, we use daily time series of river discharge and storm surge from state-of-the-art global mod- els driven with consistent meteorological forcing from reanalysis datasets. We measure the compound flood potential by analysing both variables with respect to their timing, joint statistical dependence, and joint return period. Our analysis indicates many regions that deviate from statistical indepen- dence and could not be identified in previous global studies based on observations alone, such as Madagascar, northern Morocco, Vietnam, and Taiwan. We report possible causal mechanisms for the observed spatial patterns based on existing literature. Finally, we provide preliminary insights on the implications of the bivariate dependence behaviour on the flood hazard characterisation using Madagascar as a case study. Our global and local analyses show that the depen- dence structure between flood drivers can be complex and can significantly impact the joint probability of discharge and storm surge extremes. These emphasise the need to refine global flood risk assessments and emergency planning to account for these potential interactions.

Brief communication: The role of using precipitation or river discharge data when assessing global coastal compound flooding Journal article

Authors: Emanuele Bevacqua and Michalis Vousdoukas and Theodore Shepherd and Mathieu Vrac
Journal: Natural Hazards and Earth System Sciences
Interacting storm surges and high water runoff can cause compound flooding (CF) in low-lying coasts and river estuaries. The large-scale CF hazard has been typically studied using proxies such as the concurrence of storm surge extremes either with precipitation or with river discharge ex- tremes. Here the impact of the choice of such proxies is addressed employing state-of-the-art global datasets. Although they are proxies of diverse physical mechanisms, we find that the two approaches show similar CF spatial patterns. On average, deviations are smaller in regions where assessing the actual CF is more relevant, i.e. where the CF potential is high. Differences between the two assessments increase with the catchment size, and our findings indicate that CF in long rivers (catchment >5–10,000 km2) should be analysed using river discharge data. The precipitation-based assessment allows for considering local-rainfall-driven CF and CF in small rivers not resolved by large-scale datasets.

Possible Increase of Vegetation Exposure to Spring Frost under Climate Change in Switzerland Journal article

Authors: Ondřej Lhotka and Stefan Brönnimann
Journal: Atmosphere
We assessed future changes in spring frost risk for the Aare river catchment that comprises the Swiss Plateau, the most important agricultural region of Switzerland. An ensemble of 15 bias-corrected regional climate model (RCM) simulations from the EXAR data set forced by the RCP 4.5 and RCP 8.5 concentration pathways were analysed for two future periods. Correlating actual meteorological observations and Swiss phenological spring index, we proposed and tested an RCM-compatible methodology (based on temperature data only) for estimating a start of spring and severity of frost events. In the historical climate, a significant advancement in start of spring was observed and frost events were more frequent in those years in which spring started sooner. In 2021–2050, spring is projected to start eight (twelve) days earlier, considering the RCP 4.5 (8.5) scenario. Substantial changes were simulated for the 2070–2099 period under RCP 8.5, when the total severity of frost events was projected to be increased by a factor of 2.1 compared to the historical climate. The study revealed the possible future increase of vegetation exposure to spring frost in Switzerland and that this phenomenon is noticeable even in the near future under the ‘low concentration’ RCP 4.5 scenario.

Vulnerability of hop-yields due to compound drought and heat events over European key-hop regions Journal article

Authors: Vera Potopová and Ondrej Lhotka and Martin Možny and Marie Musiolková
Journal: International Journal of Climatology
Compound climate events in which only one variable is extreme (e.g., either hot but no drought or extreme drought but not hot) and events in which both variables are extreme (e.g., drought and heat waves) may have different impacts on hop yields and alpha‐bitter acid contents. Increasing occurrences of compound drought and heat events have led to increased income variability for beer production, and also affecting the major hop growers across Europe (EU). Our study includes the key hop‐growing regions across the EU such as Hallertau (Germany); Úštěcko, Žatecko and Tršická (Czech Republic); Kent (Great Britain); Alsace (France); Lublin (Poland); Koroška (Slovenia) and León and Galicia (Spain). For these regions, we used the concurrent bivariate return period to model the joint probability distributions of daily precipitation and maximum temperature extremes and to provide risk assessments for concurrent drought‐heat waves during the hop‐growing season. We estimated the risk of lower yields from hop cones based on concurrent dry‐cool, dry‐hot, wet‐cool and wet‐hot modes over the target areas. The results show that longer and more severe drought and heat wave concurrences have increased more frequently than shorter concurrences. The degree of risk was estimated as being higher over the extensive hop‐growing areas in the Czech Republic and Germany. A total of 22.4, 12.5 and 7.2% of EU areas with conditions suitable for commercial hop production fell into the moderate, high and very high yield loss risk categories, respectively. Integrating the damage between April and August indicated that more than 62.7% of total yield losses were due to high temperatures under dry conditions and that 21.5% of the yield losses were due to dry‐cool conditions in the top hop‐farming regions. The hotter European droughts caused decreases in noble aromatic hops by 29–68%. This indicates that hop yields are very vulnerable to these events due to a slower rate of adaptation of hops compared to field crops.

Atmospheric circulation as a factor contributing to increasing drought severity in Central Europe Journal article

Authors: Ondřej Lhotka and Mirek Trnka and Jan Kysely and Yannis Markonis and Jan Balek and Martin Možny
Journal: Journal of Geophysical Research: Atmospheres
Long‐lasting and severe droughts seriously threaten agriculture, ecosystems, and society. Summer 2018 in central Europe was characterized by unusually persistent heat and drought, causing substantial economic losses, and became a part of a several years long dry period observed across this region. This study assesses the magnitude of the recent drought within a long‐term context and links the increased drought severity to changes in atmospheric circulation. Temporal variability of drought conditions since the late 19th century was analyzed at seven long‐term stations distributed across the Czech Republic using the Palmer Drought Severity Index and the Standardized Precipitation Evaporation Index. The Palmer Z Index and a variation of the Standardized Precipitation Evaporation Index were used to study rapidly emerging short‐term droughts and to link these episodes to atmospheric circulation. Changes in circulation were analyzed through circulation types calculated from flow strength, direction and vorticity in mean sea level pressure data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis for 1948–2018. Increasing drought severity across the Czech Republic with record‐low values of the drought indices during 2015–2018 was found. The trend was distinctive in both vegetation (April–September) and cold (October–March) periods. The tendency toward more severe droughts in recent decades was linked to changes in frequency of dry and wet circulation types, highlighting the important role of atmospheric circulation in regional climate. It remains an open question whether the significantly increasing frequency of dry circulation types in the vegetation period is related to climate change, or rather represents multidecadal climate variability.

Compound warm-dry and cold-wet events over the Mediterranean Journal article

Authors: P De Luca and G Messori and D Faranda and P J Ward and D Coumou
Journal: Earth System Dynamics
The Mediterranean (MED) Basin is a climate change hotspot that has seen drying and a pronounced increase in heatwaves over the last century. At the same time, it is experiencing increased heavy precipitation during wintertime cold spells. Understanding and quantifying the risks from compound events over the MED is paramount for present and future disaster risk reduction measures. Here, we apply a novel method to study compound events based on dynamical systems theory and analyse compound temperature and precipitation events over the MED from 1979 to 2018. The dynamical systems analysis quantifies the strength of the coupling between different atmospheric variables over the MED. Further, we consider compound warm–dry anomalies in summer and cold–wet anomalies in winter. Our results show that these warm–dry and cold–wet compound days are associated with large values of the temperature–precipitation coupling parameter of the dynamical systems analysis. This indicates that there is a strong interaction between temperature and precipitation during compound events. In winter, we find no significant trend in the coupling between temperature and precipitation. However in summer, we find a significant upward trend which is likely driven by a stronger coupling during warm and dry days. Thermodynamic processes associated with long-term MED warming can best explain the trend, which intensifies compound warm–dry events.

Projected changes in hot, dry and wet extreme events' clusters in CMIP6 multi-model ensemble Journal article

Authors: Martha M Vogel and Mathias Hauser and Sonia Isabelle Seneviratne
Journal: Environmental Research Letters
Concurrent extreme events, i.e. multi-variate extremes, can be associated with strong impacts. Hence, an understanding of how such events are changing in a warming climate is helpful to avoid some associated climate change impacts and better prepare for them. In this article, we analyse the projected occurrence of hot, dry, and wet extreme events' clusters in the multi-model ensemble of the 6th phase of the Coupled Model Intercomparison Project (CMIP6). Changes in 'extreme extremes', i.e. events with only 1% probability of occurrence in the current climate are analysed, first as univariate extremes, and then when co-occurring with other types of extremes (i.e. events clusters) within the same week, month or year. The projections are analysed for present-day climate (+1 °C) and different levels of additional global warming (+1.5 °C, +2 °C, +3 °C). The results reveal substantial risk of occurrence of extreme events' clusters of different types across the globe at higher global warming levels. Hotspot regions for hot and dry clusters are mainly found in Brazil, i.e. in the Northeast and the Amazon rain forest, the Mediterranean region, and Southern Africa. Hotspot regions for wet and hot clusters are found in tropical Africa but also in the Sahel region, Indonesia, and in mountainous regions such as the Andes and the Himalaya.

Shorter cyclone clusters modulate changes in European wintertime precipitation extremes Journal article

Authors: Emanuele Bevacqua and Giuseppe Zappa and Theodore G Shepherd
Journal: Environmental Research Letters
Wintertime extreme precipitation from cyclone clusters, i.e. consecutive cyclones moving across the same region, can lead to flooding and devastating socio-economic impacts in Europe. Previous studies have suggested that the future direction of the changes in these events are uncertain across climate models. By employing an impact-based metric of accumulated precipitation extremes, we show that projections of cyclone clusters are instead broadly robust, i.e. consistent in sign, across models. A novel physical diagnostic shows that accumulated precipitation extremes are projected to grow by only +1.0%/K on average across Europe, although the mean precipitation per cyclone increases by +4.7%/K. This results from a decreased number of clustered cyclones, associated with decreased wintertime storminess, the extent of which varies from northern to southern Europe and depends on the future storyline of atmospheric circulation change. Neglecting the changes in the number of clustered cyclones, i.e. assuming that accumulated precipitation extremes would change as the mean precipitation per cyclone, would lead to overestimating the population affected by increased accumulated wintertime precipitation extremes by 130-490 million across Europe.

Global hotspots for the occurrence of compound events Journal article

Authors: Nina N Ridder and Andy J Pitman and Seth Westra and Anna Ukkola and X Do Hong and Margot Bador and Annette L Hirsch and Jason P Evans and Alejandro Di Luca and Jakob Zscheischler
Journal: Nature Communications
Compound events (CEs) are weather and climate events that result from multiple hazards or drivers with the potential to cause severe socio-economic impacts. Compared with isolated hazards, the multiple hazards/drivers associated with CEs can lead to higher economic losses and death tolls. Here, we provide the first analysis of multiple multivariate CEs potentially causing high-impact floods, droughts, and fires. Using observations and reanalysis data during 1980–2014, we analyse 27 hazard pairs and provide the first spatial estimates of their occurrences on the global scale. We identify hotspots of multivariate CEs including many socio-economically important regions such as North America, Russia and western Europe. We analyse the relative importance of different multivariate CEs in six continental regions to highlight CEs posing the highest risk. Our results provide initial guidance to assess the regional risk of CE events and an observationally-based dataset to aid evaluation of climate models for simulating multivariate CEs.

More meteorological events that drive compound coastal flooding are projected under climate change Journal article

Authors: Emanuele Bevacqua and Michalis I Vousdoukas and Giuseppe Zappa and Kevin Hodges and Theodore G Shepherd and Douglas Maraun and Lorenzo Mentaschi and Luc Feyen
Journal: Communications Earth & Environment
Compound flooding arises from storms causing concurrent extreme meteorological tides (that is the superposition of storm surge and waves) and precipitation. This flooding can severely affect densely populated low-lying coastal areas. Here, combining output from climate and ocean models, we analyse the concurrence probability of the meteorological conditions driving compound flooding. We show that, under a high emissions scenario, the concurrence probability would increase globally by more than 25% by 2100 compared to present. In latitudes above 40o north, compound flooding could become more than 2.5 times as frequent, in contrast to parts of the subtropics where it would weaken. Changes in extreme precipitation and meteorological tides account for most (77% and 20%, respectively) of the projected change in concurrence probability. The evolution of the dependence between precipitation and meteorological tide dominates the uncertainty in the projections. Our results indicate that not accounting for these effects in adaptation planning could leave coastal communities insufficiently protected against flooding.

Multi-hazard dependencies can increase or decrease risk Journal article

Authors: John K Hillier and Tom Matthews and Robert L Wilby and Conor Murphy
Journal: Nature Climate Change
In risk analysis, it is recognized that hazards can often combine to worsen their joint impact, but impact data for a rail network show that hazards can also tend to be mutually exclusive at seasonal timescales. Ignoring this overestimates worst-case risk, so we therefore champion a broader view of risk from compound hazards.

Seasonal impact-based mapping of compound hazards Journal article

Authors: JK Hillier and Richard S Dixon
Journal: Environmental Research Letters
Impact-based, seasonal mapping of compound hazards is proposed. It is pragmatic, identifies phenomena to drive the research agenda, produces outputs relevant to stakeholders, and could be applied to many hazards globally. Illustratively, flooding and wind damage can co-occur, worsening their joint impact, yet where wet and windy seasons combine has not yet been systematically mapped. Here, seasonal impact-based proxies for wintertime flooding and extreme wind are used to map, at 1° × 1° resolution, the association between these hazards across Europe within 600 years as realized in seasonal hindcast data. Paired areas of enhanced-suppressed correlation are identified (Scotland, Norway), and are shown to be created by orographically-enhanced rainfall (or shelter) from prevailing westerly storms. As the hazard metrics used are calibrated to losses, the maps are indicative of the potential for damage.

Drought-related hot summers: A joint probability analysis in the Iberian Peninsula Journal article

Authors: Andreia F.S. Ribeiro and Ana Russo and Célia M. Gouveia and Carlos A.L. Pires
Journal: Weather and Climate Extremes
Droughts and hot extremes are major sources of risk to several socio-economic activities and their impacts are expected to increase under future global warming. Moreover, the simultaneous or sequential occurrence (compound events) of different climate extremes may lead to the amplification of the associated impacts. Even though the latest efforts in assessing hot and dry extremes and their interactions, the development of models describing the joint behavior of climate extremes is still a challenge. To contribute to the understanding of these compound events, we propose to assess the probability of extremely hot summer days in the Iberian Peninsula (IP) being preceded by drought events in spring and early summer, based on their joint probability distribution through copula theory. The precursor drought hazard was characterized by the Standardized Precipitation Evaporation Index (SPEI) for the months of May, June and July for different timescales (3-, 6- and 9-months). The Number of Hot Days per month (NHD) summed over the months of July and August were considered for modelling. The dependence structure between SPEI and NHD was very well identified for the most of the IP's regions by asymmetrical copulas with upper tail dependence (except in northwestern regions), suggesting that compound hot and dry extremes are strongly associated. The results show that the transition from previous wet to dry regimes increases substantially the probability of exceeding summer NHD extreme values, depending on the region and the drought timescale and target month. The results suggest a spatial heterogeneity over the IP when characterizing the influence of water deficits on following summer extreme temperatures, whereas northeastern, western and central regions were found to be the regions more prone to summer hot extremes induced by dryness, in contrast to southwestern, northwestern and southeastern regions (depending on the month and the timescale). This study provides estimates of the probability of drought-related hot extremes in the summer of the IP for different regions, which could be an important tool for responsible authorities to mitigate the impacts magnified by the interactions between the different hazards.

Risk of crop failure due to compound dry and hot extremes estimated with nested copulas Journal article

Authors: A F S Ribeiro and A Russo and C M Gouveia and P Páscoa and J Zscheischler
Journal: Biogeosciences
The interaction between co-occurring drought and hot conditions is often particularly damaging to crop's health and may cause crop failure. Climate change exacerbates such risks due to an increase in the intensity and frequency of dry and hot events in many land regions. Hence, here we model the trivariate dependence between spring maximum temperature and spring precipitation and wheat and barley yields over two province regions in Spain with nested copulas. Based on the full trivariate joint distribution, we (i) estimate the impact of compound hot and dry conditions on wheat and barley loss and (ii) estimate the additional impact due to compound hazards compared to individual hazards. We find that crop loss increases when drought or heat stress is aggravated to form compound dry and hot conditions and that an increase in the severity of compound conditions leads to larger damage. For instance, compared to moderate drought only, moderate compound dry and hot conditions increase the likelihood of crop loss by 8 % to 11 %, while when starting with moderate heat, the increase is between 19 % to 29 % (depending on the cereal and region). These findings suggest that the likelihood of crop loss is driven primarily by drought stress rather than by heat stress, suggesting that drought plays the dominant role in the compound event; that is, drought stress is not required to be as extreme as heat stress to cause similar damage. Furthermore, when compound dry and hot conditions aggravate stress from moderate to severe or extreme levels, crop loss probabilities increase 5 % to 6 % and 6 % to 8 %, respectively (depending on the cereal and region). Our results highlight the additional value of a trivariate approach for estimating the compounding effects of dry and hot extremes on crop failure risk. Therefore, this approach can effectively contribute to design management options and guide the decision-making process in agricultural practices.

The effect of surge on riverine flood hazard and impact in deltas globally Journal article

Authors: Dirk Eilander and Anaïs Couasnon and Hiroaki Ikeuchi and Sanne Muis and Dai Yamazaki and Hessel C Winsemius and Philip J Ward
Journal: Environmental Research Letters
Current global riverine flood risk studies assume a constant mean sea level boundary. In reality high sea levels can propagate up a river, impede high river discharge, thus leading to elevated water levels. Riverine flood risk in deltas may therefore be underestimated. This paper presents the first global scale assessment of the joint influence of riverine and coastal drivers of flooding in deltas. We show that if storm surge is ignored, flood depths are significantly underestimated for 9.3% of the expected annual population exposed to riverine flooding. The assessment is based on extreme water levels at 3433 river mouth locations as modeled by a state-of-the-art global river routing model, forced with a multi-model runoff ensemble and bounded by dynamic sea level conditions derived from a global tide and surge reanalysis. We first classified the drivers of riverine flooding at each location into four classes: surge-dominant, discharge-dominant, compound-dominant or insignificant. We then developed a model experiment to quantify the effect of surge on flood hazard and impacts. Drivers of riverine flooding are compound-dominant at 19.7% of the locations analyzed, discharge-dominant at 69.2%, and surge-dominant at 7.8%. Compared to locations with either surge- or discharge-dominant flood drivers, locations with compound-dominant flood drivers generally have larger surge extremes and are located in basins with faster discharge response and/or flat topography. Globally, surge exacerbates 1-in-10 years flood levels at 64.0% of the locations analyzed, with a mean increase of 11 cm. While this increase is generally larger at locations with compound- or surge-dominant flood drivers, flood levels also increase at locations with discharge-dominant flood drivers. This study underlines the importance of including dynamic downstream sea level boundaries in (global) riverine flood risk studies.

The record-breaking compound hot and dry 2018 growing season in Germany Journal article

Authors: Jakob Zscheischler and Erich M Fischer
Journal: Weather and Climate Extremes


The effect of univariate bias adjustment on multivariate hazard estimates Journal article

Authors: Jakob Zscheischler and Erich M. Fischer and Stefan Lange
Journal: Earth System Dynamics
Bias adjustment is often a necessity in estimating climate impacts because impact models usually rely on unbiased climate information, a requirement that climate model outputs rarely fulfil. Most currently used statistical bias-adjustment methods adjust each climate variable separately, even though impacts usually depend on multiple potentially dependent variables. Human heat stress, for instance, depends on temperature and relative humidity, two variables that are often strongly correlated. Whether univariate bias-adjustment methods effectively improve estimates of impacts that depend on multiple drivers is largely unknown, and the lack of long-term impact data prevents a direct comparison between model outputs and observations for many climate-related impacts. Here we use two hazard indicators, heat stress and a simple fire risk indicator, as proxies for more sophisticated impact models. We show that univariate bias-adjustment methods such as univariate quantile mapping often cannot effectively reduce biases in multivariate hazard estimates. In some cases, it even increases biases. These cases typically occur (i) when hazards depend equally strongly on more than one climatic driver, (ii) when models exhibit biases in the dependence structure of drivers and (iii) when univariate biases are relatively small. Using a perfect model approach, we further quantify the uncertainty in bias-adjusted hazard indicators due to internal variability and show how imperfect bias adjustment can amplify this uncertainty. Both issues can be addressed successfully with a statistical bias adjustment that corrects the multivariate dependence structure in addition to the marginal distributions of the climate drivers. Our results suggest that currently many modeled climate impacts are associated with uncertainties related to the choice of bias adjustment. We conclude that in cases where impacts depend on multiple dependent climate variables these uncertainties can be reduced using statistical bias-adjustment approaches that correct the variables' multivariate dependence structure.

Compound climate events transform electrical power shortfall risk in the Pacific Northwest Journal article

Authors: SWD Turner and N Voisin and J Fazio and D Hua and M Jourabchi
Journal: Nature communications
Power system reliability is sensitive to climate-driven variations in both energy demand and water availability, yet the combined effect of these impacts is rarely evaluated. Here we show that combined climate change impacts on loads and hydropower generation may have a transformative effect on the nature and seasonality of power shortfall risk in the U.S. Pacific Northwest. Under climate change, potential shortfall events occur more readily, but are significantly less severe in nature. A seasonal reversal in shortfall risk occurs: winter shortfalls are eradicated due to reduced building heating demands, while summer shortfalls multiply as increased peak loads for day-time cooling coincide with impaired hydropower generation. Many of these summer shortfalls go unregistered when climate change impacts on loads and hydropower dispatch are analyzed in isolation—highlighting an important role of compound events.

Growing Spatial Scales of Synchronous River Flooding in Europe Journal article

Authors: Wouter R Berghuijs and Scott T Allen and Shaun Harrigan and James W Kirchner
Journal: Geophysical Research Letters
Abstract River flooding is a common hazard, causing billions of dollars in annual losses. Flood impacts are shaped by the spatial scale over which different rivers flood simultaneously, but this dimension of flood risk remains largely unknown. Using annual flood data from several thousand European rivers, we demonstrate that the flood synchrony scale—the distance over which multiple rivers flood near synchronously—far exceeds the size of individual drainage basins and varies regionally by more than an order of magnitude. These data also show that flood synchrony scales have grown by about 50% over the period 1960–2010. Detrended flood synchrony values are serially correlated, implying that years with spatially extensive floods tend to follow one another. These findings reveal that flood risks are correlated well beyond the individual drainage basins for which flood hazards are typically assessed and managed.

Application of hydroclimatic drought indicators in the transboundary Prut River basin Journal article

Authors: Vera Potopová and Valeriu Cazac and Boris Boincean and Josef Soukup and Miroslav Trnka
Journal: Theoretical and Applied Climatology
The transboundary Prut River basin (PRB) is one of the most drought vulnerable areas in the Republic of Moldova, Romania, and Ukraine. The main objective of this study was to identify the response of hydrological drought to climatic conditions and cropping practice in a region with insufficient water resources. The presented work takes advantage of the development of statistical tools to analyze existing data, as well as the collection of qualitative and quantitative hydroclimatic datasets for each sub-basin region. The study also provides survey results of the impacts of climate change on agricultural water management, including agricultural water requirements and water availability, and the transition of these impacts to cropping practice. The multi-dimensional attributes of hydrological drought are defined according to the standardized streamflow index (SSI) and water-level standardized anomaly index (SWI). The standardized precipitation evapotranspiration index (SPEI) was selected for the assessment of the impact of climate drought control on hydrological drought. The streamflow/water river level is determined more by the climatic water balance deficit of the previous 6 months than over longer periods. The lag times between climatic and hydrological drought are short, which can cause a hydrological drought to occur in the same season as the climatic drought that caused it. Summer streamflow droughts are most closely linked to SPEI in the same month. Summer streamflow drought in upstream areas can impact streamflow at the outlet within the same month. Winter streamflow droughts are related to longer SPEI accumulation periods resulting from snow cover. The synthesis of findings from the river basin shown that concurrent compound climate events have much more severe impact on crop failures compared to their individual occurrence. Adjustments to sowing time (15%), the introduction of more drought resistant cultivars (11%), the use of crop protection measures (9%), and shifting to new crops (8%) seem to be minor and moderate adaptation practices employed by farmers.

Quantifying the local effect of Northern Hemisphere atmospheric blocks on the persistence of summer hot and dry spells Journal article

Authors: Matthias Röthlisberger and Olivia Martius
Journal: Geophysical Research Letters
Abstract The persistence of heat waves and droughts is a key factor in determining their societal impact. Here, the local effect of atmospheric blocks on the persistence of summer hot and dry spells is quantified by comparing their climatological daily survival probability, i.e., the probability to survive the next day, to their daily survival probability when they co-occur with a block. The survival odds of hot spells are increased by more than 50% over most of the Northern Hemisphere extra tropical land masses when co-occurring with blocks. Dry spell persistence is also strongly increased by co-located blocks over western North America, Europe and southern Russia, while it is significantly decreased over the western North Atlantic and the western North Pacific. These spatial differences in the effect of blocks on both spell types are explained by considering the spatially varying surface temperature and precipitation anomalies induced by the blocks.

Higher probability of compound flooding from precipitation and storm surge in Europe under anthropogenic climate change Journal article

Authors: Emanuele Bevacqua and Douglas Maraun and M I Vousdoukas and E Voukouvalas and M Vrac and L Mentaschi and M Widmann
Journal: Science Advances
In low-lying coastal areas, the co-occurrence of high sea level and precipitation resulting in large runoff may cause compound flooding (CF). When the two hazards interact, the resulting impact can be worse than when they occur individually. Both storm surges and heavy precipitation, as well as their interplay, are likely to change in response to global warming. Despite the CF relevance, a comprehensive hazard assessment beyond individual locations is missing, and no studies have examined CF in the future. Analyzing co-occurring high sea level and heavy precipitation in Europe, we show that the Mediterranean coasts are experiencing the highest CF probability in the present. However, future climate projections show emerging high CF probability along parts of the northern European coast. In several European regions, CF should be considered as a potential hazard aggravating the risk caused by mean sea level rise in the future.

Increased probability of compound long-duration dry and hot events in Europe during summer (1950–2013) Journal article

Authors: Colin Manning and Martin Widmann and Emanuele Bevacqua and Anne Van F Loon and Douglas Maraun and Mathieu Vrac
Journal: Environmental Research Letters
The propagation of drought from meteorological drought to soil moisture drought can be accelerated by high temperatures during dry periods. The occurrence of extremely long-duration dry periods in combination with extremely high temperatures may drive larger soil moisture deficits than either extreme occurring alone, and lead to severe impacts. In this study, we propose a framework to both characterise long-duration meteorological droughts that co-occur with extremely high temperatures and quantify their probability. We term these events as long-duration, dry and hot (DH) events and characterise them by their duration (D) and magnitude (M). D is defined as the consecutive number of days with precipitation below 1 mm, while M is the maximum daily maximum temperature during an event. A copula-based approach is then employed to estimate the probability of DH events. The framework is applied to Europe during the summer months of June, July and August. We also assess the change in probability that has occurred over the historical period 1950–2013 and find an increased probability of DH events throughout Europe where rising temperatures are found to be the main driver of this change. Dry periods are becoming hotter, leading to an increase in the occurrence of long-duration dry periods with extremely high temperatures. Some parts of Europe also show an increased probability of long-duration events although the relative change is not as strong as that seen with temperature. The results point to a predominant thermodynamic response of DH events to global warming and reaffirm previous research that soil moisture drought events are setting in faster and becoming more severe due to a change in the contributing meteorological hazards. It is hoped that the framework applied here will provide a starting point for further analysis of DH events in other locations and for the assessment of climate models.


Dependence between high sea-level and high river discharge increases flood hazard in global deltas and estuaries Journal article

Authors: Philip J Ward and Anaïs Couasnon and Dirk Eilander and Ivan D Haigh and Alistair Hendry and Sanne Muis and Ted I E Veldkamp and Hessel C Winsemius and Thomas Wahl
Journal: Environmental Research Letters
When river and coastal floods coincide, their impacts are often worse than when they occur in isolation; such floods are examples of 'compound events'. To better understand the impacts of these compound events, we require an improved understanding of the dependence between coastal and river flooding on a global scale. Therefore, in this letter, we: provide the first assessment and mapping of the dependence between observed high sea-levels and high river discharge for deltas and estuaries around the globe; and demonstrate how this dependence may influence the joint probability of floods exceeding both the design discharge and design sea-level. The research was carried out by analysing the statistical dependence between observed sea-levels (and skew surge) from the GESLA-2 dataset, and river discharge using gauged data from the Global Runoff Data Centre, for 187 combinations of stations across the globe. Dependence was assessed using Kendall's rank correlation coefficient (τ) and copula models. We find significant dependence for skew surge conditional on annual maximum discharge at 22% of the stations studied, and for discharge conditional on annual maximum skew surge at 36% of the stations studied. Allowing a time-lag between the two variables up to 5 days, we find significant dependence for skew surge conditional on annual maximum discharge at 56% of stations, and for discharge conditional on annual maximum skew surge at 54% of stations. Using copula models, we show that the joint exceedance probability of events in which both the design discharge and design sea-level are exceeded can be several magnitudes higher when the dependence is considered, compared to when independence is assumed. We discuss several implications, showing that flood risk assessments in these regions should correctly account for these joint exceedance probabilities.

A Copula-Based Bayesian Network for Modeling Compound Flood Hazard from Riverine and Coastal Interactions at the Catchment Scale: An Application to the Houston Ship Channel, Texas Journal article

Authors: Anaïs Couasnon and Antonia Sebastian and Oswaldo Morales-Nápoles
Journal: Water
Traditional flood hazard analyses often rely on univariate probability distributions; however, in many coastal catchments, flooding is the result of complex hydrodynamic interactions between multiple drivers. For example, synoptic meteorological conditions can produce considerable rainfall-runoff, while also generating wind-driven elevated sea-levels. When these drivers interact in space and time, they can exacerbate flood impacts, a phenomenon known as compound flooding. In this paper, we build a Bayesian Network based on Gaussian copulas to generate the equivalent of 500 years of daily stochastic boundary conditions for a coastal watershed in Southeast Texas. In doing so, we overcome many of the limitations of conventional univariate approaches and are able to probabilistically represent compound floods caused by riverine and coastal interactions. We model the resulting water levels using a one-dimensional (1D) steady-state hydraulic model and find that flood stages in the catchment are strongly affected by backwater effects from tributary inflows and downstream water levels. By comparing our results against a bathtub modeling approach, we show that simplifying the multivariate dependence between flood drivers can lead to an underestimation of flood impacts, highlighting that accounting for multivariate dependence is critical for the accurate representation of flood risk in coastal catchments prone to compound events

Mapping Dependence Between Extreme Rainfall and Storm Surge Journal article

Authors: Wenyan Wu and Kathleen Mcinnes and Julian O'grady and Ron Hoeke and Michael Leonard and Seth Westra
Journal: Journal of Geophysical Research: Oceans
Dependence between extreme storm surge and rainfall can have significant implications for flood risk in coastal and estuarine regions. To supplement limited observational records, we use reanalysis surge data from a hydrodynamic model as the basis for dependence mapping, providing information at a resolution of approximately 30 km along the Australian coastline. We evaluated this approach by comparing the dependence estimates from modeled surge to that calculated using historical surge records from 79 tide gauges around Australia. The results show reasonable agreement between the two sets of dependence values, with the exception of lower seasonal variation in the modeled dependence values compared to the observed data, especially at locations where there are multiple processes driving extreme storm surge. This is due to the combined impact of local bathymetry as well as the resolution of the hydrodynamic model and its meteorological inputs. Meteorological drivers were also investigated for different combinations of extreme rainfall and surge—namely rain‐only, surge‐only, and coincident extremes—finding that different synoptic patterns are responsible for each combination. The ability to supplement observational records with high‐resolution modeled surge data enables a much more precise quantification of dependence along the coastline, strengthening the physical basis for assessments of flood risk in coastal regions.

The role of atmospheric rivers in compound events consisting of heavy precipitation and high storm surges along the Dutch coast Journal article

Authors: Nina Ridder and Hylke de Vries and Sybren Drijfhout
Journal: Natural Hazards and Earth System Sciences
Atmospheric river (AR) systems play a significant role in the simultaneous occurrence of high coastal water levels and heavy precipitation in the Netherlands. Based on observed precipitation values (E-OBS) and the output of a numerical storm surge model (WAQUA/DSCMv5) forced with ERA-Interim sea level pressure and wind fields, we find that the majority of compound events (CEs) between 1979 and 2015 have been accompanied by the presence of an AR over the Netherlands. In detail, we show that CEs have a 3 to 4 times higher chance of occurrence on days with an AR over the Netherlands compared to any random day (i.e. days without knowledge on presence of an AR). In contrast, the occurrence of a CE on a day without AR is 3 times less likely than on any random day. Additionally, by isolating and assessing the prevailing sea level pressure (SLP) and sea surface temperature (SST) conditions with and without AR involvement up to 7 days before the events, we show that the presence of ARs constitutes a specific type of forcing conditions that (i) resemble the SLP anomaly patterns during the positive phase of the North Atlantic Oscillation (NAO+) with a north–south pressure dipole over the North Atlantic and (ii) cause a cooling of the North Atlantic subpolar gyre and eastern boundary upwelling zone while warming the western boundary of the North Atlantic. These conditions are clearly distinguishable from those during compound events without the influence of an AR which occur under SLP conditions resembling the East Atlantic (EA) pattern with a west–east pressure dipole over northern Europe and are accompanied by a cooling of the West Atlantic. Thus, this study shows that ARs are a useful tool for the early identification of possible harmful meteorological conditions over the Netherlands and supports an effort for the establishment of an early warning system.

Future climate risk from compound events Journal article

Authors: Jakob Zscheischler and Seth Westra and Bart J J M van den Hurk and Sonia I Seneviratne and Philip J Ward and Andy Pitman and Amir AghaKouchak and David N Bresch and Michael Leonard and Thomas Wahl and Xuebin Zhang
Journal: Nature Climate Change
Floods, wildfires, heatwaves and droughts often result from a combination of interacting physical processes across multiple spatial and temporal scales. The combination of processes (climate drivers and hazards) leading to a significant impact is referred to as a ‘compound event'. Traditional risk assessment methods typically only consider one driver and/or hazard at a time, potentially leading to underestimation of risk, as the processes that cause extreme events often interact and are spatially and/or temporally dependent. Here we show how a better understanding of compound events may improve projections of potential high-impact events, and can provide a bridge between climate scientists, engineers, social scientists, impact modellers and decision-makers, who need to work closely together to understand these complex events.

Contrasting biosphere responses to hydrometeorological extremes: revisiting the 2010 western Russian heatwave Journal article

Authors: M Flach and S Sippel and F Gans and A Bastos and A Brenning and M Reichstein and M D Mahecha
Journal: Biogeosciences
Combined droughts and heatwaves are among those compound extreme events that induce severe impacts on the terrestrial biosphere and human health. A record breaking hot and dry compound event hit western Russia in summer 2010 (Russian heatwave, RHW). Events of this kind are relevant from a hydrometeorological perspective, but are also interesting from a biospheric point of view because of their impacts on ecosystems, e.g., reductions in the terrestrial carbon storage. Integrating both perspectives might facilitate our knowledge about the RHW. We revisit the RHW from both a biospheric and a hydrometeorological perspective. We apply a recently developed multivariate anomaly detection approach to a set of hydrometeorological variables, and then to multiple biospheric variables relevant to describe the RHW. One main finding is that the extreme event identified in the hydrometeorological variables leads to multidirectional responses in biospheric variables, e.g., positive and negative anomalies in gross primary production (GPP). In particular, the region of reduced summer ecosystem production does not match the area identified as extreme in the hydrometeorological variables. The reason is that forest-dominated ecosystems in the higher latitudes respond with unusually high productivity to the RHW. Furthermore, the RHW was preceded by an anomalously warm spring, which leads annually integrated to a partial compensation of 54% (36% in the preceding spring, 18% in summer) of the reduced GPP in southern agriculturally dominated ecosystems. Our results show that an ecosystem-specific and multivariate perspective on extreme events can reveal multiple facets of extreme events by simultaneously integrating several data streams irrespective of impact direction and the variables' domain. Our study exemplifies the need for robust multivariate analytic approaches to detect extreme events in both hydrometeorological conditions and associated biosphere responses to fully characterize the effects of extremes, including possible compensatory effects in space and time.

Soil Moisture Drought in Europe: A Compound Event of Precipitation and Potential Evapotranspiration on Multiple Time Scales Journal article

Authors: Colin Manning and Martin Widmann and Emanuele Bevacqua and Anne F Van Loon and Douglas Maraun and Mathieu Vrac
Journal: Journal of Hydrometeorology
Compound events are extreme impacts that depend on multiple variables that need not be extreme themselves. In this study, we analyze soil moisture drought as a compound event of precipitation and potential evapotranspiration (PET) on multiple time scales related to both meteorological drought and heat waves in wet, transitional, and dry climates in Europe during summer. Drought indices that incorporate PET to account for the effect of temperature on drought conditions are sensitive to global warming. However, as evapotranspiration (ET) is moisture limited in dry climates, the use of such drought indices has often been criticized. We therefore assess the relevance of the contributions of both precipitation and PET to the estimation of soil moisture drought. Applying a statistical model based on pair copula constructions to data from FluxNet sites in Europe, we find at all sites that precipitation exerts the main control over soil moisture drought. At wet sites PET is additionally required to explain the onset, severity, and persistence of drought events over different time scales. At dry sites, where ET is moisture limited in summer, PET does not improve the estimation of soil moisture. In dry climates, increases in drought severity measured by indices incorporating PET may therefore not indicate further drying of soil but the increased availability of energy that can contribute to other environmental hazards such as heat waves and wildfires. We therefore highlight that drought indices including PET should be interpreted within the context of the climate and season in which they are applied in order to maximize their value.


Compounding effects of sea level rise and fluvial flooding Journal article

Authors: Hamed R Moftakhari and Gianfausto Salvadori and Amir AghaKouchak and Brett F Sanders and Richard A Matthew
Journal: Proceedings of the National Academy of Sciences
Sea level rise (SLR), a well-documented and urgent aspect of anthropogenic global warming, threatens population and assets located in low-lying coastal regions all around the world. Common flood hazard assessment practices typically account for one driver at a time (e.g., either fluvial flooding only or ocean flooding only), whereas coastal cities vulnerable to SLR are at risk for flooding from multiple drivers (e.g., extreme coastal high tide, storm surge, and river flow). Here, we propose a bivariate flood hazard assessment approach that accounts for compound flooding from river flow and coastal water level, and we show that a univariate approach may not appropriately characterize the flood hazard if there are compounding effects. Using copulas and bivariate dependence analysis, we also quantify the increases in failure probabilities for 2030 and 2050 caused by SLR under representative concentration pathways 4.5 and 8.5. Additionally, the increase in failure probability is shown to be strongly affected by compounding effects. The proposed failure probability method offers an innovative tool for assessing compounding flood hazards in a warming climate.

Multivariate Statistical Modelling of Compound Events via Pair-Copula Constructions: Analysis of Floods in Ravenna Journal article

Authors: Emanuele Bevacqua and Douglas Maraun and Ingrid Hobæk Haff and Martin Widmann and Mathieu Vrac
Journal: Hydrology and Earth System Sciences
Compound events (CEs) are multivariate extreme events in which the individual contributing variables may not be extreme themselves, but their joint – dependent – occurrence causes an extreme impact. Conventional univariate statistical analysis cannot give accurate information regarding the multivariate nature of these events. We develop a conceptual model, implemented via pair-copula constructions, which allows for the quantification of the risk associated with compound events in present-day and future climate, as well as the uncertainty estimates around such risk. The model includes predictors, which could represent for instance meteorological processes that provide insight into both the involved physical mechanisms and the temporal variability of compound events. Moreover, this model enables multivariate statistical downscaling of compound events. Downscaling is required to extend the compound events' risk assessment to the past or future climate, where climate models either do not simulate realistic values of the local variables driving the events or do not simulate them at all. Based on the developed model, we study compound floods, i.e. joint storm surge and high river runoff, in Ravenna (Italy). To explicitly quantify the risk, we define the impact of compound floods as a function of sea and river levels. We use meteorological predictors to extend the analysis to the past, and get a more robust risk analysis. We quantify the uncertainties of the risk analysis, observing that they are very large due to the shortness of the available data, though this may also be the case in other studies where they have not been estimated. Ignoring the dependence between sea and river levels would result in an underestimation of risk; in particular, the expected return period of the highest compound flood observed increases from about 20 to 32 years when switching from the dependent to the independent case.

Dependence of drivers affects risks associated with compound events Journal article

Authors: Jakob Zscheischler and Sonia I Seneviratne
Journal: Science Advances
Compound climate extremes are receiving increasing attention because of their disproportionate impacts on humans and ecosystems. However, risks assessments generally focus on univariate statistics. We analyze the co-occurrence of hot and dry summers and show that these are correlated, inducing a much higher frequency of concurrent hot and dry summers than what would be assumed from the independent combination of the univariate statistics. Our results demonstrate how the dependence structure between variables affects the occurrence frequency of multivariate extremes. Assessments based on univariate statistics can thus strongly underestimate risks associated with given extremes, if impacts depend on multiple (dependent) variables. We conclude that a multivariate perspective is necessary to appropriately assess changes in climate extremes and their impacts and to design adaptation strategies.

Multivariate anomaly detection for Earth observations: a comparison of algorithms and feature extraction techniques Journal article

Authors: M Flach and F Gans and A Brenning and J Denzler and M Reichstein and E Rodner and S Bathiany and P Bodesheim and Y Guanche and S Sippel and M D Mahecha
Journal: Earth System Dynamics
Today, many processes at the Earth's surface are constantly monitored by multiple data streams. These observations have become central to advancing our understanding of vegetation dynamics in response to climate or land use change. Another set of important applications is monitoring effects of extreme climatic events, other disturbances such as fires, or abrupt land transitions. One important methodological question is how to reliably detect anomalies in an automated and generic way within multivariate data streams, which typically vary seasonally and are interconnected across variables. Although many algorithms have been proposed for detecting anomalies in multivariate data, only a few have been investigated in the context of Earth system science applications. In this study, we systematically combine and compare feature extraction and anomaly detection algorithms for detecting anomalous events. Our aim is to identify suitable workflows for automatically detecting anomalous patterns in multivariate Earth system data streams. We rely on artificial data that mimic typical properties and anomalies in multivariate spatiotemporal Earth observations like sudden changes in basic characteristics of time series such as the sample mean, the variance, changes in the cycle amplitude, and trends. This artificial experiment is needed as there is no gold standard for the identification of anomalies in real Earth observations. Our results show that a well-chosen feature extraction step (e.g., subtracting seasonal cycles, or dimensionality reduction) is more important than the choice of a particular anomaly detection algorithm. Nevertheless, we identify three detection algorithms (k-nearest neighbors mean distance, kernel density estimation, a recurrence approach) and their combinations (ensembles) that outperform other multivariate approaches as well as univariate extreme-event detection methods. Our results therefore provide an effective workflow to automatically detect anomalies in Earth system science data.


A global quantification of compound precipitation and wind extremes Journal article

Authors: Olivia Martius and Stephan Pfahl and Clément Chevalier
Journal: Geophysical Research Letters
textcopyright2016. American Geophysical Union. All Rights Reserved. The concomitant occurrence of extreme precipitation and winds can have severe impacts. Here this concomitant occurrence is quantified globally using ERA-Interim reanalysis data. A logistic regression model is used to determine significant changes in the odds of precipitation extremes given a wind extreme that occurs on the same day, the day before, or the day after. High percentages of cooccurring wind and precipitation extremes are found in coastal regions and in areas with frequent tropical cyclones, with maxima of more than 50% of concomitant events. Strong regional-scale variations in this percentage are related to the interaction of weather systems with topography resulting in Föhn winds, gap winds, and orographic drying and the structure and tracks of extratropical and tropical cyclones. The percentage of concomitant events increases substantially if spatial shifts by one grid point are taken into account. Such spatially shifted but cooccurring events are important in insurance applications.


Analysis of a compounding surge and precipitation event in the Netherlands Journal article

Authors: Bart J J M van den Hurk and Erik van Meijgaard and Paul de Valk and Klaas-Jan van Heeringen and Jan Gooijer
Journal: Environmental Research Letters
Hydrological extremes in coastal areas in the Netherlands often result from a combination of anomalous (but not necessarily extreme) conditions: storm surges preventing the ability to discharge water to the open sea, and local precipitation generating excessive water levels in the inland area. A near-flooding event in January 2012 occurred due to such a combination of (mild) extreme weather conditions, by which free discharge of excessive water was not possible for five consecutive tidal periods. An ensemble of regional climate model simulations (covering 800 years of simulation data for current climate conditions) is used to demonstrate that the combined occurrence of the heavy precipitation and storm surge in this area is physically related. Joint probability distributions of the events are generated from the model ensemble, and compared to distributions of randomized variables, removing the potential correlation. A clear difference is seen. An inland water model is linked to the meteorological simulations, to analyze the statistics of extreme water levels and its relationship to the driving forces. The role of the correlation between storm surge and heavy precipitation increases with inland water level up to a certain value, but its role decreases at the higher water levels when tidal characteristics become increasingly important. The case study illustrates the types of analyzes needed to assess the impact of compounding events, and shows the importance of coupling a realistic impact model (expressing the inland water level) for deriving useful statistics from the model simulations.

Increasing risk of compound flooding from storm surge and rainfall for major US cities Journal article

Authors: Thomas Wahl and Shaleen Jain and Jens Bender and Steven D Meyers and Mark E Luther
Journal: Nature Climate Change
When storm surge and heavy precipitation co-occur, the potential for flooding in low-lying coastal areas is often much greater than from either in isolation. Knowing the probability of these compound events and understanding the processes driving them is essential to mitigate the associated high-impact risks1, 2. Here we determine the likelihood of joint occurrence of these two phenomena for the contiguous United States (US) and show that the risk of compound flooding is higher for the Atlantic/Gulf coast relative to the Pacific coast. We also provide evidence that the number of compound events has increased significantly over the past century at many of the major coastal cities. Long-term sea-level rise is the main driver for accelerated flooding along the US coastline3, 4; however, under otherwise stationary conditions (no trends in individual records), changes in the joint distributions of storm surge and precipitation associated with climate variability and change also augment flood potential. For New York City (NYC)—as an example—the observed increase in compound events is attributed to a shift towards storm surge weather patterns that also favour high precipitation. Our results demonstrate the importance of assessing compound flooding in a non-stationary framework and its linkages to weather and climate.

The co-incidence of storm surges and extreme discharges within the Rhine–Meuse Delta Journal article

Authors: W J Klerk and Hessel C Winsemius and W J van Verseveld and A M R Bakker and F L M Diermanse
Journal: Environmental Research Letters
The Netherlands is a low-lying coastal area and therefore threatened by both extreme river$backslash$r discharges from the Meuse and Rhine rivers and storm surges along the North Sea coastline. To date,$backslash$r in most flood risk analyses these two hazardous phenomena are considered independent. However, if$backslash$r there were a dependence between high sea water levels and extreme discharges this might result in$backslash$r higher design water levels, which might consequently have implications for flood protection policy$backslash$r in the Netherlands. In this study we explore the relation between high sea water levels at Hoek van$backslash$r Holland and high river discharges at Lobith. Different from previous studies, we use physical models$backslash$r forced by the same atmospheric forcing leading to concomitant and consistent time series of storm$backslash$r surge conditions and river discharge. These time series were generated for present day conditions as$backslash$r well as future climate projections and analysed for dependence within the upper tails of their$backslash$r distribution. In this study, dependence between the discharge at Lobith and storm surge at Hoek van$backslash$r Holland was found, and the dependence was highest for a lag of six days between the two processes.$backslash$r As no significant dependence of the threats was found for cases without time lag, there is no need$backslash$r for considering dependence in flood protection and policy making. Although future climate change is$backslash$r expected to lead to more extreme conditions in river discharges, we cannot conclude from this study$backslash$r that it will change the magnitude of the dependence for extreme conditions.


Modeling dependence between extreme rainfall and storm surge to estimate coastal flooding risk Journal article

Authors: Feifei Zheng and Seth Westra and Michael Leonard and Scott A Sisson
Journal: Water Resources Research
Accounting for dependence between extreme rainfall and storm surge can be critical for cor- rectly estimating coastal flood risk. Several statistical methods are available for modeling such extremal dependence, but the comparative performance of these methods for quantifying the exceedance probabil- ity of rare coastal floods is unknown. This paper compares three classes of statistical methods—threshold- excess, point process, and conditional—in terms of their ability to quantify flood risk. The threshold-excess method offers approximately unbiased estimates for dependence parameters, but its application for quanti- fying flood risk is limited because it is unable to handle situations where only one of the two variables is extreme. In contrast, the point process method (with the logistic and negative logistic models) and the con- ditional method describe the full distribution of extremes, but they overestimate and underestimate the dependence strength, respectively. We conclude that the point process method is the most suitable approach for modeling dependence between extreme rainfall and storm surge when the dependence is rel- atively strong, while none of the three methods produces satisfactory results for bivariate extremes with very weak dependence. It is therefore important to take the bias of each method into account when apply- ing them to flood estimation problems. A case study is used to demonstrate the three statistical methods and illustrate the implication of dependence to flood risk.

Impact of large-scale climate extremes on biospheric carbon fluxes: An intercomparison based on MsTMIP data Journal article

Authors: Jakob Zscheischler and Anna M Michalak and Christopher Schwalm and Miguel D Mahecha and Deborah N Huntzinger and Markus Reichstein and Gwenaëlle Berthier and Philippe Ciais and Robert B Cook and Bassil El-Masri and Maoyi Huang and Akihiko Ito and Atul Jain and Anthony King and Huimin Lei and Chaoqun Lu and Jiafu Mao and Shushi Peng and Benjamin Poulter and Daniel Ricciuto and Xiaoying Shi and Bo Tao and Hanqin Tian and Nicolas Viovy and Weile Wang and Yaxing Wei and Jia Yang and Ning Zeng
Journal: Global Biogeochemical Cycles
Understanding the role of climate extremes and their impact on the carbon (C) cycle is increasingly a focus of Earth system science. Climate extremes such as droughts, heat waves, or heavy precipitation events can cause substantial changes in terrestrial C fluxes. On the other hand, extreme changes in C fluxes are often, but not always, driven by extreme climate conditions. Here we present an analysis of how extremes in temperature and precipitation, and extreme changes in terrestrial C fluxes are related to each other in 10 state-of-the-art terrestrial carbon models, all driven by the same climate forcing. We use model outputs from the North American Carbon Program Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP). A global-scale analysis shows that both droughts and heat waves translate into anomalous net releases of CO2 from the land surface via different mechanisms: Droughts largely decrease gross primary production (GPP) and to a lower extent total respiration (TR), while heat waves slightly decrease GPP but increase TR. Cold and wet periods have a smaller opposite effect. Analyzing extremes in C fluxes reveals that extreme changes in GPP and TR are often caused by strong shifts in water availability, but for extremes in TR shifts in temperature are also important. Extremes in net CO2 exchange are equally strongly driven by deviations in temperature and precipitation. Models mostly agree on the sign of the C flux response to climate extremes, but model spread is large. In tropical forests, C cycle extremes are driven by water availability, whereas in boreal forests temperature plays a more important role. Models are particularly uncertain about the C flux response to extreme heat in boreal forests.


Quantifying the dependence between extreme rainfall and storm surge in the coastal zone Journal article

Authors: Feifei Zheng and Seth Westra and Scott A Sisson
Journal: Journal of Hydrology
The interaction between extreme rainfall and storm surge can be critical in determining flood risk in the coastal zone. This paper investigates the presence of dependence between these two processes along the Australian coastline using the most extensive observational records of rainfall and storm surge events currently available. A bivariate logistic threshold-excess model was employed to conduct the dependence study. Statistically significant dependence was observed for the majority of locations that were analysed, although regional variations as well as seasonal variations of the dependence strength are also apparent. The dependence remains significant even at distances of several hundred kilometres between the tide gauge and the rainfall gauge, indicating that dependence arises largely due to synoptic scale meteorological forcings. The strength of dependence varies as a function of storm burst duration, with an increase in dependence when going from one hour through to 24. h storm bursts. The dependence strength also varies with the lag between the extreme rainfall and the storm surge event, with the greatest level of dependence when extreme events occurred in the same time step for storm burst durations exceeding six hours, or for lags up to ??10. h for storm bursts durations below six hours. These findings have important implications for flood risk assessments in the coastal zone, showing that the two processes must be considered jointly if flood risk is to be quantified correctly. ?? 2013 Elsevier B.V.


A new method to assess the risk of local and widespread flooding on rivers and coasts Journal article

Authors: R Lamb and C Keef and J Tawn and S Laeger and I Meadowcroft and S Surendran and P Dunning and C Batstone
Journal: Journal of Flood Risk Management
To date, national- and regional-scale flood risk assessments have provided valuable information about the annual expected consequences of flooding, but not the exposure to widespread concurrent flooding that could have damaging consequences for people and the economy. We present a new method for flood risk assessment that accommodates the risk of widespread flooding. It is based on a statistical conditional exceedance model, which is fitted to gauged data and describes the joint probability of extreme river flows or sea levels at multiple locations. The method can be applied together with data from models for flood defence systems and economic damages to calculate a risk profile describing the probability distribution of economic losses or other consequences aggregated over a region. The method has the potential to augment national or regional risk assessments of expected annual damage with new information about the likelihoods, extent and impacts of events that could contribute to the risk.