Managing extreme weather events of unprecedented magnitude is one of the main challenges facing climate risk management and climate adaptation. Because of the unprecedented nature of these events, some authors have questioned the use of probabilistic approaches in this context. As an alternative, they introduced the so-called climate storylines approach. Climate storylines do not aim at predicting system states; rather, their focus is on revealing plausible chains of events whose impact might undermine the performance of the system.

Conceptually, climate storylines relate to - but are separate from – downward counterfactual histories. Downward counterfactual histories are plausible alternative realizations of historical events that could have turned to the worse. By constructing downward counterfactual histories in a disaster risk reduction context, some authors showed that many disasters that took societies by surprise could have in fact been anticipated.

This talk will introduce a decision-support framework to build climate storylines based on downward counterfactual histories. The framework is event-oriented, it focuses on impact and it is designed to be applied in a participatory fashion. By following the framework, the user first constructs climate storylines based on an iterative analysis of what (combinations of) counterfactuals are deemed critical (i.e., downward). Then, the user analyzes the future impact of the constructed storylines under climatic and socio-economic scenarios. Finally, the user explores the effects on the estimated impacts of the value-laden choices involved in the construction of the storylines.

The framework is applied to study the impact of tropical cyclones hitting the European Union’s outermost regions on the stability of the European Union Solidarity Fund (EUSF), a public fund that provides financial relief to Member States affected by large disasters. Contrary to what historic evidence would suggest, it is found that extreme - yet plausible - tropical cyclones might deplete the EUSF capital if they happen concurrently with large events in mainland Europe, and that a substantial recapitalization of the fund might be required to cope with future climatic and socio-economic changes.

How to cite: Ciullo, A.: A decision-support framework to construct climate impact storylines using downward counterfactuals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15450, https://doi.org/10.5194/egusphere-egu23-15450, 2023.

From mid-June until the end of August 2022, a sequence of torrential rains and deluges pummeled Pakistan, displacing more than 30 million residents with a death toll of near 2000. The accumulated amount exceeds the centennial average of 126 mm by about 7 standard deviations (50 mm), reaching a value of 487 mm and breaking its record over a century. The extraordinary extremity underscores the urgency for understanding the physical drivers of the event and the relations with human-induced climate change.

Here, we find that distinctive from the historical floods which tend to occur over the relatively wet northern mountains, the 2022 rainfall took place over arid southern Pakistan. Unlike the floods over northern mountains which had closer associations with extratropical westerly troughs, the heavy downpours in 2022 were primarily initiated by the synoptic low-pressure systems (LPS). The longevity and intensity of LPS were sustained and enhanced by the cross-equatorial monsoon flow, which has trended upward since the 1970s and is at a historical high. In combination with the zonal inflow of moisture induced by La Niña, a corridor of heavy rainfall extending from the Bay of Bengal toward southern Pakistan formed.

The signal of greenhouse-gas-forced changes in the heavy rainfall over Pakistan and the cross-equatorial monsoon flow is detectable in climate models, confirming that the likelihood of such extreme events would increase under future warming.

How to cite: You, Y. and Ting, M.: Climate change contributes to the record-shattering 2022 Pakistan rainfall, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4544, https://doi.org/10.5194/egusphere-egu23-4544, 2023.

The Murray Darling Basin, located in southeast Australia, is an agriculturally rich area, providing one-third of the country’ food supply. In 2017-2019 the region experienced one of its worst droughts since 1900. Rainfall in the Murray Darling Basin was consistently below average for three consecutive cool seasons, an unprecedented event on record. The drought set the extreme conditions that led later to the 2019-2020 Black Summer Bushfires. Previous studies suggest that the strong 2019 positive Indian Ocean Dipole intensified the conditions of the drought, however the state of the climate drivers cannot fully explain the onset and development of the Murray Darling Basin drought. In this study, we investigate processes other than remote climate drivers that may have triggered the drought. Using a Lagrangian model to backtrack moisture sources to southeast Australia, we show that local processes were crucial in explaining the onset and development of the drought. We identify the sources of moisture to the cool season precipitation over the Murray Darling Basin and show for the first time that the moisture supply from the Tasman Sea declined in 2017 and 2018. We further show that the expected moisture was instead transported northward by an anomalous anticyclonic circulation. Our results provide an explanation for the moisture and rainfall deficit that caused the 2017-19 drought. Understanding the processes that led to the 2017-2019 Murray Darling Basin drought is important for predicting and planning future multi-year droughts in Australia.

How to cite: Taschetto, A. S., Stojanovic, M., Holgate, C., Drumond, A., Evans, J., Gimeno, L., and Nieto, R.: Reduced moisture sources contributed to the 2017-2019 southeast Australian drought, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4681, https://doi.org/10.5194/egusphere-egu23-4681, 2023.

In the summer of 2022 unprecedented weather conditions in the UK lead to wildfires in many regions. Record breaking temperatures – above 40 °C for the first time - and prolonged dry conditions led to exceptional fire weather severity. On July 19th London Fire Brigade declared a major incident as firefighters battled several significant wildfires across the city. We investigate if climate change is enhancing summertime wildfire risk in the UK. 

We use reanalysis data from 1960 to the present day to analysis trends in the climatic indicators that influence the fire weather index. A large ensemble of initialised climate model simulations from the same time period are used to support the findings and identify as-yet-unrealised possible fire weather index situations in the UK. 

In the UK the term ‘wildfire’ is not limited to fires in wildland, but to any uncontrolled vegetation fire which requires action regarding suppression. Wildfires in the UK are considered a semi-natural hazard due to their close link with human activity. Though we investigate only meteorological influences, these are not the sole cause of wildfires – for example fuel availability plays a large role. 

Better understanding of the trends in the fire weather severity and chance of exceptional conditions for the UK will enable improved understanding of the risks. This information can feed into relevant policy and contingency planning, allowing society to become better prepared for the future as the planet continues to warm. 

How to cite: Thompson, V., Mitchell, D., Bloomfield, H., Dunstone, N., and Kay, G.: Increasing chances of summer wildfires in the UK?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2330, https://doi.org/10.5194/egusphere-egu23-2330, 2023.

It comes as no surprise that the future holds record-breaking weather and climate events. As global warming continues, temperature records will continue to be broken. Also heavy precipitation records are likely to be broken due to the increased water holding capacity of the atmosphere, in combination with changing atmospheric stability and circulation patterns. Improved estimates on the range of possible record-breaking precipitation events – now and in the future – are a first step to inform adequate adaptation policies for heavy precipitation. Of particular interest are events that break records by large margins – record-shattering events –, since these are likely to incur most damage and losses. 

In order to improve estimates of record shattering precipitation events in the present and future climate we use initial condition large ensemble simulation data (CESM2, SSP370) and statistical models. We evaluate record-shattering events in Rx1d (day with most precipitation per chosen time period (year or season)). In a stationary climate, the probability of Rx1d record-breaking is known to decrease with the number of data points since the start of measurements (inversely proportional). We find, however, that in our nonstationary climate, the decay in Rx1d record breaking and shattering probability is slowed down and even reversed in most world regions. Regional changes in record shattering probability are attributable to a changing underlying probability distribution of Rx1d, which also is region specific. We elucidate the contributions of changes in mean (distribution shift), and in variability (distribution widening/narrowing) to increasing record shattering probability by using a statistical model to create counterfactual realities representative of the regions of interest.

We focus on regions of a size relevant for national and cross-border policy that show differently driven changes in record shattering precipitation probabilities. For example, the annual probability of a record shattering precipitation event somewhere in the Benelux-Germany region which was hit by severe floods in summer 2021 increases from ~2% now to ~4.5% at the end of the century in CESM2. This increase results from a non-linear interaction between mean and variability increases, and is primarily driven by increasing variability. At lower latitudes, for example in Central America, the effect of variability is even stronger, where we find increasing record shattering probability despite a negative long-term trend in Rx1d levels.

Very unlikely events are, paradoxically, arguably the most important to know about, since their unimaginability often means that critical infrastructure is not sized to withstand these events. Our results may thus prove invaluable for regional policy. 

How to cite: de Vries, I., Sippel, S., Fischer, E., Zeder, J., Humphrey, V., and Knutti, R.: Increasing probability of extreme records in heavy precipitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14539, https://doi.org/10.5194/egusphere-egu23-14539, 2023.

Societally relevant weather impacts typically result from compound events, which are rare combinations of weather and climate drivers. For example, compound hot-dry events frequently cause damage to human and natural systems, often exceeding separate impacts from heatwaves and droughts. Focussing on four event types arising from different combinations of climate variables across space and time, we illustrate that robust analyses of compound events – such as frequency and uncertainty analysis under present-day and future conditions, event attribution, and exploration of low-probability-high-impact events – require very large sample sizes. In particular, the required sample is much larger than that needed for routinely considered univariate extremes. We demonstrate how large ensemble simulations from multiple climate models are crucial for advancing our assessments of compound events and for constructing robust model projections. For example, among the case studies, we focus on compound hot-dry events and show that large ensemble model simulations allow for identifying plausible extremely dry climates that, if occurring in a warmer world, would be associated with high risk from compound hot-dry events. Overall, combining large ensemble simulations with an improved physical understanding of compound events will ultimately provide practitioners and stakeholders with the best available information on climate risks.

How to cite: Bevacqua, E., Suarez-Gutierrez, L., Jezequel, A., Lehner, F., Vrac, M., Yiou, P., Zappa, G., and Zscheischler, J.: Advancing research on compound weather and climate events via large ensemble model simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2376, https://doi.org/10.5194/egusphere-egu23-2376, 2023.

The UK is committed to substantially increasing offshore wind capacity in its drive to decarbonise electricity production and achieve net zero. If low wind episodes – or “wind drought” events – occur during high energy demand periods, energy security may be threatened without alternative supply. To ensure resilience of the power system now and in the coming years as offshore wind generation grows, better understanding of the severity, frequency and duration of low wind episodes would be useful. Variability in winds is likely to dominate over trends in the next few decades, and hence having improved information on present day characteristics of wind drought is valuable.

Here we focus our attention on the North Sea as a centre of current and planned offshore wind resource for the UK and a number of other European countries, and on the winter season, given the occurrence of weather patterns that risk security of supply. We use a large ensemble of initialised climate model simulations to provide a synthetic but realistic event set that greatly increases the sample size of extreme events compared with that available from reanalysis data, and gives more robust information about their likelihood and properties. Using the basic unit of a week of low winds as the timescale of analysis, we report on the frequency and duration of wind drought events. In addition, we examine the wider conditions associated with wind drought events to investigate what remote factors may contribute to prolonged wind drought.

How to cite: Kay, G., Dunstone, N., Maidens, A., Scaife, A., Smith, D., Thornton, H., Dawkins, L., and Belcher, S.: Variability in North Sea wind energy and the potential for prolonged winter wind drought, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13309, https://doi.org/10.5194/egusphere-egu23-13309, 2023.

Extreme wind events are among the costliest natural disasters in Europe. Significant effort is dedicated to understanding the risk of such events, usually analysing observed storms in the modern era. However, it is likely that some historical windstorms were more extreme and/or followed different tracks from those in the modern era. Producing plausible reanalyses of such events would improve the quantification of current and future windstorm risks.

Billions of historical climatological observations remain unavailable to science as they exist only on paper, stored in numerous archives around the world. We demonstrate how the rescue of such paper observations has improved our understanding of an extreme windstorm that occurred in February 1903 and its significant impacts. By assimilating newly rescued atmospheric pressure observations into the 20th Century Reanalysis system, the storm is now credibly represented in an improved reanalysis of the event. In some locations this storm produced stronger winds than any event during the modern era. As a result, estimates of risk from severe storms, based on modern period data, may need to be revised. Simulations of the storm surge resulting from this storm show a large coastal surge of around 2.5m, comparing favourably with newly rescued tide gauge observations and increasing our confidence in the quality of the reconstruction.

In addition, we use novel reanalysis experiments to translate this windstorm into a warmer world to quantify how it might be different both in the present and in the future. We find that the same storm produces more intense rainfall and stronger winds in a warmer climate, providing a new approach to quantifying how extreme weather events will change as the world is warming.

How to cite: Hawkins, E., Brohan, P., Burgess, S., Burt, S., Compo, G., Gray, S., Haigh, I., Hersbach, H., Kuijjer, K., Martinez-Alvarado, O., McColl, C., Schurer, A., Slivinski, L., and Williams, J.: Quantifying windstorm risks by translating historical extreme events into the future, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9290, https://doi.org/10.5194/egusphere-egu23-9290, 2023.

The 2021 summer flooding was an extremely rare event, driven by precipitation extremes that exceed Dutch design levels for flood protection in regions away from the main rivers and coastline. However, similar events in neighboring locations cannot be ruled out even in the near future. The implications of such extreme rainfall amounts will vary by region, subject to local topography, hydraulic flow patterns, water management, and societal exposure. We explore the geographic distribution of potential flood impacts induced by a similar event by constructing impact-oriented event storylines for different subregions in the Netherlands. The plausibility of the storylines is underlined by using physical evidence, proven impact-modelling concepts, and expert judgment successfully assessing the (sometimes unexpected) outcomes. The approach supports impact assessment for extraordinary events.

The presentation will illustrate the development of the storylines, and its uptake and interpretation by governing authorities responsible for water safety, civil protection and water management.

How to cite: Van Den Hurk, B., de Bruijn, K., Slager, K., Hegnauer, M., and Rongen, G.: Storylines of the impacts in the Netherlands of alternative realizations of the Western Europe July 2021 floods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1212, https://doi.org/10.5194/egusphere-egu23-1212, 2023.