How can the role of regional water vapor dynamics in causing precipitation intensity in the region be realized? Are water vapor dynamics changing with climate change in the current conditions of the Earth's climate?

While sub-daily precipitation extremes cause flash flooding and pose risk to life, longer precipitation extremes threaten infrastructure such as water supply dams. Frequent storm or floods events replenish water supplies, ensuring the health of our ecosystems, while rarer larger storms or floods cause damage to property and life. These differing impacts depend on both storm rarity and duration and are largely dependent on coincident atmospheric water vapour. Using a novel metric that quantifies the extent of concurrency that exists between precipitation and total water vapour extremes, large regional variations are identified across the globe. Tropical regions such as Northeast Africa and South/East Asia consistently exhibit greater concurrency across all precipitation durations. In contrast, areas of the extra-tropics, such as the Mediterranean and Northwest Americas, show a rapid decline in concurrency with increasing duration. However, for rare events of long duration, nontropical regions maintain high concurrency. With the link between climate change and increasing total water vapour well established, these results suggest that flood events will increase globally, with increases most apparent for longer and rarer events. This work underscores the need for tailored regional strategies in managing extreme precipitation and flood events in the future.Climate change is progressing at a rate unparalleled in recent geological timescales. While the Paleocene–Eocene Thermal Maximum around 55.5 million years ago saw rapid climate shifts (Kennett and Stott, 1991), Earth has experienced significant climate variability throughout its history. However, the current rate of human-induced warming is unprecedented in recent geological timescales (Bowen et al., 2015; IEA, 2021). Human-induced warming has reached approximately 1.3 ◦C above pre-industrial levels since the mid-19th century (IPCC, 2021), leading to widespread climate disasters (Bindi et al., 2018). At one extreme floods are especially devastating (Thomas and Lopez, ´ 2015). In recent years, countries around the world have faced catastrophic flooding events. For example, in 2022, Australia was struck by the Great Deluge with the highest monthly total rainfalls on record, and Pakistan experienced its worst floods in history (Climate Council, 2022; Nanditha et al., 2023). Flash floods in Afghanistan and Pakistan in 2023 further highlighted the catastrophic impact of extreme rainfalls and floods resulting in dozens of people losing their lives (Al Jazeera, 2023). As the world grapples with abrupt and relentless environmental changes these events, marked by significant loss and destruction, emphasize the urgent need for climate action, disaster preparedness, and resilience building. With regard to less extreme floods, dams capture flood waters to secure water supplies, and smaller more frequent floods are critical for sustaining the biodiversity and health of our floodplains. However, discussions about water shortages in Hoover Dam (Chesky, 2022; Sutton and McCleary, 2021) and the degradation of the health of our waterways emphasize the importance of a comprehensive understanding of changes in our regional climate patterns (Xie et al., 2015). The key driver of these catastrophic flood events is extreme precipitation (Komori et al., 2012; O’Connor and Costa, 2004; Syvitski and Brakenridge, 2013; Woldemeskel and Sharma, 2016). Climate change exacerbates this driver, inducing regional and global increases in extreme precipitation throughout the last century (Alexander et al., 2006; Donat et al., 2013; Donat et al., 2016; Sun et al., 2021). The situation is only projected to worsen. With extreme precipitation becoming more severe and frequent due to global warming (Bao et al., 2017; Prein et al., 2017; Sillmann et al., 2013; Westra et al., 2014), the population exposed to pluvial and riverine flooding is projected to increase substantially by the end of this century (Hirabayashi et al., 2013; Jongman et al., 2012; Smith et al., 2019; Ward et al., 2013). While total water vapor is a critical factor in driving extreme precipitation, the association between the two is not well understood. In the context of climate change characterized by increasing atmospheric temperatures, as shown in Fig. S1, we observe a dominant upward trend in total water vapor, particularly in the tropics, with this trend consistent regardless of the data set used to quantify this trend (Borger et al., 2022; Ren et al., 2023). This means that, if precipitation extremes are linked to the available moisture in the atmosphere, extreme precipitation will also increase (Alexander and Arblaster, 2017; Ali et al., 2018; Donat et al., 2016; Papalexiou and Montanari, 2019; Roderick et al., 2020). But the question remains, how well are precipitation extremes and total water vapor linked? Here we use the Concurrent Extreme Index (CEI) to build on previous work (Kim et al., 2022), to assess this relationship for varying storm durations, over a longer period of time, across multiple data sets, to deepen our understanding of the total precipitation water – extreme precipitation relationship and its implications for climate change. Global variability of precipitation extremes and total water vapor The CEI assesses the expected difference in the Cumulative Distribution Function (CDF) of extreme precipitation and coincident total water vapour, where a CEI equal to 1 indicates that extreme precipitation events coincide with maxima in the total water vapour, while a CEI equal to zero indicates there is no relationship. Here, we focus on the ERA5 water vapour and illustrate the variation in CEI for two contrasting rainfall durations (3 h and up to 10 days) in Fig. 1. For short 3-hour annual maxima (Figure a) the CEI is high across most of the globe indicating 3-hour precipitation annual maxima generally coincide with the maxima in total water vapour. The exception of tropical rainforests, including the Amazon, Congo Basin, and Indonesian Rainforests, is likely due to the dense vegetation’s evapotranspiration influencing localized precipitation patterns (Gimeno et al., 2016; Otto et al., 2013). Similarly, areas near the Mediterranean also exhibit lower CEI values. This can be attributed to large-scale atmospheric flow patterns, such as the influence of the North Atlantic Oscillation (NAO), which affects the regional climate, and regional climatic changes that modify moisture availability and transport (Rebora et al., 2013; Tramblay and Somot, 2018). In contrast, for 10-day maxima (Fig. 1b), only the tropics largely maintain their high CEI values with many regions outside the tropics seeing a decline in CEI at this longer duration. While Trenberth et al. (2003) highlighted the role of increasing atmospheric moisture in intensifying precipitation events, particularly in regions undergoing significant warming, the results presented here highlight the variability of the CEI over short and long durations, revealing potential regional differences in the impact of changes in atmospheric water vapour on extreme precipitation events. 3. Impact of storm duration on the relationship between moderate precipitation extremes and total water vapour There are clear regional differences as well as differences between storm durations. Fig. 2a presents the CEI for the most extreme average annual event (1E) across a range of durations, grouped by the updated IPCC WGI regions (Iturbide et al., 2020). The tropical belt, as well as regions like Southern Africa and Central and East Asia, consistently show a high CEI that remains high or increases with longer durations. On the other hand, regions in the mid to high latitudes such as the Mediterranean (MED) and Western Central Asia (WCA), see a decline in CEI as durations lengthen. The regions of Northern Australia (NAU), Northern Europe (NEU) and Madagascar (MDG) have the highest CEI values indicating an increase in the water vapour (Fig. 1) would lead to an increase in precipitation events in these regions. The U ratio allows us to unpack where across the globe these trends with duration are significant (Fig. 3). Consider Fig. 3b which presents the U ratio for events that occur on average once per year (1E). In the tropic belt, a U ratio greater than 1 indicates that CEI values in these regions increase with longer durations. This is in contrast with the mid-high latitude regions, where a U ratio less than 1 indicates that CEI values decrease with extended durations. The distinction in behaviour becomes more pronounced as rarity decreases. For example, Fig. 3a presents the U ratio for events that occur on average once every two years (0.5E), while Fig. 3d presents the U ratio for events that occur on average 10 times per year (10E). As rarity increases the magnitude of the U ratio increases. Notably, Northern Australia (NAU) exhibits a marked increase in the U ratio, highlighting the strong dependence of the CEI with duration in this region. Events of the order of the annual maxima (1E) provide insight into potential impacts on water supplies. Dykman et al. (2023) found that annual streamflow volumes are influenced by a range of events with different exceedance frequencies. Infrastructure like the Grand Ethiopian Renaissance Dam (GERD) in Northeastern Africa (NEAF) and the Farakka Barrage in South Asia (SAS) are located in areas with high CEI values across longer durations (Fig. 2). This indicates that these regions may face more frequent extreme events, necessitating a reassessment of water management strategies. On the other hand, areas like Western North America (WNA) and the Mediterranean (MED) tell a different story. Here, the increase in water vapour was less evident, and the CEI tends to decrease for longer durations. Already declining water resources in these regions will continue to decline (Gudmundsson et al., 2021). Supporting these findings, Lee et al. (2023) observed increased rainfall extremes in places like China, due to the warming Indo-Pacific warm pool (IPWP). This increase is attributed to heightened atmospheric moisture and deep convective rainfall caused by global.warming-induced land-sea temperature differences. In central western Europe, Meyer et al. (2022) associated flash floods with extreme rainfall, indicating changed atmospheric conditions leading to increased precipitation extremes. In the tropics, from Fig. S2a to d, the CEI remains consistently high regardless of the event rarity represented by E. This indicates the tropics’ frequent exposure to simultaneous extreme events. The tight confidence interval further suggests a regular extreme event pattern in this region. On the other hand, the non-tropics show more variation. The CEI decreases with longer durations, particularly for events with larger E values from Fig. S2a to d. This variation, highlighted by a wider confidence interval, indicates a diverse extreme event pattern in these areas. The distinction in CEI behaviour between the two regions becomes more pronounced with less frequent events, pointing to the unique weather patterns of each area. For non-tropical regions, the variations can arise from several factors. Guan et al. (2022) highlighted the combined effects of intra-seasonal oscillations from both mid-high latitudes and the tropics in North China, affecting continuous extreme rainfall events. While the tropics consistently experience significant extreme rainfall due to increased atmospheric moisture driven by high sea surface temperatures and convective activity, the variations in non-tropics result from factors like atmospheric oscillations, including the El Nino-Southern ˜ Oscillation (ENSO), and regional climate dynamics that affect moisture distribution and weather patterns. It is important to note the limitations of using ERA5 reanalysis data, as different sources of information may lead to different results (Borger et al., 2022). 4. Variability in the dynamics between total water vapor and extreme precipitation with event rarity As rarer precipitation events are more likely to have more devastating consequences Fig. 4 presents the trend in CEI with decreasing rarity conditioned by duration. Ranging from the shortest duration of 3 h (Fig. 4a) to the longest duration of 10 days (Fig. 4f) the U ratio is predominantly less than 1. This indicates that as the rarity decreases (or as E increases), the CEI values in these regions also decrease. That is, the rarer the event the stronger the relationship between extreme precipitation and total water vapour. As the duration increases, the U ratio approaches 1, suggesting that the decreasing trend in CEI values with increased event frequency becomes less pronounced. This transition is evident as regions transition from a pronounced blue shade in the 3-hour duration to a more muted hue by the 10-day duration. For the shortest duration of 3 h (3H) in Fig. S3a, both the tropics and non-tropics show similar CEI variations with E, indicating a consistent relationship between W and EP for such short events. As we move from Fig. S3a to f, representing durations from 3 h to 10 days, a pattern becomes evident. In the tropics, the CEI remains consistently high across all rarities, indicating a narrow confidence interval. This suggests that the tropics frequently experience simultaneous extreme events, regardless of their rarity. In contrast, the non-tropics reveal a more intricate pattern. While the CEI is high for rare events (E = 0.5), it decreases with less rarity (increasing E values). This reduction is more evident with longer durations, pointing to the unique weather patterns in these regions. The wider confidence interval in non-tropical areas further indicates the variability of extreme events in these regions. Escobar-Gonzalez ´ et al. (2022) highlighted the continuous extreme rainfall events in tropical regions, driven by factors such as increased atmospheric moisture and convective processes. Meanwhile, a study by Singh et al. (2023) in nontropical regions emphasized the role of large-scale climate oscillations in rainfall extremes, suggesting the impact of atmospheric oscillations and regional climate conditions on event variability. Australia stands out with a clear decreasing tendency, emphasizing the region’s distinct response to different rarities and durations. As highlighted in Fig. S1, regions situated within the tropic belt, such as NWS, CAF, and SEA, exhibit a distinct increasing trend in total water vapor, both in terms of trend visibility and magnitude. Despite a slight reduction in the CEI within these regions, a decrease in the event rarity (or as E increases) demonstrates a consistently high CEI value across all durations, alongside narrow confidence intervals, as illustrated in to f. This finding supports the notion that tropical belt regions could experience a quantitative increase in extreme precipitation events, spanning a wide range of rarities and durations. Conclusions There is a clear link between precipitation extremes and total water vapor, implying that an increase in total water vapor with climate change . can potentially lead to more frequent and severe precipitation events, presenting challenges for regions already susceptible.to water-related disasters. Regions like Northeast Africa and South/East Asia consistently display high CEIs across storm durations. In contrast, areas such as the Mediterranean and Northwest Americas exhibit a marked decrease in CEI with increasing duration. This suggests that while certain regions, like those in the tropics, might grapple with consistent challenges tied to increasing extreme precipitation, others could face a diverse set of changes in extreme events. In the extra-tropics the decreasing CEI with increasing duration for frequent extremes suggests these regions could continue to experience commensurate drying in water supplies while extreme flooding increases. Our findings emphasize the need to develop region-specific adaptation strategies to manage the varying impacts of extreme precipitation events. Given the significant regional differences in the relationship between total water vapor and extreme precipitation, the development of effective climate resilience strategies will require the use of tailored approaches. Notably, for longer precipitation events, non-tropical regions tend to show lower CEI values. This suggests that while these regions may experience a drop in CEI for longer events, they remain vulnerable to extreme precipitation increases for shorter duration events. Conversely, in tropical regions, the high CEI values across all durations indicate that these areas consistently face a high risk of extreme precipitation. The implications of increased water vapor extend to various extreme events at different spatial and temporal scales, affecting infrastructure such as dams which are designed for water supply and/or flood control, and infrastructure related to floodplain management and urban drainage design. Such infrastructure must account for a range of short and long duration flood events, as well as frequent and rare occurrences, necessitating comprehensive adaptation strategies. However, it is important to recognize the inherent uncertainties in projecting climate change impacts, particularly in quantifying these threats. Policymakers and authorities must consider these uncertainties in their decision-making processes and prioritize adaptive management strategies that can accommodate a range of possible future scenarios over different planning periods relevant to the service life of the assets. While this study provides essential insights into the relationship between extreme precipitation and total water vapor, it also points to areas that warrant further exploration. Exploring factors such as water vapor transport, its dissipation, and turnover time, as investigated by Borger et al. (2022), will offer a comprehensive perspective on the challenges brought forth by climate change. Understanding these dynamics is crucial for predicting the frequency and intensity of extreme precipitation events. As the global climate landscape continues to shift, such knowledge will be instrumental in formulating strategies to mitigate the impacts of extreme weather events.Precipitation data for this study is sourced from the Multi-Source Weighted-Ensemble Precipitation (MSWEP) (Beck et al., 2019). Total water vapor is sourced from four reanalysis datasets: the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis version 5 (ERA5) (Hersbach et al., 2020), the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) (Gelaro et al., 2017), the Japanese 55-year Reanalysis (JRA-55) (Kobayashi et al., 2015), and the National Centers for Environmental Prediction (NCEP) (Saha et al., 2010; Saha et al., 2014). To ensure uniformity across the datasets, they were harmonized to a 1-degree spatial resolution and a 3-hour temporal resolution. For trend analysis, the study encompasses a period of 41 years, from January 1, 1980, to December 31, 2020. However, for the CEI analysis, the timeframe is limited to 37 years, from January 1, 1980, to December 31, 2016, due to the data availability constraints of the MSWEP dataset. It should be noted that this temporal trend analysis is based on a relatively short-time dataset, which may impact the robustness of longterm trends. It is also important to acknowledge the uncertainties inherent in our analyses, particularly those related to data quality, model assumptions, and the short duration of some datasets. These uncertainties can influence the interpretation of trends and their implications for water resource management. Details of these datasets are presented in Table 1. The selection of these four datasets is motivated by the need to investigate the consistency and concurrency in the total water vapor, as assessed through the Data Concurrence Index (DCI) (Anabalon´ and Sharma, 2017; Kim et al., 2021). This approach allows for a robust examination of the spatial distribution and correlation of total water vapor trends across different data sources. The details of the DCI methodology and its application to these datasets are provided in the following subsection. For the rest of the analysis, the ERA5 dataset is specifically chosen, given its extensive verification and widespread utilization in existing climate studies (Martens et al., 2020; Tetzner et al., 2019). Results are summarized using the 46 land regions from the updated Intergovernmental Panel on Climate Change (IPCC)’s Working Group I (WGI) reference regions version 4 (Iturbide et al., 2020). The full names and spatial boundaries of these regions can be found at: https://essd. copernicus.org/articles/12/2959/2020/. For a more streamlined interpretation, the globe was also divided into tropics and non-tropics. The tropics encompass areas within ± 30◦ latitude, while the non-tropics cover regions outside this latitude range. This division was adopted as the CEI values over the tropics and non-tropics are visually distinct, enhancing the clarity of the data presentation. While the detailed breakdown across the 46 regions offers a comprehensive view, simplifying the presentation into these two broader regions facilitates a more concise and intuitive understanding of the CEI behaviours in relation to rarity, duration, and spatial distribution.