The research team confronted a long-standing puzzle. Central Europe sits in a climatological transition zone, wedged between the drying Mediterranean and the wetting Nordic countries, and different studies have reached contradictory conclusions about whether the region is becoming wetter or drier. Part of the disagreement stems from the metrics used. The researchers tackled this by employing two complementary drought indices computed over three-month accumulation windows: the Standardized Precipitation Index, which considers rainfall alone, and the Standardized Precipitation Evapotranspiration Index, which also accounts for atmospheric evaporative demand, the combined thirst of the atmosphere for water from soil and vegetation. The gap between these two measures turned out to be the key to the entire story.

To extend the record back to the pre-industrial era, the team harnessed three independent reanalysis datasets, each blending numerical weather models with historical observations. ERA5, the most modern and high-resolution product, assimilates satellites, aircraft, ships and station data but only reaches back to 1940. The Twentieth Century Reanalysis version 3 relies primarily on surface pressure observations and offers temporal consistency from 1836 onward, while the Modern Era Reanalysis incorporates proxy records, documentary sources and early instrumental measurements. Before trusting any of them, the researchers rigorously evaluated each dataset against century-long weather station records from De Bilt, Frankfurt am Main, Paris-Le Bourget and Uccle, using moving-window error metrics and correlation coefficients to identify the periods where each reanalysis could be considered reliable.

The evaluation revealed telling weaknesses. ERA5 showed spurious dryness in its earliest years, likely because upper-air temperature observations were scarce before 1946, while the Twentieth Century Reanalysis displayed excessive wetness before the late 1910s, when the number of assimilated Northern Hemisphere observations jumped sharply. ModE-RA proved dependable from 1844 onward. The final analysis therefore combined reliable windows from each dataset: 1947 to 2023 for ERA5, 1918 to 2015 for the Twentieth Century Reanalysis, and 1844 to 2008 for ModE-RA. Encouragingly, the datasets agreed well with one another, with correlation coefficients for drought-event identification reaching 0.79 between ERA5 and ModE-RA.

The historical perspective delivered some surprising verdicts. The most severe drought in the 180-year record was not the celebrated disaster of 2018 but the great drought of 1921, which persisted for an extraordinary fourteen months in the ModE-RA reconstruction, producing a severity more than double the threshold defining the worst ten percent of events. Comparable clusters of successive dry years to the 2011-2023 period occurred during 1857-1874 and 1947-1960, separated by conspicuously wet intervals in 1912-1920 and 1977-1988. This pronounced multidecadal oscillation means that any assessment of recent trends must reckon with the phase of natural variability, a caution the authors emphasize throughout.

Yet one finding stood out with unsettling clarity. When evaporative demand was included, the 2010s emerged as the driest decade since the pre-industrial era, with 35 drought months in the ERA5 record, nearly a third of the decade, and western Germany locally exceeding 40 drought months. When only precipitation was considered, that same decade looked entirely unremarkable, with just 21 drought months. This divergence is the fingerprint of warming. Since the 1990s, western Central Europe has warmed at 0.42 degrees Celsius per decade, faster than the global average, and the resulting rise in atmospheric evaporative demand has progressively offset the region’s increasing precipitation. In practical terms, droughts that once required a rainfall deficit now can be manufactured, at least in part, by heat alone.

The seasonal anatomy of this shift is equally revealing. Winter and autumn have become wetter, as robust precipitation increases outpace the modest warming-driven rise in evaporative demand during the energy-limited cold season. Summer, by contrast, shows the strongest increase in evaporative demand, which largely cancels out rainfall gains and produces drying in the water-balance index. Most strikingly, spring, historically an intermediate season, has recently tipped toward drying as well, with precipitation rising far below the roughly seven percent per degree expected from basic thermodynamic scaling while evaporative demand climbed more than twice as fast. Spring drying, the analysis shows, is a distinctly recent development, absent from the longer historical record.

To explain these patterns dynamically, the researchers turned to the atmosphere itself. They applied a k-means clustering algorithm to seasonal anomalies of 500 hectopascal geopotential height, the height of the pressure surface roughly five to six kilometers aloft, for every drought event across the Euro-Atlantic sector. Four recurrent circulation patterns emerged: a Baltic High, a British Isles High, a North-South Dipole combining high pressure over Northern Europe with low pressure to the south, and a European High centered directly over the study region. Each carries a distinct hydroclimatic signature. The Baltic High draws warm continental air from the southeast, inflating evaporative demand, while the British Isles High brings cool northerly flow and its droughts are driven almost purely by rainfall deficits. The North-South Dipole dominates winter droughts and, linked to cold easterly air masses, produces the weakest events.

The European High is the villain of the modern chapter. Droughts associated with this pattern, which combines strong anticyclonic subsidence, reduced cloud cover and the largest positive evaporative demand anomalies, are the most intense in the record, particularly over western Belgium and northern France, and they occur predominantly in spring. Crucially, the fraction of droughts tied to the European High has been rising across all reanalyses and both indices, with moderate to high statistical confidence, while associations with the British Isles High and the North-South Dipole have declined since 1947. This dynamical shift dovetails neatly with the observed hydroclimatic trends: the fading winter dipole aligns with winter wetting under increasingly positive North Atlantic Oscillation phases, and the ascending European High provides a coherent explanation for the emergence of spring drying.

The broader implication is that western Central Europe’s droughts are entering a new regime. Severity alone no longer tells the full story, because the 1921 benchmark can still be matched or exceeded by persistence, while the character of droughts, increasingly hot, evaporative and spring-rooted, has shifted beneath the apparent stability of the long-term water balance. The authors note that compound hot and dry conditions, which inflict impacts exceeding either hazard alone, are becoming the regional norm, and that multi-year droughts of the kind seen in 2018 are projected to intensify further as evaporative demand continues to climb. For the roughly 100 million people who live in this economic heartland of Europe, and for the croplands, grasslands and forests it supports, the message of the past 180 years is clear: the droughts of the future will not simply be repeats of the past, and monitoring systems that ignore the atmosphere’s growing thirst risk underestimating the threat.

Subject of Research: Long-term drought variability and associated atmospheric circulation patterns in western Central Europe since 1844

Article Title: Droughts in western Central Europe and associated atmospheric circulation patterns since 1844

Article References: Neimry, E., Goosse, H., & Jonard, M. (2026). Droughts in western Central Europe and associated atmospheric circulation patterns since 1844. Weather and Climate Dynamics, 7(3), 1759-1778. https://doi.org/10.5194/wcd-7-1759-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1759-2026

Keywords: drought, western Central Europe, atmospheric circulation, evaporative demand, climate change, reanalysis, SPEI, anticyclone, European High, multidecadal variability, spring drying, hydroclimate

The research, published in Atmospheric and Oceanic Science Letters, examined more than six decades of records, drawing on observations and atmospheric reanalysis data spanning 1961 to 2024. Rather than treating all heatwaves as a single category, the team separated them into three types: events that only push daytime temperatures to extremes, events that only produce anomalously hot nights, and the compound events in which both days and nights remain dangerously hot. Comparing the three across China, the researchers found that compound events are particularly concentrated over the Yangtze River Basin, the densely populated and agriculturally vital region that follows the country’s longest river.

One of the study’s most striking findings concerns rhythm. Compound heatwaves over the basin, the analysis showed, display a pronounced 10–30-day periodicity, a subseasonal cycle often referred to in atmospheric science as quasi-biweekly variability. Daytime-only and nighttime-only heatwaves are less tightly coupled to this rhythm. In other words, the events that deny residents any nighttime relief are not random occurrences scattered through summer; they tend to arrive on a recognizable subseasonal clock, driven by slowly evolving patterns in the large-scale atmospheric circulation.

That rhythmic structure matters for forecasting. Variability on the 10–30-day scale sits in the sweet spot of subseasonal-to-seasonal prediction, the forecast range that lies beyond the two-week limit of reliable weather prediction but shorter than seasonal outlooks. If forecasters can identify the precursors of the compound-heatwave cycle, they may gain a valuable head start in warning hospitals, power grid operators, and agricultural planners that a multi-day stretch of unrelenting heat is becoming more likely.

The study goes beyond identifying the rhythm and reconstructs the physical sequence that produces it. The researchers describe a two-stage process. In the first stage, about a week before a compound heatwave strikes, the region experiences increased upward motion and rainfall. That may sound counterintuitive for a heat event, but the wetter soils left behind by this pre-heatwave precipitation turn out to be a crucial ingredient. Moisture stored in the ground becomes fuel for the atmosphere in the days that follow.

In the second stage, as the event approaches, an atmospheric wave pattern propagates eastward toward the basin. This wave favors anticyclonic conditions over the Yangtze region, the kind of large-scale sinking motion that suppresses cloud formation. With fewer clouds overhead, more solar radiation reaches the surface, intensifying daytime heating. At the same time, the wetter soil supports stronger evapotranspiration, the process by which plants and soil release water vapor into the air. Southwesterly winds add a third ingredient, transporting warm and moist air into the region from lower latitudes.

The combined effect is a lower atmosphere that is simultaneously warm and humid, a state that sustains extreme heat through both halves of the day. During daylight hours, strong incoming solar radiation drives surface temperatures upward. After sunset, the abundant water vapor in the boundary layer, supplied by local evapotranspiration and by moisture transported on southwesterly flow, traps heat near the ground and slows radiative cooling, keeping nighttime temperatures anomalously high. Humid air also raises the heat index, making hot nights feel even more oppressive and dangerous to human health than dry heat of the same temperature.

“Compound heatwaves are not simply daytime heat continuing into the night,” said Prof. Yitian Qian, corresponding author of the study. “They result from the coordinated evolution of atmospheric circulation, radiation, soil moisture and moisture transport over a period of several days.” That framing represents a shift in how scientists think about these events. Rather than viewing a compound heatwave as a daytime extreme that merely lingers, the study presents it as the end product of a multi-day coupling between the sky and the land, in which rainfall a week earlier helps set the stage for humid, sleepless nights to come.

The Yangtze River Basin is a particularly consequential place for this kind of research. Home to hundreds of millions of people and a cornerstone of China’s rice production, the region has repeatedly experienced severe summer heat in recent years, including episodes that strained power supplies and threatened crops. Because compound heatwaves concentrate there more than elsewhere in China, according to the new analysis, understanding their subseasonal precursors carries direct implications for protecting both urban populations and food systems in one of the world’s most economically important river valleys.

The practical payoff of the study lies in early warning. The researchers note that changes in rainfall, soil moisture and large-scale circulation before heatwave onset may provide useful early signals of compound heat extremes. A week of unusually wet weather followed by the arrival of an eastward-moving wave pattern and building anticyclonic conditions could, in principle, be recognized in forecast models as a compound-heatwave signature well before the first extreme day arrives. Soil moisture observations, satellite estimates of evapotranspiration, and monitoring of quasi-biweekly circulation anomalies could all feed into such an early-detection framework.

There is also a broader scientific agenda ahead. The authors indicate that future work will examine how tropical and midlatitude atmospheric variability contribute to these events, and whether the precursor signals identified here can genuinely improve subseasonal prediction skill. If they can, the payoff would extend beyond the Yangtze: many of the world’s major heatwave-prone regions feature similar interactions between soil moisture, moisture transport and slowly evolving circulation patterns. For now, the study offers a clear message about the anatomy of sleepless heat. The nights that offer no relief are not accidents of a single hot afternoon, but the culmination of a week-long collaboration between rain-soaked ground, clearing skies, and rivers of warm, wet air converging over the basin.

Subject of Research: Subseasonal mechanisms sustaining day–night compound heatwaves over the Yangtze River Basin

Article Title: Why do some heatwaves last through the night in the Yangtze River Basin?

Article References: Why do some heatwaves last through the night in the Yangtze River Basin?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: compound heatwaves, Yangtze River Basin, subseasonal variability, quasi-biweekly oscillation, soil moisture, evapotranspiration, anticyclone, moisture transport, atmospheric circulation, nighttime heat, early warning, Atmospheric and Oceanic Science Letters

The urgency of the problem was underscored during the pre-monsoon season of 2026, when temperatures climbed above 46 degrees Celsius across many parts of India. Northern and central regions of the country endured prolonged and intense heat waves, while compound heatwave-PM2.5 pollution events during the pre-monsoon season became increasingly frequent. The convergence of scorching temperatures and elevated particle concentrations during this period drew renewed attention from public health authorities and atmospheric scientists alike, raising a fundamental question: why are these compound extreme events becoming more common, and what mechanisms are driving their formation?

To answer that question, the research team, led by Professor Meng Gao of the Department of Geography at Hong Kong Baptist University, examined pre-monsoon heat-PM2.5 compound extreme events across India between 2017 and 2024. The study combined three complementary lines of evidence: ground-based observations, satellite measurements, and atmospheric chemistry simulations. This integrated approach allowed the researchers to track not only when and where heat and pollution overlapped, but also the physical and chemical processes that linked them. Their analysis revealed that the frequency of compound extreme events increased markedly after 2020, a shift that stands out clearly against the baseline of the preceding years.

The scale of the overlap during individual episodes is striking. In April 2022, more than 70 percent of severe PM2.5 pollution episodes across northern India occurred simultaneously with heat waves. That figure illustrates how tightly the two hazards have become coupled in the region. Rather than alternating between a heat crisis and an air quality crisis, populations in northern India increasingly face both at once, a pattern that public health researchers consider substantially more dangerous because extreme heat amplifies the physiological stress imposed by polluted air, stressing cardiovascular and respiratory systems simultaneously.

At the heart of the new findings is the role of a specific large-scale circulation feature: an anomalous anticyclonic circulation over northern India. Anticyclones, characterized by sinking air and suppressed vertical mixing, are well known for trapping heat near the surface and clearing skies of clouds, which allows more solar radiation to reach the ground and drives temperatures even higher. The study found that this anomalous anticyclone strengthened heat conditions over northern India while simultaneously generating anomalous southeasterly winds that transported additional pollution precursors into the region. In other words, a single meteorological pattern was working on both sides of the compound-extreme equation at once, intensifying the heat while feeding the chemical ingredients needed to build particle pollution.

The chemical consequences of these conditions proved to be just as important as the meteorological ones. The combination of extreme heat and an influx of pollution precursors accelerated atmospheric chemical processes, substantially enhancing the formation of secondary organic aerosols, or SOA, a major component of PM2.5. Unlike primary particles emitted directly from vehicles, industry, or biomass burning, secondary aerosols are manufactured in the atmosphere itself, as gaseous precursor compounds undergo chemical reactions and condense into particulate form. The atmospheric chemistry simulations conducted by the team showed that SOA production increased by approximately 2.3 times during compound events, contributing significantly to the elevated particle concentrations observed under extreme heat.

This finding carries significant implications for how scientists and policymakers understand air quality in a warming world. Secondary organic aerosol formation is highly sensitive to temperature, radiation, and the availability of reactive precursor gases, all of which are modulated by the large-scale circulation patterns described in the study. As heat waves intensify under climate change, the chemical environment of the boundary layer changes in ways that can amplify particle production even if emissions of primary pollutants remain constant. The Indian case demonstrates that climate variability can influence air quality through this pathway, meaning that emission control strategies alone may not fully capture the future trajectory of pollution extremes in densely populated regions.

Lead author Huibin Dai emphasized that the study reframes how compound extremes should be investigated. Most studies have focused on heat waves and air pollution separately, Dai noted, but this research shows that large-scale atmospheric circulation can simultaneously intensify heat and promote secondary aerosol formation, creating favorable conditions for compound heat-PM2.5 extremes. According to Dai, the results highlight an important pathway through which climate variability can influence both weather extremes and air quality, linking two research communities that have often worked along parallel but separate tracks.

The broader context makes these findings particularly consequential for India. The country is home to some of the world’s most densely populated urban corridors, and its pre-monsoon season has historically been associated with severe heat stress on agriculture, water resources, and human health. The demonstration that the same circulation anomalies that drive dangerous heat can also supercharge the atmospheric chemistry that produces PM2.5 suggests that the health burden of future warming may be compounded in ways that standard heat wave or air quality forecasts do not capture. As climate warming is expected to increase the occurrence of extreme heat, these coupled meteorological and chemical processes may further raise the risk of compound environmental extremes across South Asia.

Looking ahead, the research team plans to extend its analysis into the future. The next step, Dai said, is to investigate how compound heat-PM2.5 extremes will evolve under future climate change and to quantify their impacts on human health. The ultimate goal of the work, according to the researchers, is to better understand the mechanisms behind these compound events and to provide scientific evidence that can support early prevention, risk reduction, and climate adaptation strategies. The study was supported by grants from the National Natural Science Foundation of China and the Research Grants Council of the Hong Kong Special Administrative Region, China, and its findings arrive at a moment when the overlap of heat and pollution is emerging as one of the defining environmental health challenges of a rapidly warming century.

Subject of Research: Compound heatwave and PM2.5 pollution extremes driven by atmospheric circulation over India

Article Title: Climate-driven atmospheric circulation fuels dangerous heat-PM2.5 extremes over India

Article References: Climate-driven atmospheric circulation fuels dangerous heat-PM2.5 extremes over India. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: India, heat waves, PM2.5, compound extremes, atmospheric circulation, anticyclone, secondary organic aerosols, air pollution, climate change, pre-monsoon, atmospheric chemistry, public health