Extreme weather and climate events, such as droughts, heatwaves, and rainstorms, pose serious threats to human health, agricultural production, and energy supplies. These events often occur at the same time, and such "compound extreme events" can cause far more damage than any single event.
A new study led by Prof. WANG Aihui from the Institute of Atmospheric Physics of the Chinese Academy of Sciences has revealed that land–atmosphere coupling (the coupling of the land surface and the atmosphere through processes such as evaporation, transpiration, and heat exchange) may have played an important role in the persistent compound extreme events in the summer of 2022 in eastern China.
Eastern China is not only a typical monsoon region, but also a hotspot of land–atmosphere coupling, both of which contribute to the challenge of accurately predicting extreme weather and climate events in this region. In this respect, land surface features are important sources of predictability on various time scales. Moreover, significant asymmetry exists in the feedback between these features and the atmosphere, which often plays a significant role in amplifying extreme weather and climate events.
In the summer of 2022, persistent high temperature and drought compound extreme events occurred in central and eastern China, affecting a wide area and lasting for a long time. Overall, the extreme heat lasted for 79 days, which was the longest since 1961, and the intensity was also the highest on record. Furthermore, the end time of these high-temperature events was later than it would normally be.
The researchers found that the persistent high temperature, precipitation deficit, and soil drought occurred in eastern China during the warm season of 2022. Among them, the above compound extreme events maintained in the middle and lower reaches of the Yangtze River and southeast China from July to September, significantly deviating from the historical situation in the same period.
"Dry soil can be regarded as an important prior signal of subsequent high-temperature events, and the intraseasonal variation of land–atmosphere feedback can strongly regulate the persistence of such extreme events," said Prof. WANG.
In humid regimes, such as the middle and lower reaches of the Yangtze River valley and the southeast China, evapotranspiration is mainly limited by the available energy over the land surface. In other words, the soil water content in these areas is abundant, and in general, the more radiation energy absorbed by the land surface, the stronger the evapotranspiration.
From mid-summer to early autumn of 2022, the available energy increased to such an extent that the limiting effect of soil water content on evapotranspiration gradually exceeded its long-term average. High temperatures reduced the soil moisture, while the dry land surface further heated the atmosphere in return through strong feedback. Meanwhile, high temperatures accelerated the loss of soil water by regulating evapotranspiration, which inhibited the formation of rain after July.
The findings of this study enhance our understanding of the processes underlying persistent extreme events and potentially offer insights into better predicting them.