Amplification effect of intra-seasonal variability of soil moisture on heat extremes over Eurasia

Drying soil has been conducive to a high frequency of extreme high-temperature events over many regions worldwide in recent decades. However, changes in the intraseasonal variability of soil moisture can also influence the likelihood of extremely high temperatures. Although previous investigators have examined the association between extremely high temperatures and large-scale atmospheric circulation variability, the role of land-atmosphere coupling dominated by soil moisture variability in extremely high temperatures, particularly over the Eurasian continent, is not well understood. In this study, on the basis of the Land Surface, Snow, and Soil Moisture Model Intercomparison Project, we found that land-atmosphere feedback amplified the variability of soil moisture in most regions of Eurasia during summer from 1980 to 2014. This amplification of soil moisture variability is closely correlated with more intensive intraseasonal variability of surface air temperature and more frequent occurrences of extreme high-temperature events, particularly in Europe, Siberia, Northeast Asia, and the Indochina Peninsula. This correlation implies that increasing the intraseasonal variability of soil moisture results in a high likelihood of heat extremes during summer in most parts of Eurasia except Asian desert areas. On the intraseasonal timescale, the land-atmosphere coupling increases the variability of surface sensible heat flux and net long-wave radiation heating the atmosphere by intensifying the soil moisture variability, thus amplifying the variability of surface air temperature and enhancing the extreme high-temperature days. This finding demonstrates the importance of changes in intraseasonal soil moisture variability for the increasing likelihood of heat extremes in summer.