Asymmetric response of short- and long-duration dry spells to warming during the warm-rain season over Eastern monsoon China

Changes in dry spell duration have been analyzed over China and worldwide. However, the asymmetric response (i.e. skewed distribution) of short- and long-duration dry spells to warming temperature and possible mechanisms remain unknown. Here, we investigate this response based on daily weather observations during the warm-rain season (May-September) over 1961–2019 from 1540 quality-controlled stations across Eastern Monsoon China (EMC). Our results show a 1℃ warming in surface air temperature (SAT) and a considerable elongation of dry spells. Specifically, the regional average, maximum, and total dry spell duration over EMC increased by +7.06%, +6.17%, and +5.16%, respectively over 1961–2019. The areas in EMC with long dry spells and high SAT are also the areas that have experienced faster increases in the duration of dry spells per degree of warming. We find increases in the frequency of long-duration dry spells, but significant decreases in the frequency of short-duration dry spells. The increasing frequency of long-duration dry-spells can be explained by the slight increase in land evaporation (+1.6%/℃) over EMC and the decrease in integrated moisture vapor transport (-3.8%/℃) with warming, such that atmospheric moisture sources cannot meet the warming-induced demand, resulting in significant decreases in relative humidity (−2.4%/℃) and precipitation (−7.0%/℃). In contrast, the decreasing frequency of short-duration dry spells during hotter warm-rain seasons occurs when the saturation vapor pressure deficit is enhanced, leading to prolonged periods where the atmosphere is replenished to necessary saturation levels for local precipitation. Relative to cold warm-rain seasons, hot warm-rain seasons with higher SAT witness cold-dry air advection in the upper level, lower convective available potential energy, and stronger vapor divergence over EMC, which tends to suppress precipitation occurrence. With future warming, the increasing frequency and duration of dry spells in the warm-rain season is likely to be conducive to the occurrence of severe droughts and heatwaves in EMC, with negative impacts on socio-economic development in China.