Compound hot drought events in the Mei-yu region: influences from polar and tropical regions.

In the Mei-yu region, there has been noticeable increase in the occurrence of compound hot drought (CHD) events in recent years. However, the underlying causes of these occurrences remain poorly understood. To address this knowledge gap, we conducted a comprehensive study utilizing observational datasets, reanalysis datasets, and four numerical experiments to investigate the associated physical mechanisms. Our findings indicated that the prevalence of CHD events in the Mei-yu region is influenced strongly by two key factors: the decline in Barents Sea ice during February and the presence of a La Niña-like pattern of sea surface temperature (SST) in April. The decline in Barents Sea ice generates an anomalous Rossby wave in the Arctic that propagates southeastward. The La Niña-like SST pattern regulates a Rossby wave over western America, propagating along the subtropical jet stream. These two Rossby waves induce northward movement and strengthened intensity of the subtropical westerly jet in East Asia. The local circulation patterns in the Mei-yu region are influenced by the position and intensity of the subtropical jet, leading to downward motion in accordance with the secondary circulation theory for high-altitude jet streams. Consequently, these local circulation patterns might contribute to occurrence of CHD events. Moreover, our analysis revealed that the impact of Barents Sea ice and the La Niña-like SST pattern can explain approximately two-thirds of the mild CHD events in the Mei-yu region, and that the influence of each is relatively independent. This research underscores influences of polar-tropical systems on climate extremes in eastern Asia.