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Decadal changes of Meiyu rainfall around 1991 and its relationship with two types of ENSO

This study investigates Meiyu (June‐July) rainfall from 1980 to 2000 over Yangtze‐Huaihe River valley (YHRV) and reveals a decadal change around 1991 with a seesaw‐like distribution. This decadal change is closely associated with negative phase of preceding winter El Niño–Southern Oscillation (ENSO) Modoki (EM) in the pre‐1991 epoch and positive phase of conventional ENSO (CE) in the post‐1991 epoch. In response to negative EM (pre‐1991 epoch), the source of water vapor is western North Pacific, associated with the eastward retreat of western Pacific subtropical high (WPSH) and the northward shift of East Asian westerly jet (EAWJ). On the other hand, in response to positive CE (post‐1991 epoch), water vapor generally comes from the South China Sea, associated with the enhancement and westward advance of the WPSH and southward shift of the EAWJ. Two possible mechanisms are proposed to explain the relationship between preceding winter sea surface temperature (SST) and Meiyu rainfall over YHRV in the two epochs. In the pre‐1991 epoch, due to enhanced precipitation, solar radiation decreases, resulting in SST cooling over western North Pacific in Meiyu period. An anomalous anticyclone associated with the SST cooling suppresses precipitation over northern part of YHRV. Meanwhile, an anomalous cyclone accompanied with the anticyclone locates over East China Sea and brings more precipitation to the southern part of YHRV. In the post‐1991 epoch, the persistent simultaneous warming in eastern equatorial Pacific and Indian Ocean leads to abundant moisture brought to southern China. Further analysis suggests that the two types of ENSO have asymmetric features with respect to the impact of their positive and negative phases on the seesaw‐like Meiyu rainfall, which is closely related with EM La Niña (CE El Niño) in the pre‐1991 epoch (post‐1991 epoch).