Factors responsible for the increasing trend of mei-yu season rainfall during 1979-2020 over the western and eastern mei-yu domain

近几十年来,江淮流域梅雨监测区(MMD)的梅雨期(6-7月)降水呈增加趋势.本文基于1979-2020年台站观测降水资料和ERA5再分析数据,从大气环流变异的角度揭示了这种长期增加趋势的主要影响因素.发现在MMD范围内,梅雨期降水趋势的增幅东部大于西部.水汽收支定量诊断表明,异常的蒸发和水汽平流对MMD西部和东部降水增加趋势的相对贡献是不同的.MMD西部(东部)的降水趋势主要归咎于增强的局地蒸发(增强的垂直水汽平流),后者又取决于MMD对流层中,低层的异常气旋环流.这种位于气候平均的西太平洋副热带高压西北侧的异常气旋有助于MMD东部600 hPa以上的水汽辐散增加,伴随加强的850 hPa水汽辐合,从而导致垂直水汽平流的增强.相反,该异常气旋则有利于增强MMD西部的局地蒸发.

The mei-yu season(June-July)rainfall over the mei-yu monitoring domain(MMD)in the Yangtze-Huaihe Basin has shown an increasing trend in recent decades.This study examines the dominant factors responsible for this increasing trend for the period 1979-2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation.Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD,the magnitude of the trends is slightly larger over the eastern MMD(EMMD)than over the western MMD(WMMD).Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD.The increasing rainfall trend over the WMMD(EMMD)is attributable to increased evaporation(enhanced vertical moisture advection),which is dependent on an anomalous cyclonic circulation in the middlelower troposphere over the MMD.Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 60o hPa over the EMMD,leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa.By contrast,the anomalous cyclone favors increasing local evaporation over the WMMD.