多因子协同作用对1992年和1998年黄淮地区夏季降水异常的影响

1991年5月和1997年4月赤道中东太平洋均发生了El Ni?o事件，但是1992年夏季黄淮地区降水异常偏少，而1998年夏季却异常偏多。分析结果显示，1992年夏季西北太平洋副热带高压（以下简称副高）偏东，中高纬阻塞高压偏弱，黄淮地区降水异常偏少；而1998年夏季，副高偏西，中高纬阻塞高压活动频繁，黄淮地区降水异常偏多。对海温外强迫信号的诊断和数值模式试验显示:当西太平洋对流活动偏弱时，有利于副高西伸；鄂霍茨克海及以东海温偏高时，其上空的阻塞高压增强；北大西洋中纬度地区海温偏高时，有利于后期乌拉尔山高压脊明显增强。即在赤道中东太平洋发生El Ni?o事件的背景下，西太平洋对流、鄂霍茨克海附近亲潮区域和北大西洋中纬度区域海温异常可能是导致黄淮区域1992年夏季和1998年夏季降水差异大的主要原因。该工作显示仅根据El Ni?o事件的发生时间和强度无法完全预测黄淮地区夏季降水变化，需要综合考虑西太平洋对流、鄂霍茨克海附近海域和北大西洋中纬度区域海温异常对季风环流的影响，从多因子协同作用的角度诊断和预测黄淮地区夏季降水异常趋势，提高预测能力。

Combined Effect of Multiple Factors on the Summer Rainfall Anomalies over the Huanghe-Huaihe Valley in 1992 and 1998

El Niño events developed in the equatorial central-eastern Pacific in the spring of both 1991 and 1997. The former began in May and the latter in April. However, a dry summer occurred over the Huanghe-Huaihe valley in 1992, but a wet summer prevailed in 1998. Also, the summer circulation anomalies showed different patterns over the Eurasia-Northwest Pacific region between 1992 and 1998. In the summer of 1992, the Northwest Pacific subtropical high (NWPSH) shifted eastward and the blocking high activities over the Eurasian midlatitudes were weaker than normal, resulting in below-normal rainfall anomalies over the Huanghe-Huaihe valley. In contrast, the NWPSH shifted westward and the blocking high activities over the Eurasian midlatitudes were intensified in the summer of 1998, leading to a wet summer over the Huanghe-Huaihe valley. Further diagnosis and model experiment results indicate that the contrasting pattern of summer rainfall anomalies over the Huanghe-Huaihe valley between 1992 and 1998 was probably due to the Northwest Pacific convection activities anomalies, as well as SST anomalies over the Sea of Okhotsk, its adjacent region, and the midlatitudes of the North Atlantic. The summer Northwest Pacific convection activities were intensified (weakened) in 1992 (1998), which favored an eastward (westward) shift of the NWPSH. Additionally, the warm SST anomalies over the Sea of Okhotsk and adjacent region favored the intensified blocking high activities, whereas the warm SST anomalies over the midlatitudes of the North Atlantic favored the intensified blocking high activities over the Urals. These results reveal that a large proportion of the interannual variability of the summer rainfall anomalies over the Huanghe-Huaihe valley cannot be explained solely by the El Niño signals; rather, it is also related to the Northwest Pacific convection activities and SST anomalies over the Sea of Okhotsk and the midlatitudes of the North Atlantic. Therefore, the combined effect of multiple factors should be considered for improving the predictability of summer rainfall anomalies over the Huanghe-Huaihe valley on the seasonal to interannual timescale.