2020年与1998年梅汛期洪涝的异常大气环流及相关海表温度强迫的差异性分析

2020年梅汛期长江流域强降雨范围超越1998年,且雨量中心偏北,这与两年的异常大气环流和海表温度强迫差异有密切联系。与1998年相比,2020年西北太平洋异常反气旋(WNPAC)偏北偏强,中心偏东,东亚双阻形势偏弱,使得副高北抬加强,北侧的西南气流亦偏北偏强,中高纬反气旋和气旋对的存在使得30°N以北为异常东北风控制,冷空气偏强,长江流域上空的水汽含量亦偏多,这些环流差异直接导致2020年降水较1998年偏多且中心偏北。这两年在对流层中层都存在大西洋—西太平洋的波列,但2020年波列偏南,有利于东亚反气旋和气旋对的维持以及WNPAC和副高的北抬加强,而1998年波列偏北且偏强,有利于双阻形势的稳定。2019/2020年(1997/1998年)冬季发生中部型(东部型)El Ni?o事件,前者强迫的6—7月WNPAC北界位置较后者偏北;同时2019/2020年印度洋—太平洋中部一致增暖,使得WNPAC加强,中心东移。2020年(1998年)同期处在北大西洋强(弱)负三极子模态,可能是两年中高纬度环流形势差异的主要原因之一。2020年(1998年)南太平洋中部暖海温异常(冷异常)能加强(减弱)越赤道气流,有利于WNPAC偏东偏北(偏西偏南)和水汽输送加强(减弱)。综上,2020年与1998年梅汛期降水差异可能由多洋盆海温强迫协同作用所致。

Differences in the associated atmospheric circulation and SST anomalies for the floods during Meiyu season in 2020 and 1998

The area of heavy rainfall in the Yangtze River basin during the Meiyu period in 2020 was larger than that in 1998, and the rainfall center was to the north, which was closely related to the two-year abnormal atmospheric circulation and the forcing difference of Sea Surface Temperature(SST). Compared with 1998, the Northwest Pacific Anomaly Anticyclone(WNPAC) at low latitudes in 2020 was stronger and northward with the center eastward, and East Asia''s dual resistance was relatively weak, which strengthened the northward uplift of the subtropical high and the southwest airflow on the north side. The existence of the East Asian anticyclone and cyclone pair in high latitude made the north of 30°N controlled by the abnormal northeast wind, and the cold air in the south was also stronger. The water vapor content over the Yangtze River basin was also higher. These circulation differences directly led to more precipitation in 2020 than in 1998 and a northerly center. The Atlantic-West Pacific wave train existed in the middle troposphere for two years, but the wave train was southerly in 2020, which was beneficial to the maintenance of the East Asian anticyclone and cyclone pair and the strengthening of WNPAC and the subtropical high. In 1998, the wave train was northerly and stronger, which was conducive to the stability of the dual blocking pattern. 2019/2020(1997/1998) experienced the central type(eastern type) El Niño event. The former was beneficial to norther WNPAC from June to July than the latter. At the same time, there was a consistent warming between the Indian Ocean and the central Pacific Ocean in 2019/2020, which strengthened WNPAC, and the center shift to the east. In the same period of 2020/1998, there were strong(weak) negative tripole SST over the North Atlantic, which might be one of the main reasons for the difference of the atmospheric circulation anomalies at mid-high latitudes. In 2020(1998), the warm sea surface temperature anomaly(cold anomaly) in the central South Pacific can strengthen(weaken) the cross equatorial flow, which is conducive to WNPAC east and north(west and south)and strengthened(weakened) water vapor transport. In summary, the difference in precipitation during the Meiyu period between 2020 and 1998 might be caused by the synergistic effect of multi-oceanic sea temperature forcing.