1.南京大学 大气科学学院;2.复旦大学 大气与海洋科学系
Department of Atmospheric and Oceanic Sciences, Fudan University
厄尔尼诺(El Ni?o)事件对东亚和南亚次年夏季降水影响及其机理已经得到充分研究，但其对夏季青藏高原降水是否有显著影响还不清楚。本研究根据1950年后El Ni?o事件次年衰减期演变速度，对比分析衰减早型与晚型El Ni?o事件对南亚季风区与青藏高原夏季 (6-9月) 季节平均和月平均气候影响差异。结果显示在衰减早型次年夏季热带太平洋海温转为拉尼娜 (La Ni?a) 型且持续发展，引起Walker环流上升支西移，印度洋和南亚季风区上升运动加强，同时激发异常西北太平洋反气旋(NWPAC)，阿拉伯海异常气旋和伊朗高原异常反气旋性环流响应，增加7-9月对流层偏南气流和印度洋水汽输送，导致南亚和高原西南侧降水偏多。衰减晚型次年6-8月热带太平洋El Ni?o型海温仍维持，印度洋暖异常海温显著，对应的印度洋和南亚季风区上升运动较弱，NWPAC 西伸控制南亚季风区, 阿拉伯海和中西亚分别呈现异常反气旋和气旋性环流，导致青藏高原西风加强，水汽输送减少，南亚北部和高原降水一致偏少。结果表明：(1) El Ni?o显著影响次年青藏高原西南部夏季季节和月平均降水与温度，是印度和高原西南部夏季降水显著相关的重要原因; (2) El Ni?o衰减快慢速度对南亚和青藏高原西南部夏季季节内降水的影响有着重要差异。
Impacts of El Ni?o events on boreal summer rainfall over the East Asian Monsoon and South Asian Monsoon (SAM) regions and the associated mechanisms have been extensively examined. Nevertheless, the varying impacts of an El Ni?o on the Tibetan Plateau (TP) regional seasonal and monthly rainfall and circulation have not been systematically examined. Based on the timing of El Ni?o decay with respect to the boreal summer season, El Ni?o decay phases are classified into two types in this study using 1950-2018 sea surface temperature (SST) data, which are as follows: (1) early decay and (2) late decay. If El Ni?o decays below the threshold before spring, a La Ni?a sea surface temperature anomaly (SSTA) pattern usually develops during summer with increasing anomaly amplitudes from June to September, which causes an enhanced westward shift of the Walker circulation with a strong ascending branch over the tropical Indian Ocean (TIO) and the SAM, and induces concurrent heavy rainfall over the SAM and southwestern TP area from July to September. Meanwhile, the developing La Ni?a SSTA forces a response of the anomalous North Western Pacific anticyclonic (NWPAC), an anomalous cyclonic circulation over the Arabian Sea and an anticyclonic circulation over the Western Asian region, which induce a strengthening southerly wind anomaly, enhance water vapor transport to the Indian and TP regions from tropical Indian Ocean, and thus increase summer precipitation over northern India and southwestern TP. In contrast, if El Ni?o decays below the threshold after September, the eastern Pacific El Ni?o SSTA pattern and strong SST warming over the tropical Indian Ocean persists in June to July, then gradually weakens from August to September. This causes the anomalous ascending branch of the Walker circulation over the eastern TIO with a weak ascending branch over the western TIO and SAM, an anomalous eastwardly extended NWPAC, an anomalous anticyclonic circulation over the Arabian Sea and an cyclonic circulation over the Western and Central Asian region, which induce a strengthening westerly wind anomaly and reduces water vapor transport over the TP. The above responses result in deficient rainfall and warm surface temperatures in the central and northern SAM regions in June, but relatively increased rainfall and cool SAT in most of the SAM region during August and September. This coincides with dryness over northeastern India and the southwestern TP in June, and then with increasing precipitation over northwestern India and the western TP in September. Our results demonstrate that the decaying El Ni?o has a significant impact on summer seasonal and monthly precipitation and temperature over the TP, which may explain the positive correlation of precipitation between Indian and southwestern TP recently discussed in some studies. Our results also suggest that the differences in an El Ni?o decay phase have strong impacts on the seasonal and intraseasonal rainfall over the SAM region and over the southwestern TP.