| 观测分析El Ni?o衰减早晚对南亚与青藏高原夏季降水和气温的影响 |
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| 引用本文： | 梁涵洲,吴其冈,任雪娟,姚永红,刘师佐.观测分析El Ni?o衰减早晚对南亚与青藏高原夏季降水和气温的影响[J].大气科学,2021,45(4):777-798. |
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| 作者姓名： | 梁涵洲 吴其冈 任雪娟 姚永红 刘师佐 |
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| 作者单位： | 1.南京大学大气科学学院，南京 210023 |
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| 基金项目： | 国家自然科学基金项目91837206 |
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| 摘 要： | El Ni?o（厄尔尼诺）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La Ni?a（拉尼娜）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。
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| 关 键 词： | 衰减El Ni?o 南亚夏季风 青藏高原 夏季降水 西北太平洋反气旋 |
| 收稿时间： | 2020-04-06 |
Impacts of Decay of Different El Ni?o Types on Boreal Summer Rainfall and Surface Air Temperature in South Asian Monsoon Region and Tibetan Plateau |
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| LIANG Hanzhou,WU Qigang,REN Xuejuan,YAO Yonghong,LIU Shizuo.Impacts of Decay of Different El Ni?o Types on Boreal Summer Rainfall and Surface Air Temperature in South Asian Monsoon Region and Tibetan Plateau[J].Chinese Journal of Atmospheric Sciences,2021,45(4):777-798. |
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| Authors: | LIANG Hanzhou WU Qigang REN Xuejuan YAO Yonghong LIU Shizuo |
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| Affiliation: | 1.School of Atmospheric Sciences, Nanjing University, Nanjing 2100232.Department of Atmospheric and Oceanic Sciences, Fudan University, Shanghai 2004383.China Innovation Center of Ocean and Atmosphere System, Zhuhai Fudan Innovation Research Institute, Zhuhai, 518057 |
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| Abstract: | In this paper, we extensively examine the impacts of El Ni?o events on boreal summer rainfall over the East Asian Monsoon and South Asian Monsoon (SAM) regions and their associated mechanisms. To date, the various impacts of an El Ni?o event on the Tibetan Plateau (TP) regional seasonal and monthly rainfall and circulation have not been systematically examined. Based on the timing of the El Ni?o decay with respect to the boreal summer season and 1950–2018 sea surface temperature (SST) data, the El Ni?o decay phases are classified into two types: (1) early decay and (2) late decay. If the El Ni?o decays to 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. This causes an enhanced westward shift in 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 areas from July to September. Meanwhile, the developing La Ni?a SSTA forces a response by the anomalous North Western Pacific anticyclone (NWPAC), an anomalous cyclonic circulation over the Arabian Sea, and anticyclonic circulation over the Western Asian region. These induce a strengthening southerly wind anomaly, enhance water vapor transport to the Indian and TP regions from the TIO, and thus increase summer precipitation over northern India and the southwestern TP. In contrast, if El Ni?o decays below the threshold after September, the eastern Pacific El Ni?o SSTA pattern and the strong SST warming over the TIO persists into June to July, then gradually weakens from August to September. This causes an 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 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 surface air temperature over most of the SAM region during August and September. This coincides with dryness over northeastern India and the southwestern TP in June, and then increasing precipitation over northwestern India and the western TP in September.Our results confirm that a 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 between Indian and southwestern TP precipitation recently discussed in some studies. Our results also suggest that differences in the El Ni?o decay phase have strong impacts on the seasonal and intraseasonal rainfall over the SAM region and the southwestern TP. |
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