Abstract
Located at the southern boundary of the tropical rainfall belt within the South Africa monsoon regime, Rodrigues Island, ∼2500 km east of East Africa, is ideally located to investigate climatic changes over the southwest Indian Ocean (SWIO). In this study, we investigate the climatic controls of its modern interannual rainfall variability in terms of teleconnection and local effects. We find that increased rainfall over the SWIO tends to occur in association with anomalously warm (cold) SSTs over the equatorial central Pacific (Maritime Continent), resembling the central Pacific El Niño, closely linked with the Victoria mode in the North Pacific. Our analyses show that the low-level convergence induced by warm SST over the equatorial central Pacific leads to anomalous low-level divergence over the Maritime Continent and convergence over a large area surrounding the Rodrigues Island, which leads to increased rainfall over the SWIO during the rainy season. Meanwhile, the excited Rossby wave along the tropical Indian Ocean transports more water vapor from the tropical convergence zone into the SWIO via intensified northwest wind. Furthermore, positive feedback induced by the Rossby wave response to the increased rainfall in the region contributes to the large interannual variations over the SWIO.
摘 要
西南印度洋是南非季风区的重要组成部分, 加强西南印度洋的现代气候变化研究对理解其当地降水的变化规律及古气候研究有一定的指导意义. 位于赤道辐合带南部边界的罗德里格斯岛 (罗岛) 距离东非赤道地区约 2500 千米, 是研究南非季风区西南印度洋气候变化的理想地点. 本文利用观测数据和再分析资料, 以罗岛为中心研究了西南印度洋雨季 (1-4 月)降水变化特征, 并从遥相关和局地环流两个角度探讨了影响西南印度洋雨季降水年际变化的主控因子和变化机制. 结果表明, 西南印度洋雨季降水增加, 常常对应着同期中太平洋型厄尔尼诺海温异常 (赤道中太平洋海表温度偏暖、 海洋性大陆地区海表温度偏冷). 一方面, 偏暖的赤道中太平洋海表温度会产生低空辐合 (高空辐散), 并引起海洋性大陆的低空辐散 (高空辐合) 和西南印度洋的低空辐合 (高空辐散), 进而引起西南印度洋出现异常上升运动和降水增加. 另一方面, 海洋性大陆的冷海温和降水负异常会激发Gill响应, 冷源西侧的赤道罗斯贝波会把热带印度洋水汽通过增强的东北风越赤道转为西北风后输送至西南印度洋, 进而增加西南印度洋地区的降水. 此外, 一旦西南印度洋降水由于上述两种机制出现增加, 就会通过释放凝结潜热并在其西侧进一步激发出赤道罗斯贝波响应, 从而增强热带印度洋向该地的水汽输送. 由于这一正反馈的存在, 西南印度洋局地雨季降水的变率十分明显. 本研究表明, 大尺度大气环流和局地正反馈机制的共同作用可能是造成以罗岛为中心的西南印度洋雨季降水剧烈波动的主要原因. 本研究将为理解西南印度洋的古气候变化提供一定的理论参考.
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Acknowledgements
The authors thank editor Dr. Buwen DONG and two anonymous reviewers for their constructive comments and suggestions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41888101, 41731174 and 41561144003 to Hai CHENG, and 41472140), and the Chinese Academy of Sciences “PIFI Program” (Grant No. 2020VCA0019) to Ashish SINHA. Qiong ZHANG acknowledges the support from Swedish Research Council (Vetenskapsrådet, Grant Nos. 2013-06476 and 2017-04232). Peng HU acknowledges the Special Research Assistant Project of Chinese Academy of Sciences.
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Article Highlights
• The interannual wet-season precipitation variability over the SWIO significantly correlates with SST over the tropical central Pacific.
• Weakened Walker Circulation sets up the favorable dynamic environment for the pluvial over the SWIO.
• A Gill response to the Maritime Continent’s diabatic cooling favors the thermodynamic environment of the pluvial over the SWIO.
• Additional anomalous low-level cyclone induced by the increased latent heat over the SWIO enlarges the rainfall variability there.
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Li, H., Hu, P., Zhang, Q. et al. Understanding Interannual Variations of the Local Rainy Season over the Southwest Indian Ocean. Adv. Atmos. Sci. 38, 1852–1862 (2021). https://doi.org/10.1007/s00376-021-1065-3
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DOI: https://doi.org/10.1007/s00376-021-1065-3
Key words
- southwest Indian Ocean
- interannual rainfall variability
- Pacific
- Walker Circulation
- Rossby wave
- positive feedback