赤道印度洋上层环流辐合辐散的年际变异成因分析

印度洋上层海气相互作用对印度洋和太平洋气候系统有重要影响。目前针对印度洋气候态环流特征已有较为全面的研究,但针对印度洋环流的年际变化及其季节性差异的特征分析和具体作用机制,仍缺乏深入的研究。本文利用1979—2007年Simple Ocean Data Assimilation(SODA)再分析资料研究了赤道印度洋表层辐合辐散的年际变异及其季节依赖性。结果表明,以赤道为中心,印度洋上层异常海流,在经向上形成显著的辐合(辐散)现象,究其原因主要是赤道纬向风异常形成的Ekman流所导致。进一步分析表明,热带印度洋异常纬向风的成因与太平洋-印度洋的热力强迫过程作用有关,并且不同的热力强迫过程呈现出显著的季节差异性。此热力强迫过程,具体可分为3种类型:第一类是太平洋纬向海表热力差异的遥强迫作用,主要发生在冬末春初,热带太平洋的纬向热力差异通过调节Walker环流,在印度洋激发出一个异常的次级环流,对应的大气低层形成纬向风异常;第二类是东-西印度洋海表热力差异的局地强迫作用导致的局地环流,使赤道印度洋上空形成纬向风异常,此过程在春末夏初较为显著;第三类是太平洋-印度洋热力差协同作用的结果,使赤道印度洋盛行异常的纬向风,此过程在秋季起主导作用。

THE MECHANISM OF INTERANNUAL VARIABILITY OF UPPER-LAYER OCEAN CIRCULATION CONVERGENCE AND DIVERGENCE IN EQUATORIAL INDIAN OCEAN

The upper-layer air-sea interactions in the Indian Ocean have important effects on the Indian and Pacific climate systems. At present, the climatological circulation features of the Indian Ocean have been comprehensively investigated by previous studies. However, there is still a lack of research on the interannual variability of circulation in the Indian Ocean and its seasonal differences. As well, the related mechanism is not so clear. We studied the interannual variability of the surface convergence and divergence of the equatorial Indian Ocean and its seasonal differences using the Simple Ocean Data Assimilation Reanalysis (SODA) datasets from 1979-2007. The results show that the anomalous flow in the upper Indian Ocean converge or diverge in the meridional direction centered on the equator, which is mainly formed by the Ekman flow due to the equatorial zonal wind anomaly. Further analysis indicates that the anomalous equatorial zonal wind on Indian Ocean is related to the thermal forcing process in the Pacific-Indian Ocean, and different processes show significant seasonal differences. This thermal forcing process can be divided into three types. The first type is the remote forcing caused by the sea surface zonal thermal gradient in the Pacific Ocean, mainly occurring in late winter and early spring. This zonal thermal difference regulates the Walker Circulation and induces the anomalous secondary circulation in the Indian Ocean, forming zonal wind anomaly in the corresponding lower atmosphere. The second type is the local forcing caused by the zonal thermal gradient in the surface of Indian Ocean, which arouses the local atmospheric circulation and finally leads to the zonal wind anomaly over the equatorial Indian Ocean. This type is more pronounced in late spring and early summer. The last type is the result of the combined effect of the Pacific-Indian Ocean thermal difference, which leads to anomalous zonal wind in the equatorial Indian Ocean. This type is dominant in autumn.