印度洋赤道潜流年际变化特征及其与印度洋偶极子的联系

INTERANNUAL VARIABILITY OF INDIAN OCEAN EQUATORIAL UNDERCURRENT AND ITS LINK TO THE INDIAN OCEAN DIPOLE

印度洋赤道潜流(equatorial undercurrent,EUC)是赤道流系的重要组成部分,对印度洋物质输运和能量交换有着重要意义。基于SODA3.4.2海洋再分析数据,对印度洋EUC的三维空间结构和年际变化特征进行分析,并揭示其年际变率与印度洋偶极子(Indian Ocean dipole, IOD)的联系。结果显示,气候态上,印度洋EUC出现在冬末春初(2~4月)和夏末秋初(8~10月)季节,且空间结构关于赤道呈南北对称分布,其半年周期变化是由赤道纬向东风所引起的向东的次表层压强梯度力(pressure gradient force, PGF)所驱动。年际尺度上,印度洋EUC的结构和强度显著受到IOD的调控,即EUC在正IOD期间几乎能够维持一整年,其强度春季增强、夏季减弱,再随着正IOD的成熟而达到最强,同时其流核中心往赤道东南稍稍偏移。动力诊断结果表明,赤道印度洋次表层向东的PGF是表征印度洋EUC年际变化的重要指标,主导了EUC的年际变化。因此,在IOD期间,风-温跃层-海表温度之间的正反馈机制通过影响印度洋次表层PGF进而调制了EUC的年际变化,同时加强(减弱)的EUC也将通过补偿赤道东印度洋的上升流进而维持正(负) IOD的发展。

The Indian Ocean equatorial undercurrent(EUC) is an important part of the equatorial current system and plays a vital role in Indian Ocean mass transport and heat exchange. Based on the SODA 3.4.2 ocean reanalysis data, we studied the three-dimensional spatial structure of the EUC in the Indian Ocean and its interannual variability, and the relationship between the EUC and Indian Ocean dipole(IOD) on the interannual timescale. Results show that the climatological Indian Ocean EUC usually appears in late winter and early spring from February to April and late summer and early autumn from August to October, and displays a symmetric zonal current anomaly distribution over the equator.Semiannual variation of the EUC is primarily attributed to the eastward subsurface zonal pressure gradient force(PGF) that is driven by the equatorial easterly wind. On the interannual scale, the structure and intensity of the EUC in the Indian Ocean is significantly regulated by the IOD. During the positive IOD developing years, the Indian Ocean EUC occurs almost a whole year, and its intensity increases in spring and weakens in summer. The EUC finally reaches the peak amplitude, and its core is slightly moved southward to the equator during the mature of positive IOD. Dynamic diagnosis reveals that, the eastward subsurface PGF in the equatorial central Indian Ocean dominates the interannual variability of the Indian Ocean EUC and is an important proxy of the interannual variation of the EUC. Therefore, IOD mainly affects the Indian Ocean subsurface PGF via the positive feedback mechanism of the wind-thermocline-sea surface temperature anomalies, and further modulates the interannual variation of EUC. Meanwhile, the enhanced EUC can compensate the upwelling in the equatorial eastern Indian Ocean and further maintain the IOD.