Nakazato Mai, Kido Shoichiro, Tozuka Tomoki
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
Application Laboratory (APL), Research Institute for Value-Added-Information Generation (VAiG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan.
Sci Rep. 2021 Nov 25;11(1):22546. doi: 10.1038/s41598-021-01619-2.
The Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing.
印度洋偶极子（IOD）是热带印度洋的一种年际气候模态。尽管已知在正IOD期间东极的负海表面温度（SST）异常比负IOD期间的正SST异常更强，但对于导致这种不对称性的各种机制的相对重要性尚未达成共识。基于使用区域海洋模型的封闭混合层热量收支分析，我们首次表明，除了先前研究提出的非线性平流和温跃层反馈的贡献外，垂直混合在导致SST异常的这种不对称性方面也起着重要作用。垂直混合项的分解表明，与次表层温度异常和异常垂直混合系数相关的异常垂直温度梯度中的非线性是这种不对称性的主要驱动因素。次表层温度的这种变化是由沿印度尼西亚海岸的异常东南信风引起的，这些信风通过沿岸上升流/下降流调节温跃层深度。因此，温跃层反馈对SST不对称性的贡献不是如先前所建议的通过垂直平流，而是通过垂直混合。
