Abstract The empirical orthogonal function (EOF) analysis of subsurface temperature shows a dominant north-south mode of interannual variability in the Tropical Indian Ocean (TIO) at around 100 m depth (thermocline). This subsurface mode (SSM) of variability evolves in September-November (SON) as a response to Indian Ocean Dipole and intensifies during December-February (DJF) reinforced by El Nino and Southern Oscillation (ENSO) forcing. The asymmetry in the evolution of positive and negative phases of SSM and its impacts on the modulation of surface features are studied. The asymmetry in the representation of anomalous surface winds along the equator and off-equatorial wind stress curl anomalies are primarily responsible for maintaining the asymmetry in the subsurface temperature through positive and negative phases of the SSM. During the positive phase of SSM, downwelling Rossby waves generated by anticyclonic wind stress curl propagate towards the southwestern TIO (SWTIO), the thermocline ridge region of mean upwelling. The warmer subsurface water associated with the downwelling Rossby waves upwells in the region of mean upwelling and warms the surface resulting in strong subsurface-surface coupling. Such interaction processes are however weak during the negative phase of SSM. The asymmetry in the subsurface-surface interaction during the two phases of SSM and its impact on the modulation of surface features of TIO are also reported. In addition to the ENSO forcing, self-maintenance of SSM during DJF season is evident in the positive SSM (PSSM) years through modulation of subsurface surface coupling and air-sea coupling. This positive feedback during PSSM years is maintained by the deepening thermocline, warm SSTs and convection. The asymmetry in the thermocline evolution is more evident in the SWTIO and southern TIO.

中文翻译：

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热带印度洋地下温度变化的不对称性

摘要 地下温度的经验正交函数 (EOF) 分析显示热带印度洋 (TIO) 在约 100 m 深度（温跃层）的年际变化主要是南北模式。作为对印度洋偶极子的响应，这种地下模式 (SSM) 在 9 月至 11 月 (SON) 中演变，并在 12 月至 2 月 (DJF) 期间因厄尔尼诺现象和南方涛动 (ENSO) 强迫而加剧。研究了 SSM 正负相演化的不对称性及其对表面特征调制的影响。沿赤道的异常地表风和赤道外风应力旋度异常表示的不对称性是通过 SSM 的正负相位维持地下温度不对称性的主要原因。在 SSM 的正相位期间，由反气旋风应力卷曲产生的向下涌的罗斯贝波向西南 TIO (SWTIO) 传播，即平均上升流的温跃层脊区。与向下涌的罗斯比波相关的较暖的地下水在平均上升流区域上升并加热地表，导致强烈的地下-地表耦合。然而，在 SSM 的负面阶段，这种相互作用过程很弱。还报告了 SSM 两个阶段中地下-表面相互作用的不对称性及其对 TIO 表面特征调制的影响。除了 ENSO 强迫之外，在正 SSM (PSSM) 年，通过调节地下表面耦合和海气耦合，在 DJF 季节期间 SSM 的自我维护也很明显。PSSM 年的这种正反馈是由不断加深的温跃层、温暖的 SST 和对流维持的。温跃层演化的不对称性在 SWTIO 和南部 TIO 中更为明显。