A north-south subsurface dipole mode (SDM) is found to be the leading mode of interannual variability in the thermocline and subsurface temperature over the tropical Indian Ocean. The present study reports the existence of a decadal SDM with a north-south dominant structure. The mechanisms responsible for the formation of the decadal SDM and its close association with the evolution of local atmospheric and ocean circulation over tropical Indian Ocean are examined. Though the interannual SDM also has a north-south dominant dipole structure, it is imprinted with a weak east-west dipole structure as well. Unlike the interannual SDM, decadal SDM is characterized by a pure north-south pattern with the northern mode covering the entire longitudinal extent of equatorial Indian Ocean. The decadal variability in the surface winds along the equatorial Indian Ocean and the associated wind stress curl and their persistence are found to be the forcing mechanisms responsible for the decadal evolution of the north-south mode. Persistent positive wind stress curl anomalies south of 8^oS intensify the downwelling Rossby waves in the south during the positive phase of the decadal SDM. On the other hand, the northern cooling is driven mostly by the equatorial upwelling Kelvin waves, the Ekman divergence and the equatorially trapped reflected upwelling Rossby waves. It is found that the phase transition in the subsurface mode is determined by the strength of the surface winds and the associated changes in the Ekman transport. Consistent with SDM, upper 50-300 m and 500 m oceanic heat contents reveal conventional north-south dipole structure highlighting the importance of SDM on the tropical Indian Ocean heat redistribution. The meridional overturning circulation associated with SDM modulates the conventional shallow meridional overturning circulation in the Indian Ocean (amplifying it during the positive phase and weakening it during the negative phase) and the associated meridional heat transport.

发现南北地下偶极子模式（SDM）是热带印度洋上温跃层和地下温度年际变化的主要模式。本研究报告了具有南北主导结构的年代际 SDM 的存在。研究了造成年代际 SDM 形成的机制及其与热带印度洋局部大气和海洋环流演变的密切关联。年际SDM虽然也具有南北主导偶极子结构，但也带有东西弱偶极子结构的印记。与年际 SDM 不同，年代际 SDM 的特征是纯南北模式，北方模式覆盖赤道印度洋的整个纵向范围。发现沿赤道印度洋表面风的年代际变化以及相关的风应力卷曲及其持续性是造成南北模式年代际演化的强迫机制。8 号以南持续存在正风应力旋度异常^○在年代际 SDM 的正相位期间，S 加强了南部向下涌的罗斯比波。另一方面，北方降温主要是由赤道上升开尔文波、埃克曼辐散和赤道被困反射上升罗斯比波驱动的。发现地下模式的相变由地表风的强度和埃克曼输运的相关变化决定。与 SDM 一致，上层 50-300 m 和 500 m 海洋热含量揭示了常规的南北偶极子结构，突出了 SDM 对热带印度洋热量再分布的重要性。