A new mode of decadal variability in the Tropical Indian Ocean subsurface temperature and its association with heat redistribution

Abstract

<p>Similar to the Pacific and Atlantic, Tropical Indian Ocean (TIO) has its own internal climate mode of variabilities such as Indian Ocean Dipole (IOD) and subsurface mode (SSM). A typical interannual SSM is characterized by the meridional gradient in opposing subsurface temperature anomalies in the eastern equatorial IO and in the southwestern IO. Here in the present study, we have explored the structure and the underlying dynamics for the SSM in decadal time scale which has not been reported before. By analyzing different reanalysis products we observe that decadal SSM is characterized by a pure north-south pattern with the northern mode covering the entire equatorial belt which is different from interannual SSM. A north-south SSM is the leading mode of decadal variability in the thermocline and subsurface temperature over the TIO. Our preliminary analysis suggests that the decadal variability in the surface winds along the equatorial IO and the associated wind stress curl are found to be the primary forcing mechanisms for the decadal evolution of the north-south mode. Positive wind stress curl anomalies south of 8<sup>o</sup>S intensify the downwelling Rossby waves in the south during the positive phase of the decadal SSM. On the other hand, the northern cooling is driven mostly by the equatorial upwelling Kelvin waves and the Ekman divergence. Further, the phase transition in the SSM is primarily determined by the strength of the surface wind and the associated Ekman transport. The equatorial easterlies (westerlies) diverge (converge) the meridional Ekman transport, transporting heat towards the off-equatorial (equatorial) region during the positive (negative) phase. Consistently with SSM, upper 500m oceanic heat content reveals a conventional north-south dipole highlighting the importance of SSM on the TIO heat redistribution. This is further supported by the modulation of meridional overturning circulation and the meridional heat balance across the southern Indian Ocean (SIO). Overall the present study explores the underlying mechanism responsible for decadal SSM and its association with the heat distribution across the SIO.</p>