Mechanism of intraseasonal oceanic signature in the region off southern tip of India during boreal summer

摘要

Sea surface temperature (SST) in the region off southern tip of India (STI, 75-83°E, 5-8°N) exhibited a prominent variability in the intraseasonal time scale (both 30-90 d and 10-30 d band) during boreal summer. Mechanisms associated with this intraseasonal variability are studied using three-dimensional ocean general circulation model (OGCM) sensitivity experiments, satellite observed outgoing longwave radiation, SST and winds for the period 1998-2007. The background oceanic structure of the STI characterized by a shallow thermocline and moderate mixed layer provided ideal conditions for strong oceanic sub-surface processes. The model mixed layer heat budget reveals that the oceanic processes such as horizontal advection and vertical processes are the dominant mechanisms in the STI region as compared with air-sea flux. Sensitivity experiments with the OGCM reveals that the ocean dynamical processes contribute to most of the intraseasonal SST variability and the wind stress contributes to 85% of the variability whereas surface flux contributes to only 15% for the 30-90 d SST variability. Higher amplitude of surface flux perturbation and its contribution to SST in the 10-30 d as compared with the 30-90 d band are evident in the model experiment and are consistent with the observational analysis. There is year-to-year variability in the relative role of horizontal and vertical processes for different intraseasonal SST events over STI.