The Bay of Bengal upper‐ocean response to tropical cyclone forcing during 1999

Abstract

The regional upper‐ocean dynamical and thermodynamical responses to two consecutive, strong tropical cyclones (TCs)—04B (15–19 October) and 05B (25 October to 3 November) in 1999 (hereafter, TC1 and TC2) in the Bay of Bengal (BoB) and the associated oceanic processes are investigated using an eddy‐permitting Hybrid Coordinate Ocean Model. The TC winds induce positive sea surface height anomalies (SSHA) along the northern BoB coastline and Andaman Sea due to onshore convergence, and negative SSHA along the TC tracks due to Ekman divergence, which in turn induce geostrophic flow. The TC‐associated radiation and precipitation have negligible effects on the northwestern BoB top 30 m averaged temperature (T0–30 m), while the strong TC winds significantly enhance turbulent heat flux causing T0–30 m decrease. Due to the existence of the barrier layer and subsurface warm advection in the northwestern BoB, vertical mixing may induce near‐surface warming by entraining warm water from below. As a result, the proportion of the T0–30 m cooling caused by turbulent heat flux is likely elevated in the 6° × 6° footprints of the TCs. Both TC wind‐induced vertical mixing and upwelling significantly cool T0–30 m in TC1's wake, while upwelling dominates the maximum cooling region in TC2's wake, likely due to the preceding deepening of the mixed layer by TC1. The near‐surface cooling for both TCs has rightward bias because of the higher winds and the resonant response on the right, and Ekman divergence extends the cooling areas outward. TC1 and TC2's sizes and prestorm oceanic conditions are found to be the most influential factors for near‐surface cooling.