Ocean Mixed Layer Response to Two Post‐Monsoon Cyclones in the Bay of Bengal in 2018

Abstract

AbstractThe Bay of Bengal (BoB) is characterized by a shallow (∼10–20 m deep) fresh layer associated with 40–60 m deep warm near‐surface layer during the post‐monsoon season (October–November). We use hourly observations from a moored buoy at 15°N, 90°E along with satellite and ocean analysis data sets to understand the evolution of the near‐surface layer during the passage of two category‐3 cyclonic storms: Cyclone Titli (7–11 October 2018) and cyclone Gaja (10–15 November 2018). The mooring was ∼200 km away to the right of the two cyclone tracks. A 15 day (22 September–7 October) break in the Indian summer monsoon resulted in clear skies, calm winds, and sea surface temperature warming (SST) by 2.8°C before Titli. During Titli, near‐surface thermal stratification restricted vertical mixing to 20 m depth and limited SST cooling at the mooring. During passage of cyclone Gaja, fresh Irrawaddy River water associated with a subsurface warm layer was advected to the mooring by mesoscale eddy flow. Cyclone winds could not break through the near‐surface density stratification associated with this river water. Shear associated with near‐inertial currents led to delayed mixed layer deepening after the cyclone's passage but there was no cold wake along the track of Gaja due to mixing of subsurface warm water to the surface. We illustrate the relevant processes at work for the two cyclones based on mixed layer salt and heat balance at 15°N, 90°E. This study demonstrates the importance of ocean preconditioning and thermohaline stratification on SST evolution under the influence of post‐monsoon cyclones in BoB.