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

The Bay of Bengal (BoB) is characterized by a shallow (similar to 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 degrees N, 90 degrees 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 similar to 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 degrees 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 degrees N, 90 degrees E. This study demonstrates the importance of ocean preconditioning and thermohaline stratification on SST evolution under the influence of post-monsoon cyclones in BoB. Plain Language Summary The Bay of Bengal (BoB) is an active zone for genesis of tropical cyclones. During the post-monsoon season, the near-surface ocean in the Bay is stable due to presence of a thin, fresh layer of river water and the subsurface waters are generally warm. In this study, we use in-situ mooring, satellite, and ocean analysis data sets to understand how the preexisting temperature and the salinity structure of the ocean can influences the ocean's response to two severe tropical cyclones, Titli and Gaja, that occurred in the Bay during October-November 2018. In the case of Titli, we observe that warming of the sea surface prior to arrival of cyclone resulted in strong thermal stratification, which restricted vertical mixing to a depth of 20 m under strong cyclone winds. During the passage of Gaja, river water with a subsurface warm layer advected to the mooring location resulted in stable stratification, which did not allow cyclone-induced mixing. We observe a delayed deepening of the mixed layer to 40 m depth due to shear induced by inertial currents and near-surface warming after the passage of cyclone Gaja. This study demonstrates the importance of preexisting upper ocean temperature and salinity stratification for mixed layer evolution under post-monsoon tropical cyclones in the BoB.