Presenter: Sreelekha Jarugula (NOAA PMEL)

Description:
This study will focus on how ocean pre-conditioning affects mixed layer temperature and salinity variability under the influence of post-monsoon cyclones in the northern Bay of Bengal. The Bay is characterized by a shallow (about 10 m deep) fresh layer which is often associated with a subsurface warm layer during the post-monsoon season (October-November). We use hourly observations from the 15^oN 90^oE RAMA mooring along with satellite and ocean analysis datasets to understand the evolution of 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. The maximum wind stress measured at the mooring during passage of cyclones was 0.4 N m^-2 and 0.3 N m^-2 respectively. A 15-day (22 September-7 October 2018) break in the Indian summer monsoon resulted in clear skies, calm winds and warming of sea surface temperature (SST) by 2.8^oC. Strong near-surface thermal stratification associated with this warming restricted vertical mixing to 20 m depth. Subsequently, satellite SST maps show a cold wake along the path of cyclone Titli track with maximum cooling of 3.5^oC close to the east coast of India. In the case of Gaja, Irrawaddy river water associated with subsurface warm layer was advected to the mooring location by mesoscale eddy flow during 10-13 November. The strong winds associated with cyclone Gaja could not break through the stable near-surface density stratification owing to the arrival of this river water. A week after the cyclone passed, the inertial currents set up by the cyclone winds led to deepening of mixed layer to about 50 m depth. There is no cold wake along the track of Gaja due to mixing of subsurface warm water into the mixed layer. We illustrate the relevant processes at work for the two cyclones based on mixed layer salt and heat balance at the 15°N, 90°E buoy location.