Influence of tropical cyclone Jawad on the surface and sub-surface circulation in the Bay of Bengal: ocean-atmosphere feedback

The anomalous post-monsoon tropical cyclone (TC) Jawad has been simulated using the Coupled Ocean-Atmosphere Wave Sediment Transport (COAWST) model using Global Forecasting System (GFS) Analyses and Forecasts as the atmospheric initial and boundary conditions (IC and BC) along with two contrasting ocean IC and BCs, viz., HYCOM (experiment name GFS-HYCOM) and INCOIS (experiment name GFS-INCOIS), to evaluate the influence of TC Jawad on the ocean surface and sub-surface characteristics. Both experiments captured the track of the simulated TC, including its recurvature, with significant accuracy. Validation of surface and sub-surface temperatures from two buoys, (1) BD11 (west of the TC track) and (2) BD13 (east of the TC track) suggests that a proper contrast in temperature exists in the buoy observations between the two sides of the TC track with the eastern side-showing higher sub-surface warming, which is captured by GFS-HYCOM but with significant overestimation. The lower temperature on the western side of the TC track can be attributed to the weak upwelling associated with the cyclonic circulation caused by the interaction of the TC with the southward coastal currents. The vertical distribution of the temperature across the longitudes showed an unusually higher downwelling on the eastern side of the TC track suggesting the existence of a strong clockwise circulation near the location of BD13. This circulation was found to be more rigorous in GFS-HYCOM, which also simulated higher current magnitude in the sub-surface than GFS-INCOIS. Further analysis showed that the interaction of the cyclonic wind flow of TC Jawad (westerly) near the surface with the easterly flow caused the generation of the clockwise circulation over the ocean surface on the eastern side of the TC track leading to intense downwelling and warming of sub-surface temperature. This study highlights the importance of the employment of coupled ocean-atmosphere models to simulated TCs for a better understanding of the air-sea interaction processes and their responses to the passage of an anomalous TC like Jawad.