Impact of Ocean–Atmosphere Coupling on the Simulation of a Monsoon Depression Over the Bay of Bengal

In this study, we aimed to analyze the role of air–sea coupling with one-dimensional (1D) and 3D ocean coupled models in simulating the monsoon depression (MD) that formed over the Bay of Bengal from 13 to 17 August 2018. This synoptic system produced substantial heavy rainfall amounts over the east and west coasts of India, which led to a flooding situation along its path. In view of its characteristic features, we considered this MD as an ideal case and conducted six numerical simulations. The first three experiments were initialized by GFS data using a stand-alone Weather Research and Forecasting model (WRFCNTRL) coupled with its 1D ocean mixed layer (OML) model (OMLCNTRL) and 3D Price–Weller–Pinkel (PWP) ocean model (PWPCNTRL). The other three experiments (WRF3DVAR, OML3DVAR, and PWP3DVAR) were conducted with the same set of models by assimilating all available observations using the 3D variational data assimilation (3DVAR) method. The evaluation of results of coupled model simulations with observed estimates clearly suggested that the WRF model enabling ocean surface feedback has a positive impact on the simulation of MD and associated heavy rainfall. These improvements were mainly seen in the simulated tracks of the coupled model experiment, which were found to be in the order of 23–29% and 31–38% with OMLCNTRL and PWPCNTRL, respectively, as compared with WRFCNTRL at 48–72 forecast hours. The improvements were significant with the ocean coupled models and assimilation of observations, suggesting an overall reduction in the track error of about 13–17%, 27–32%, and 37–44% with WRF3DVAR, OML3DVAR, and PWP3DVAR, respectively, as compared with WRFCNTRL at 48–72 forecast hours. The positive impact of observational assimilation and ocean coupling could be due to the improved simulation of the depression-induced ocean surface variations, enhanced sea surface cooling, and increased surface enthalpy flux, leading to the changes in dynamical fields and the moisture convergence process. Our analysis suggests that the WRF-PWP model with assimilation of available observations provides improved estimates of the track and intensity of the MD over the Bay of Bengal and its associated heavy rainfall.