On the wave-current interaction during the passage of a tropical cyclone in the Bay of Bengal

The impact of waves on currents and vice-versa is an integral part of the complex coastal dynamics. The wave-current interaction is prominent in the coastal shelf-slope regions, particularly during the passage of a tropical cyclone. In the present study, we describe the importance of wave-current interaction in the northern Bay of Bengal (BoB) using a fully coupled three-dimensional Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modelling system. The COAWST model is applied to the case of a very severe cyclonic storm Phailin that passed over the BoB basin during 10-15 October 2013. To represent the interaction between waves and currents, we utilized the newly implemented vortex-force method in the coupled model. Numerical experiments with different coupling configuration within the COAWST modelling framework were performed to separately identify the effects of wind, tide, and wave-current interaction process. A comparison of model results with the buoy observations of water elevations, currents, and wave measurements shows a good agreement between model and observed data. Various terms of the momentum balance were calculated from the model simulated and diagnosed parameters. A comparison of the horizontal momentum balance term identifies the wave-breaking induced acceleration as one of the leading terms along the storm affected the east coast of India. Further, an increase in the apparent bed roughness caused by waves found to affect the values and distribution of the bottom shear stress. The pressure gradient term showed significant changes to the pure tidal case. The study highlights that the changes in the momentum balance caused by waves includes contributions from the variations in the water level and currents. The most relevant effect on the coastal hydrodynamics was in the form of a wave-induced setup near the location of landfall of the cyclone.