Pathways of forward energy cascades in the northern Bay of Bengal

Abstract We investigate the governing pathways of energy cascades in the northern Bay of Bengal (BoB) region of the Indian Ocean using a hierarchy of nested Regional Ocean Modelling System (ROMS) simulations. The analysis is focused on one month each in the pre-monsoon (April) and post-monsoon (October) seasons, motivated by the contrasting upper ocean dynamics in these months. In April, a swift, meandering western boundary current and its separation produces an energetic mesoscale eddy field in the northern BoB. By contrast, the post-monsoon month of October is characterized by an equatorward East India Coastal Current (EICC) and recurrent episodes of surface submesoscale frontogenesis driven by freshwater fluxes from monsoon rainfall and river runoff. Using a coarse-graining analysis for computing scale-to-scale kinetic energy fluxes, we find that at scales smaller than ≈ 15 km, surface forward fluxes in October are almost twice as large as in April, producing a fine-scale field of eddies, fronts and filaments. We show that in both months, forward energy fluxes at submesoscales (<≈ 25 km) strongly correlate with convergent, frontogenetic mechanisms. Additionally, we provide evidence that remotely generated, semi-diurnal internal tides have a crucial role in stimulating cross-scale energy fluxes in this region. Comparing simulations with and without barotropic tidal forcing, we demonstrate that internal tides are able to catalyze downscale cascades through three distinct mechanisms — direct energy extraction from the eddy field, scattering of the internal wave field by eddies and nonlinear wave-wave interactions.