Bay of Bengal Circulation Features at Different Spatio-temporal Scales

Abstract

The Bay of Bengal is a semi-enclosed tropical basin shaped by monsoon winds and massive freshwater input, producing a warm, low salinity, stratified upper ocean with a shallow mixed layer and a thick barrier layer in the north. These dynamical settings foster the development of energetic mesoscale eddies (O[10s–100s km; days–months]) and ubiquitous submesoscale buoyancy fronts (O[<10 km; hours–days]), which strongly modulate upper ocean variability. Despite its importance for regional climate, biogeochemistry, and extreme weather, the combined influence of monsoon forcing, mesoscale eddies, river plumes, and submesoscale processes on the BoB circulation remains poorly understood. This thesis is motivated by the need to unravel these multiscale interactions, which are critical for improving predictions of storm response, freshwater redistribution, and fine-scale turbulence in the northern Indian Ocean.

The study integrates in situ moored buoy and satellite observations, reanalysis products, and high-resolution ROMS simulations (~1–2 km) to address these questions. The research is organised around three themes: (1) the role of monsoon winds and mesoscale eddies in shaping subsurface thermocline structure, (2) the response of river plumes to tropical cyclone forcing, and (3) the modulation of submesoscale fronts and filaments by extreme cyclonic winds.

The first part identifies a previously undocumented intra thermocline bulge, formed by winter monsoon winds interacting with westward propagating anticyclonic eddies. This mechanism provides a new pathway for subsurface ventilation and vertical salt redistribution. The second part, using Cyclone Phailin (8-14 October 2013) as a case study, demonstrates that river runoff amplifies cyclone-induced ocean responses, producing widespread haline wakes and altering mixed-layer dynamics. The third part shows that cyclones suppress submesoscale activity by homogenising buoyancy gradients, with only partial post-storm recovery.

Together, these results establish the tight coupling among the monsoon, eddies, river plumes, and cyclones in regulating the BoB circulation. The findings underscore the importance of realistic freshwater forcing, fine-scale resolution, and process-based diagnostics in ocean models, with direct implications for climate prediction, extreme event forecasting, and parameterisation of submesoscale turbulence in Earth system models.