Meeting Documents

Exploring Tidal Influences on Surface Current and Mesoscale Eddies in the Northern Bay of Bengal Using CROCO Modeling

Abstract

The Bay of Bengal (BoB), a tropical basin in the northern Indian Ocean enclosed on three sides by land, is characterized by high freshwater input from rivers, wind-driven upwelling and downwelling, strong rainfall, and pronounced seasonal variability in temperature and salinity. These features make it distinct from other marine regions. While earlier studies have examined wind-driven circulation, river runoff, Kelvin and Rossby waves, and their combined effects on tides and mesoscale eddies, the direct influence of tidal forcing on surface currents and eddy dynamics in the BoB has not been fully addressed. To investigate this, we applied the CROCO model at a horizontal resolution of 0.25° × 0.25° with 50 vertical layers. Model outputs were analyzed using multivariate empirical orthogonal function (EOF) decomposition, Pearson’s correlation, and the Okubo-Weiss parameter, alongside observational and reanalysis data from CMEMS, SMOS, ECCO, QuickSCAT, and WindSAT. Results reveal that both cyclonic and anticyclonic eddies intensify in the southwestern BoB under tidal forcing, whereas such patterns are weaker in the northwestern basin. Mesoscale eddy activity follows a seasonal cycle, with significant strengthening from March and a peak in June-July, linked to enhanced zonal wind stress and the semi-reversing East India Coastal Current (EICC). In contrast, removing tidal forcing diminishes this seasonal variability, highlighting tides as a critical driver of eddy activity. Additionally, in the eastern BoB, smaller-scale eddies are primarily generated by coastal Kelvin waves and radiated Rossby waves. This study indicates the crucial role of tidal forces in modulating surface currents and mesoscale eddies, with broader implications for heat and salinity redistribution, tropical cyclone development, and the regional carbon cycle. The findings highlight the necessity of including tidal dynamics in future oceanographic assessments and forecasting efforts for the Bay of Bengal, especially in the context of climate variability and regional ocean stability.

Plain-language Summary: Previous research in the Bay of Bengal has explored the effects of winds, waves, precipitations and river runoff on the surface currents, but the specific role of tides in shaping surface currents and eddy activity has not been well studied. This is why this study is designed to observe the impact of tides on surface currents.

In this study, we used CROCO model together with observational datasets to examine how tidal forces affect ocean circulation in the northern Bay of Bengal. The results show that tides play a major role in strengthening both cyclonic and anticyclonic eddies, especially in the southwestern part of the bay. Eddy activity increases sharply in spring and peaks in early summer, linked to stronger winds and the East India Coastal Current. Without tides, these seasonal patterns become much weaker.

The findings highlight how tides are essential in controlling surface currents, redistributing heat and salt, and influencing cyclones and regional climate. This research provides new insights into the dynamics of the Bay of Bengal and will help improve ocean forecasting and management in Bangladesh.