Processes controlling the upper ocean variability in the central Bay of Bengal on a seasonal time scale

The Bay of Bengal (BoB) has several distinct oceanographic features that make it a natural laboratory for learning about theinteractions between the ocean and atmosphere. In spite of several past studies, a holistic understanding of the upper ocean variability is incomplete, mainly due to inadequate coverage of spatial data. Present study explores the seasonal cycle of the upper waters of the central BoB and its spatial structure using high-resolution spatial data from recently released World Ocean Atlas 2023. The upper ocean response was studied using monthly climatology of three surface parameters viz., sea surface temperature (SST), sea surface salinity (SSS) and sea surface density (SSD), and three subsurface parameters viz., isothermal layer thickness (ILT), mixed layer depth (MLD) and barrier layer thickness (BLT). A suite of data was used to determine atmospheric forcing via wind speed used as a proxy for momentum flux, net heat flux (NHF) and fresh water flux. All three surface and three subsurface parameters showed a seasonal cycle with bi-modal distribution of varying magnitudes. The processes that controlled the seasonal cycle of SST and ILT was the NHF and the wind speed. The seasonal cycle of SSS and BLT closely followed the freshwater flux and wind speed. The seasonal cycle of both SSD and MLD was regulated by NHF and wind during the winter monsoon, while freshwater flux and wind controlled it during summer monsoon. The role of cloud cover, which was hither to unexplored, provided additional cooling mechanism during winter and summer monsoons via different processes. In summary, the study provides an in-depth understanding of how the latitudinal structure of the upper ocean's seasonal patterns vary and highlights the role of atmospheric forces, including cloud cover, in shaping this variability.