Arabian Sea High Salinity Core in the Bay of Bengal

Abstract

The Summer Monsoon Current (SMC) advects high salinity water from the Arabian Sea into the Bay of Bengal (BoB) during the summer monsoon. This high salinity water subducts and enters the BoB as a subsurface high salinity core (HSC) sandwiched between fresher BoB water on its top and bottom. This thesis presents the structure of the HSC and its impact on salinity and oxygen distributions in the BoB using shipboard observations, Argo profiles, and reanalysis products. Shipboard measurements were conducted during the Bay of Bengal Boundary Layer Experiment (BoBBLE) in the southern BoB in July 2016, crossing the SMC and Sri Lanka dome (SLD), characteristic circulation features in the southern BoB. HSC was present in vertical profiles collected across the strong northeastward flowing SMC system and absent in profiles collected within the SLD. Microstructure observations were used to quantify the small-scale mixing processes across these dynamic features. Dissipation rates of turbulent kinetic energy (ϵ) and temperature variance (χ) were estimated using shear and temperature gradient microstructure measurements, respectively. Turbulent diffusivities were calculated using ϵ and χ. Subsurface turbulent diffusivity values suggest that the SMC is an energetic region and is conducive to mixing salt from HSC into the BoB. In contrast, SLD is relatively calm and stable. The presence of warmer and high salinity water, HSC, also formed regions suitable for salt fingering, enhancing the vertical salt fluxes. Concurrent Dissolved Oxygen (DO) profiles at these stations suggested that the HSC is relatively rich in DO compared to the ambient BoB water. Vertical DO fluxes obtained from simultaneous profiles of DO and diffusivity suggested that the HSC loses DO to subsurface layers, replenishing the layers below the HSC. Considering the implications of HSC in salt and DO budget in the BoB, the spread of HSC is investigated using Argo and Bio-Argo profiles scattered in the BoB. Signatures of HSC observed during BoBBLE in the southern BoB are identified in Argo profiles as far north as 19 N. The vertical fluxes at the locations where HSC is present appear to oxygenate the layers above the BoB Oxygen Minimum Zone. Several model outputs and reanalysis data sets are investigated for their performance in simulating the HSC and to understand its spread in the BoB. A general algorithm is developed and implemented to identify HSC modeled in the ocean reanalysis products. Results from this analysis suggested that HSC enters as an intrusion at the beginning of the summer monsoon, whose thickness varies between 50-100 m till the end of the year. A maximum thickness of around 140 m was observed at the beginning of the summer monsoon in June-July. The northward extent of HSC is maximum at the beginning of the year, up to 19 N, and is limited to 15 N until the end of the monsoon. This thesis encourages further investigation of HSC to understand its physical and biological implications.