A Long-term Study of Interannual Variability in the Upper Ocean Thermohaline Structure of the Bay of Bengal.

This study investigates the interannual variability (IAV) of the Bay of Bengal (BoB) thermohaline structure in terms of mixed layer depth (MLD), isothermal layer depth (ILD), and barrier layer thickness (BLT) over a 65-year period spanning from 1958 to 2022. This study offers a comprehensive understanding of the mentioned IAV and underlying mechanism using the ORAS5 and ERA5 reanalysis data products. Although previous studies have explored seasonal and year-to-year variability in this region, this study delves into unexplored dynamics like differential spatial response to the plausible drivers of the IAV. An empirical orthogonal function (EOF) analysis conducted on monthly anomalies of MLD, ILD, and BLT reveals that the first EOF accounts for 44.5% of the variance in ILD, 23.7% in MLD, and 22.3% in BLT, and the first principal component (PC) shows a good correlation to Niño3.4 index and dipole mode index (DMI). This analysis further reveals that the influence of El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) is restricted to the southern and eastern boundaries of the bay. The composite analysis shows that the ILD exhibits negative (positive) anomalies in the equatorial and the eastern BoB during El Niño (La Niña) years, whereas the MLD does not show a distinct response to the ENSO events. The negative (positive) ILD anomalies are also prominent in the eastern BoB during positive (negative) IOD events. Unlike the ENSO years, negative (positive) MLD anomalies are visible in the Southern BoB during positive (negative) IOD years. The above anomalous variation in the MLD and ILD results in an anomalous decrease (increase) in BLT in the eastern side during El Niño and positive IOD years (La Niña and Negative IOD years). The response mentioned above in the MLD, ILD, and BLT is linked to the interannual response of the Kelvin waves and associated Rossby wave radiation to the ENSO and IOD forcing. In the northern, central, and western BoB, salinity exerts a strong influence on barrier layer formation, likely driven by the freshwater influx through evaporation, precipitation, and river runoff; however, the exact role of river discharge in modulating the IAV of BLT remains poorly understood. Since southern BoB acts as a persistent heat source, meridional heat transport (MHT) via the eastern boundary plays a critical role in enhancing the deepening of ILD, with heat being advected northward by oceanic currents. The role of the East India Coastal Current (EICC) is also evident, with freshwater advection contributing significantly to MLD variability along the east coast of India. The influence of monsoon current is observed in the second EOF of both MLD and ILD, capturing 9% and 8.5% of the variance respectively. A strong monsoon current brings high-saline, relatively cooler Arabian Sea water into the southwest BoB, resulting in deeper MLD and shallow ILD.