Hydrological characteristics of the Bay of Bengal water column using δ18O during the Indian summer monsoon

Abstract

The Bay of Bengal, the eastern part of the northern Indian Ocean, experiences a reversal of monsoonal winds that control its surface and subsurface hydrological characteristics. Due to the limited number of studies, many aspects of the Bay of Bengal water column, particularly its deep hydrodynamic characteristics, are not thoroughly understood. For this purpose, oxygen isotopic compositions of water (δ18O), a potential tracer of oceanographic study, were measured at twelve discrete depths at eight locations in the central Bay of Bengal during the summer monsoon. The water columns were sampled to include surface (≤ 10 m), subsurface zone (25, depth of Deep Chlorophyll Maxima, 75, and 100 m), intermediate zone (150, 200, 300, and 500 m) and deep zone (1000, 1500, and 2000 m). Slopes of δ18O-salinity relationships at the surface and subsurface zones were similar. The δ18O and salinity showed an increasing southward trend at the surface, subsurface, and intermediate zones, which could be attributed to the influx of relatively saline and 18O enriched water from the Arabian Sea through Southwest Monsoon Current. The TS characteristics along with the negative slope of δ18O-salinity in the intermediate zone (− 0.51 ± 0.14) indicated the presence of Indonesian Throughflow water in the Bay of Bengal. Temporal variation in δ18O and slope of δ18O-salinity relationship in the intermediate zone was also observed, possibly indicating variable contribution of Indonesian Throughflow water mass through time. The δ18O of the deeper water showed the signature of existence of North Atlantic Deep Water in the Bay. Mesoscale eddies, a frequent phenomenon in the Bay of Bengal, also seem to modulate the δ18O-salinity relationship in the basin.