A modeling study of the processes of surface salinity seasonal cycle in the Bay of Bengal

Abstract

In response to the Indian Monsoon freshwater forcing, the Bay of Bengal exhibits a very strong seasonal cycle in sea surface salinity (SSS), especially near the mouths of the Ganges-Brahmaputra and along the east coast of India. In this paper, we use an eddy-permitting (25 km resolution) regional ocean general circulation model simulation to quantify the processes responsible for this SSS seasonal cycle. Despite the absence of relaxation toward observations, the model reproduces the main features of the observed SSS seasonal cycle, with freshest water in the northeastern Bay, particularly during and after the monsoon. The model also displays an intense and shallow freshening signal in a narrow (100 km wide) strip that hugs the east coast of India, from September to January, in good agreement with high-resolution measurements along two ships of opportunity lines. The mixed layer salt budget confirms that the strong freshening in the northern Bay during the monsoon results from the Ganges-Brahmaputra river discharge and from precipitation over the ocean. From September onward, the East India Coastal Current transports this freshwater southward along the east coast of India, reaching the southern tip of India in November. The surface freshening results in an enhanced vertical salinity gradient that increases salinity of the surface layer by vertical processes. Our results reveal that the erosion of the freshwater tongue along the east coast of India is not driven by northward horizontal advection, but by vertical processes that eventually overcome the freshening by southward advection and restore SSS to its premonsoon values. The salinity-stratified barrier layer hence only acts as a ''barrier'' for vertical heat fluxes, but is associated with intense vertical salt fluxes in the Bay of Bengal.

Fifteen Years (1993–2007) of Surface Freshwater Storage Variability in the Ganges-Brahmaputra River Basin Using Multi-Satellite Observations

“…For instance, these new observations are in accordance with the results of [65], which investigate how the occurrence (or co-occurrence) of different climate modes (El-Niño, La Niña and Indian Ocean Dipole (IOD) events) affects the variability of precipitation in the GB basin, as well as the occurrences of major flood and drought events: for instance, major droughts, such as the one observed in 1994, are linked to a positive Indian Ocean Dipole (pIOD) mode, whereas major floods, such as the one in 1998 for the entire GB system, might be linked to a negative Indian Ocean Dipole (nIOD) mode. Ocean Dipole (pIOD) mode, whereas major floods, such as the one in 1998 for the entire GB system, might be linked to a negative Indian Ocean Dipole (nIOD) mode.…”

“…It can be used for a better understanding of the complex relationship between the water cycle, climate variability and human activities, for estimating the sub-surface water storage and discharge to the ocean and their impact on key parameters for oceanography of the Bay of Bengal such as salinity and temperature [65][66][67] and for the validation of regional/global hydrological models.…”

Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

“…The oceanic portions of the eastern, northern, and southern boundaries use radiative open boundaries (Treguier et al, 2001), constrained with a 150-day timescale relaxation to physical and biogeochemical inputs from a global simulation (Dussin et al, 2009). The circulation and thermodynamics of this regional configuration have been extensively evaluated and found to perform well in several Indian Ocean regions (Praveen Kumar et al, 2014), including the Arabian Sea (Nisha et al, 2013;Vialard et al, 2013;Keerthi et al, 2015Keerthi et al, , 2016 and the Bay of Bengal (Akhil et al, 2014(Akhil et al, , 2016.…”

Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

“…The oceanic portions of the eastern, northern, and southern boundaries use radiative open boundaries (Treguier et al, 2001), constrained with a 150-day relaxation timescale to outputs from a global simulation (Dussin et al, 2009). The circulation and thermodynamics of this regional configuration have been extensively evaluated and reproduce observed variations of key physical parameters well in several Indian Ocean regions Akhil et al, 2014Akhil et al, , 2016Praveen Kumar et al, 2014), including the AS (Nisha et al, 2013;.…”