The effects of freshwater on the Bay of Bengal

A  new study from the Indian Institute of Science (IISc), Bengaluru finds that the river discharge and rainfall differently affect the sea surface temperature, salinity and ocean currents in different parts of the Bay of Bengal. The team headed by P N Vinayachandran at the Centre for Atmospheric and Oceanic Sciences, IISc has used the Ocean General Circulation Model, a mathematical model to depict the physical and thermodynamical processes in oceanic circulation.

The Bay of Bengal, unlike the Arabian Sea, maintains a warm sea surface temperature of 280 degree celsius and higher throughout the summer monsoon. Owing to the enormous freshwater input from river discharge, the salinity of the upper layer of the Bay of Bengal is remarkably low. This is especially true near the river mouths of Ganga, Brahmaputra and the coastal regions. This results in strong vertical stratification – the separation of the waters of the Bay into layers, based on salinity. When the surface waters become relatively less saline due to incoming freshwater, a barrier layer is sometimes formed, which inhibits cooling of the salt-water beneath.

This prevents the mixing of fresh and salt-water layers. Earlier studies have found that this barrier layer causes warming of about 0.5 to 10 degree celsius of the sea surface temperature during summer in the northwestern part of the Bay of Bengal, primarily caused by the river discharge from the Ganga-Brahmaputra system.

The researchers used a model that simulates the Bay of Bengal surface salinity. The model is forced with surface winds, rainfall, atmospheric heat flux and radiation, and river discharge for major rivers along the coasts of India and Sri Lanka. They designed experiments to investigate individual and combined effects of rainfall and river discharge on the Bay of Bengal. “Freshwater input increases the sea surface height along the east coast of India in the northwestern Bay of Bengal during winter, and strengthens the equator-ward East India Coastal Current (EICC),” says Ambica Behara, one of the investigators. This current transports low salinity water to the southern parts of the Bay of Bengal and the south-eastern Arabian Sea after the summer monsoons, and reverses direction to flow pole-ward between February to May.

The surface salinity of the Bay of Bengal, during summer, is clearly marked with lower salinities in the northeastern bay, which gradually increases towards the southwestern part. The experiments revealed that rainfall has a basin wide impact on surface salinity.

Rainfall decreases the salinity by 0.5 psu (practical salinity units) in the western Bay of Bengal, and one psu and higher in eastern part. Unlike rainfall, river discharge tends to stay closer to the coast and results in stronger salinity gradients in the Bay, with a salinity decrease of about six psu near the mouths of large rivers.

The freshwater input allows the northwestern bay to stay warmer owing to the greater heat trapped within the shallow mixed layer. However, in the northeastern Bay, the warming due to greater heat absorption within the shallow mixed layer is dominated by the greater shortwave heat loss below the thin mixed-layer, which results in a cooling tendency during summer in the presence of freshwater.

The surface temperature further cools in the subsequent winter. The absence of compensating processes in the subsequent seasons to inhibit the dominant winter cooling in the northeastern Bay results in cooler Sea Surface Temperature (SST) throughout the year in the presence of strong vertical stratification.

This cooling impact owing to the salinity stratification observed in the northeastern Bay of Bengal has not been found in previous modelling studies. The flux forced model experiments developed by the team helps to understand the amount of work done by the atmosphere on the ocean to offset this cooling through the air-sea heat flux terms. “The study has found that freshwater input enhances the impact of warming in the northwestern Bay and cooling in the northeastern Bay. These results suggests that accurate regional SST forecast in the Bay of Bengal would require accurate estimates of each component of the freshwater input and their spatio-temporal patterns,” says Ambica.