An international team of ocean researchers has now generated a comprehensive in situ observational dataset of the physical, chemical and biological parameters of the southern Bay of Bengal, air–sea interface and the overlying atmosphere. The ocean–atmosphere interaction plays a major role in controlling the weather systems associated with the Indian summer monsoon (June–September).
The field programme was carried out as a part of the Bay of Bengal Boundary Layer Experiment (BoBBLE) to collect the dataset, onboard one of India’s research ships Sindhu Sadhana, and the findings were recently published in the Bulletin of the American Meteorological Society.
The two-month study — June to July 2016 — was carried out on multiple platforms (ship, ocean gliders and Argo floats) to measure salinity, conductivity, temperature, dissolved oxygen and chlorophyll content in the sea water.
Salinity contrast
“There is a huge salinity contrast between Arabian Sea and the Bay of Bengal. There are no major rivers in the western side of India. So Arabian Sea does not get much fresh water. But [since] rivers [such as] Ganga, Brahmaputra, Mahanadi and Godavari empty huge amounts of fresh water [into the Bay of Bengal] and with heavy rainfall, the salinity of Bay of Bengal is comparatively less. The exchange between these two basins takes place in the southern Bay,” explains Prof. P.N. Vinayachandran from the Centre for Atmospheric and Oceanic Sciences at Indian Institute of Science, Bengaluru. The southern Bay region hosts a salt pump which draws high salinity water from the Arabian Sea and supplies to the Bay of Bengal.
The research was mainly focussed on the waters east of Sri Lanka which are marked by intense Summer Monsoon Current and at the Sri Lankan Dome, which is a patch of ocean with anti-clockwise circulation with upwelling in its centre. “The Experiment shows that a barrier layer of about 40 metres thickness can be found here. This boundary layer exists between the upper warm fresh layer and the bottom layer.
The layer insulates the upper layer of the ocean from cooling from below while maintaining high sea surface temperature thus helping rapid build-up of weather systems,” he says in an e-mail to The Hindu.
Gliders equipped with photosynthetically active radiation sensors were used for studying the biological components of the water. “Phytoplankton was studied, as the southern Bay of Bengal is a biologically productive region [which is] rich in chlorophyll content,” he says. “The physical processes such as upwelling at the Sri Lankan Dome and [the] nutrient carried by monsoon currents support the biological process.”
Carbon dioxide
The study region exhibited high carbon dioxide and the report says that southern Bay of Bengal could be a possible source of carbon dioxide to the atmosphere during summer.
The data collected during BoBBLE programme found that the sea surface temperature in this region increases steadily during the break period of the monsoon.
The paper states that as the rainfall over the entire Asian landmass during the monsoon is linked to moisture and heat exchange over the Indian Ocean, it is essential to get a detailed understanding of this region.
The researchers are now analysing the data to understand how the monsoon system and ocean atmospheric systems in the southern Bay of Bengal are linked.
