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Submesoscale fronts linked to persistent fresh surface layer in the Bay of Bengal

The persistence of a thin, surface layer of fresh water in the Bay of Bengal (BoB) is known to have a great influence on weather and climate across South Asia. However, the mechanism that sustains this low-salinity layer has remained a puzzle for ocean scientists.

Prof. Debasis Sengupta and his colleagues G. N. Bharath Raj and J. Sree Lekha of the Center for Atmospheric and Oceanic Sciences, Indian Institute of Science (IISc), Bangalore, with M.Ravichandran of the Indian National Center for Ocean Information Services, Hyderabad and Fabrice Papa of the Indo-French Cell for Water Sciences, IISc, have presented the first evidence that submesoscale fronts may be the key to the sustenance of a fresh layer in the north BoB.

 “A front is a region of sharp change in the ocean (e.g., saltier water on one side and fresher water on the other). A ‘submesoscale’ front is marked by abrupt jumps in salinity within 1-10 km” explains Prof. Sengupta, the lead author of the paper.

Low surface salinity persists through the year in the north Bay of Bengal. Summer monsoon rainfall and river runoff from the Ganga-Brahmaputra-Meghna and Irrawady are the major contributors to low sea surface salinity. To answer the question “What keeps the thin layer of fresh water from mixing away due to winds, tides and surface cooling?” the scientists. deployed moorings in the north BoB, nearly 500 km away from the mouth of Ganga-Brahmaputra-Meghna river, recording data every 10 min for two years, November 2009-November 2010 and January-December 2013. Apart from the moored observations, supporting data sets like sea surface salinity from the Aquarius satellite were used to study salinity across a range of time scales.

The scientists noticed frequent and abrupt fluctuations in salinity at the moorings, which were unrelated to local rainfall. The observations suggest that submesoscale fronts cause the rapid salinity changes: Velocity data indicates that the salinity jumps are due to fronts with spatial scales of 200 m to 10 km moving past the mooring. “At submesoscale fronts, saltier (denser) water slides under lighter (fresher) water. This process, called ‘slumping’, may help to sustain the fresh surface layer”, concludes Prof. Sengupta from the findings.

The sea surface temperature (SST) of the Bay of Bengal has significant influence on weather phenomena like tropical cyclones. The evolution of SST is known to depend sensitively on the presence of the fresh layer and the vertical salinity gradient. Therefore this new finding of submesoscale salinity fronts in the Bay of Bengal adds a new dimension to the understanding of the upper ocean. It also suggests a need for significant changes in present day monsoon forecast models, which have mostly overlooked submesoscale effects.

Talking about the implication this study could have on ocean modeling, Prof. Sengupta replied, “Our finding of salinity fronts from moored and satellite observations serve as a starting point in understanding phenomena at 1-10 kilometre scales in the river-dominated Bay of Bengal. Present day ocean models have horizontal resolution of 10-25 km, not enough to resolve submesoscale fronts; the surface layer in these models is often too deep. We are learning that fronts are found in almost all seasons in the Bay of Bengal, therefore frontal processes have to be properly represented in ocean models”

In a major new initiative, a team of scientists from several universities and institutes in India and USA is engaged in making observations with high spatial resolution in the BoB. The Indian programme “Ocean Mixing and Monsoon (OMM)” is supported by the National Monsoon Mission, Ministry of Earth Science. “We have conducted several research cruises as part of OMM. The new high-resolution data from ships and autonomous instruments is invaluable - we are learning more about submesoscale eddies and fronts in the upper ocean, as well as their possible role in monsoon air-sea interaction.” says an optimistic Prof. Sengupta.  

About the Authors:

Prof. Debasis Sengupta, Mr. G.N. Bharath and Ms. J. Sree Lekha work at the Center for Atmospheric and Oceanic Sciences, Indian Insitute of Science, Bangalore.

Contact (Lead Author): Ph : 91-80-360 0450     Email: dsen@caos.iisc.ernet.in

Dr. M. Ravichandran is at the Indian National Center for Ocean Information Services, Hyderabad and Dr. Fabrice Papa is currently at Indo-French Cell for Water Sciences, Indian Institute of Science, Banglaore.

About the Paper:

Sengupta, D., G. N. Bharath Raj, M. Ravichandran, J. Sree Lekha, and F. Papa (2016), Near-surface salinity and stratification in the north Bay of Bengal from moored observations, Geophys. Res. Lett., 43, doi:10.1002/ 2016GL068339.

 

http://onlinelibrary.wiley.com/doi/10.1002/2016GL068339/abstract?campaign=wolsavedsearch