Quasi-Biweekly Mode of the Asian Summer Monsoon Revealed in Bay of Bengal Surface Observations

Sree Lekha, J and Lucas, AJ and Sukhatme, J and Joseph, JK and Ravichandran, M and Suresh Kumar, N and Farrar, JT and Sengupta, D (2020) Quasi-Biweekly Mode of the Asian Summer Monsoon Revealed in Bay of Bengal Surface Observations. In: Journal of Geophysical Research: Oceans, 125 (12).

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Abstract

Asian summer monsoon has a planetary-scale, westward propagating �quasi-biweekly� mode of variability with a 10�25 day period. Six years of moored observations at 18°N, 89.5°E in the north Bay of Bengal (BoB) reveal distinct quasi-biweekly variability in sea surface salinity (SSS) during summer and autumn, with peak-to-peak amplitude of 3�8 psu. This large-amplitude SSS variability is not due to variations of surface freshwater flux or river runoff. We show from the moored data, satellite SSS, and reanalyses that surface winds associated with the quasi-biweekly monsoon mode and embedded weather-scale systems, drive SSS and coastal sea level variability in 2015 summer monsoon. When winds are calm, geostrophic currents associated with mesoscale ocean eddies transport Ganga-Brahmaputra-Meghna river water southward to the mooring, salinity falls, and the ocean mixed layer shallows to 1�10 m. During active (cloudy, windy) spells of quasi-biweekly monsoon mode, directly wind-forced surface currents carry river water away to the east and north, leading to increased salinity at the moorings, and rise of sea level by 0.1�0.5 m along the eastern and northern boundary of the bay. During July�August 2015, a shallow pool of low-salinity river water lies in the northeastern bay. The amplitude of a 20-day oscillation of sea surface temperature (SST) is two times larger within the fresh pool than in the saltier ocean to the west, although surface heat flux is nearly identical in the two regions. This is direct evidence that spatial-temporal variations of BoB salinity influences sub-seasonal SST variations, and possibly SST-mediated monsoon air-sea interaction. © 2020. American Geophysical Union. All Rights Reserved.

Item Type:	Journal Article
Publication:	Journal of Geophysical Research: Oceans
Publisher:	Blackwell Publishing Ltd
Additional Information:	Copyright to this article belongs to Blackwell Publishing Ltd
Department/Centre:	Division of Mechanical Sciences > Divecha Centre for Climate Change Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Date Deposited:	12 Feb 2021 10:50
Last Modified:	12 Feb 2021 10:50
URI:	http://eprints.iisc.ac.in/id/eprint/67524

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