Quasi-Biweekly Mode of the Asian Summer Monsoon Revealed in Bay of Bengal Surface Observations
Sree Lekha J*, Debasis Sengupta*, Andrew J Lucas, Jai Sukhatme, Thomas J Farrar, Jossia K Joseph, Suresh Kumar N
Indian Institute of Science
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The Asian summer monsoon has a planetary-scale, westward propagating quasi-biweekly mode of variability with 10-25 day period. Six years of observations from open ocean moorings at 18N, 89.5E in the north Bay of Bengal reveal distinct quasi-biweekly oscillations in sea surface salinity (SSS) during summer and autumn, with peak-to-peak amplitude of 3-8 psu. These large-amplitude oscillations are not due to quasi-biweekly variations of surface freshwater flux or river runoff. We show from moored wind, salinity and current data, satellite SSS, and reanalyses, that surface wind changes associated with the quasi-biweekly mode of the monsoon and embedded weather-scale systems, drive the SSS and coastal sea level oscillations in the summer of 2015. When winds are calm, 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 the quasi-biweekly monsoon mode, directly wind-forced surface Ekman-like currents carry river water away to the east and north, leading to increased salinity at the mooring, as well as 0.1-0.5 m rise in sea level along the northeastern boundary of the bay. In July-August 2015, a shallow pool of low-salinity river water lies in the northeastern bay. The amplitude of a 20-day SST oscillation 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 space-time variation of Bay of Bengal salinity influences sub-seasonal SST variations, and possibly SST-mediated monsoon air-sea interaction.