Quasi-biweekly mode of the Asian summer monsoon revealed in Bay of Bengal surface observations

dc.contributor.author Lekha, J. Sree
dc.contributor.author Lucas, Andrew J.
dc.contributor.author Sukhatme, Jai
dc.contributor.author Joseph, Jossia K.
dc.contributor.author Ravichandran, M.
dc.contributor.author Kumar, N. Suresh
dc.contributor.author Farrar, J. Thomas
dc.contributor.author Sengupta, Debasis
dc.date.accessioned 2021-03-29T19:02:25Z
dc.date.available 2021-05-16T06:16:33Z
dc.date.issued 2020-11-16
dc.description Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(12),(2020): e2020JC016271, https://doi.org/10.1029/2020JC016271. en_US
dc.description.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. en_US
dc.description.embargo 2021-05-16 en_US
dc.description.sponsorship The authors thank the Ministry of Earth Sciences (MoES) institutes NIOT and INCOIS, and the Upper Ocean Processes (UOP) group at WHOI for design, integration, and deployment of moorings in the BoB. The WHOI mooring was deployed from the ORV Sagar Nidhi and recovered from the ORV Sagar Kanya—we thank the officers, crew and science teams on the cruises for their support. Sengupta, Ravichandran and Sukhatme acknowledge MoES and the National Monsoon Mission, Indian Institute of Tropical Meteorology (IITM), Pune, for support; Lucas and Farrar acknowledge the US Office of Naval Research for support of ASIRI through grants N00014‐13‐1‐0489, N0001413‐100453, N0001417‐12880. We thank S. Shivaprasad, Dipanjan Chaudhuri and Jared Buckley for discussion on ocean currents and Ekman flow, and Fabien Durand for discussion on sea level. JSL would like to thank the Divecha Center for Climate Change, IISc., for support. DS acknowledges support from the Department of Science and Technology (DST), New Delhi, under the Indo‐Spanish Programme. en_US
dc.identifier.citation Sree Lekha, J., Lucas, A. J., Sukhatme, J., Joseph, J. K., Ravichandran, M., Suresh Kumar, N., Farrar, J. T., & Sengupta, D. (2020). Quasi-biweekly mode of the Asian summer monsoon revealed in Bay of Bengal surface observations. Journal of Geophysical Research: Oceans, 125(12), e2020JC016271. en_US
dc.identifier.doi 10.1029/2020JC016271
dc.identifier.uri https://hdl.handle.net/1912/26881
dc.publisher American Geophysical Union en_US
dc.relation.uri https://doi.org/10.1029/2020JC016271
dc.title Quasi-biweekly mode of the Asian summer monsoon revealed in Bay of Bengal surface observations en_US
dc.type Article en_US
dspace.entity.type Publication
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