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Opposite Changes in Monsoon Precipitation and Low Pressure System Frequency in Response to Orographic Forcing

Thomas, T.M. and Bala, G. and Vemavarapu, S.V. and Boos, W.R. (2023) Opposite Changes in Monsoon Precipitation and Low Pressure System Frequency in Response to Orographic Forcing. In: Journal of Climate, 36 (18). pp. 6309-6329.

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Official URL: https://doi.org/10.1175/JCLI-D-22-0476.1


Monsoon low pressure systems (LPS) are synoptic-scale disturbances that form along the quasi-stationary trough of the larger-scale South Asian summer monsoon, producing a large fraction of total monsoon precipitation. Here, we use an Earth system model to investigate the influence of Tibetan and Himalayan orography (THO) on mean monsoon strength and LPS activity. The influence of THO height on LPS activity has not been investigated before. The model simulates decreased mean monsoon circulation and precipitation when THO is removed, but the number of LPS and the meridional extent of LPS activity increase; this is an unexpected, important finding given that LPS are principal rain-bearing systems of the monsoon. The decreased mean monsoon circulation and precipitation on removal of THO are attributed to enhanced mixing of dry extratropical air into the humid monsoon domain, as demonstrated by prior studies. The increase in LPS frequency and the meridional extent of LPS activity is attributed to the increase in magnitude and meridional extent of the low-level meridional shear of zonal wind, which previous work showed amplifies LPS through barotropic growth. Specifically, as monsoon precipitation decreases, the sensible heat-driven low-level trough intensifies and shifts equatorward; this strengthens the shear zone in which LPS grow. Conversely, increasing THO height decreases the magnitude and meridional extent of cyclonic shear over India, decreasing LPS frequency and the spatial extent of LPS activity while increasing total monsoon precipitation. These results demonstrate that LPS activity and total monsoon rainfall can undergo large, opposing changes in response to imposed forcings. © 2023 American Meteorological Society.

Item Type: Journal Article
Publication: Journal of Climate
Publisher: American Meteorological Society
Additional Information: The copyright for this article belongs to the Authors.
Keywords: Atmospheric thermodynamics; Earth system models; Shear flow, Barotropic flows; Low pressure systems; Monsoon; Monsoon circulations; Monsoon precipitation; Orographic effects; Orographic forcing; System frequency; Tibetans; Wind shears, Rain, barotropic motion; climate forcing; low pressure system; monsoon; orographic effect; precipitation (climatology); wind shear
Department/Centre: Division of Interdisciplinary Sciences > Interdisciplinary Centre for Water Research
Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Division of Mechanical Sciences > Civil Engineering
Date Deposited: 10 Nov 2023 03:49
Last Modified: 10 Nov 2023 03:49
URI: https://eprints.iisc.ac.in/id/eprint/83321

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