Variability in the onset of summer monsoon over Gangetic West Bengal, India: an investigation with climatology of tropopause characteristics

ABSTRACT The inter‐annual variation in the onset of summer monsoon over Gangetic West Bengal (GWB) (22.5°–25°N; 85°–90°E), India is investigated by analyzing temperature gradient (Tg) at tropopause (100 and 70 hPa) and mid‐tropospheric (500–200 hPa) levels, outgoing long‐wave radiation (OLR), wind and its divergence, vorticity at 150, 100 and 70 hPa pressure levels, low‐level (850 hPa) convergence along with vertical motion of air during the period from 1982 to 2012. The objective is to observe the dynamical variability at tropopause and its impact on lower troposphere during variable onset of summer monsoon over GWB. The years under study are classified as delayed, normal and advanced monsoon years by estimating the mean date of onset over GWB. Decreasing and increasing trends in Tg during the onset over GWB is observed at 100, 70 and (500–200) hPa levels, respectively. The sharp changes in Tg might have aided in dragging the monsoon wind over GWB during the advanced onset years in contrast to the normal and delayed onsets. The position of South Asian High (SAH) at 150 hPa level over Bay of Bengal is observed to change during advanced, normal and delayed onset years. The SAH is found to tilt with height towards warm air forming anti‐cyclonic vortices at 100 and 70 hPa levels. The changing position of SAH at 150 hPa level leads to dissimilarity in divergence of wind at upper levels which in turn influences the low‐level convergence and the position of vorticity during advanced, normal and delayed onset years. The result further reveals that OLR is quite low on the onset day during advanced monsoon years compared to normal and delayed years. The anomaly in OLR depicts different features during advanced, normal and delayed onset years.