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Abstract

Despite much progress, the mechanism behind the northward propagation of the BSISO over South Asia remains elusive. Here, an attempt is made to understand this mechanism connecting vorticity and column-integrated moisture budgets. The moisture budget shows that when organized intraseasonal convection first appears over the Indian Ocean, easterlies associated with the Rossby wave response act on the zonally oriented climatological moisture gradient over the Arabian Sea (AS), resulting into rapid moistening of the southern AS. This process is not as dominant over Bay of Bengal (BOB). Thus,convection moves into AS and the characteristic northwest-southeast oriented convection belt comes into being. Now, as a latitudinally tilted convection is present behind dry anomalies associated with subsidence, a large vortex tilting term gets generated north of the convection along the tilted meridional gradient in anomalous vertical velocity in presence of vertical shear of the mean monsoon wind . The vorticity budget clearly shows that tilting matches the tendency and leads the vorticity anomalies. This leads to northward movement of the Rossby gyre with a latitudinal tilt. As a result, anomalous winds over the northern Indian landmass, particularly beyond 20N become south-easterly, which tap into the north-westward mean moisture gradient over the northern Indian landmass , and help move convection further north. Over the BOB, mean monsoonal winds act on intraseasonal moisture gradient to move the convection northward. Thus, horizontal moisture advection and vortex tilting together pull the convection band northward over the South Asian monsoon region and generate its characteristic structure.