Northward Propagating Versus Non-Propagating BSISO Over South Asia: Horizontal Advection Driven Moisture Mode Within a Vertically Sheared Background

Abstract

The Boreal Summer Intraseasonal Oscillation (BSISO) is a pronounced mode of tropical variability. Here, we identify two types of BSISO events, one which propagates northward over South Asia from the equatorial Indian Ocean (EIO), and the other which doesn't. Contrasting their behavior shows that the northward propagation occurs in multiple stages after convection is initiated over the EIO. First, the convection moves into the southern Arabian Sea (AS) due to moistening of the free troposphere via horizontal BSISO anomalous winds acting on the background moisture distribution, and forms a northwest-southeast (NW-SE) oriented convection band. Subsequently, in the presence of an easterly vertical shear of monsoon winds and meridional gradient of anomalous vertical velocity, a NW-SE oriented tilting term is generated that results in a tilted gyre north of the existing convective anomaly and south-easterly BSISO winds over the South Asian landmass. In the second stage, these winds tap the ambient north-westward moisture gradient and help move the convection further north over land. Moreover, the background winds advect anomalous moisture to initiate convection over the Bay of Bengal. For the non-propagating events, though a Rossby gyre results as a response to the nascent EIO convection, it is smaller, thus the BSISO advection of background moisture is weaker and does not initiate convection over the southern AS. In turn, the meridional gradient of the anomalous vertical velocity is weak, and the background vertical shear cannot generate sufficient tilting over the northern AS. Therefore, the convective wind response stalls, and large-scale convection does not propagate north of 15N. Thus, free-tropospheric moisture advection and vortex tilting due to the background vertical shear work together for robust northward propagation of the BSISO, and strong BSISO easterlies over the southern AS is critical behind this propagation.