  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 (SA) from the equatorial Indian Ocean
(EIO), and the other which doesn't. Contrasting their behaviour shows that
northward propagation occurs in multiple stages after convection is initiated
over the EIO. First, 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 convection further north over
land. Moreover, background winds advect anomalous moisture to initiate
convection over the Bay of Bengal. For non-propagating events, though a Rossby
gyre results as a response to nascent EIO convection, it is smaller, thus BSISO
advection of moisture is weaker and does not initiate convection over the
southern AS. In turn, the meridional gradient of anomalous vertical velocity is
weak, and the background vertical shear does not generate sufficient tilting
over the northern AS. Thus, 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.
