Northward Prpogating BSISO over South Asia: connection between moisture advection and vortex tilting

Abstract

The mechanism behind the northward propagation of the Boreal Summer Intraseasonal Oscillation (BSISO) over South Asia is still elusive, though many theories have been proposed to explain it. In this study, column- integrated moisture budget and vorticity budget analyses are used on NCEP-NCAR1 and ERA5 reanalyses data to understand the dominant propagation mechanism. While traditional theories have focused on the importance of monsoon winds' vertical easterly shear (via vortex tilting) in preconditioning the atmosphere north of the convection centre by inducing a barotropic vorticity anomaly that supports boundary layer convergence, recent work based on the 'moisture mode' framework suggests that moistening ahead of convection by horizontal moisture advection is necessary for the convection to move northward. In this study, we show that both vortex tilting and moisture advection are essential for northward propagation. The moisture budget analysis shows that when the organized convection first appears over the Indian Ocean, easterlies associated with the Rossby wave response to the convection act on the climatological moisture distribution. This moisture gradient is large along the anomalous wind direction over the Arabian Sea and leads to rapid moistening in the southern Arabian Sea region, while over the Bay of Bengal, this process is not as effective as the moisture gradient is not aligned with the anomalous wind direction over the whole Bay. Thus, the characteristic northwest-southeast oriented convection observed in the BSISO over South Asia comes into being. Hence, a tilted belt of strong convection is present behind the dry anomalies associated with subsidence, and as the convection is associated with strong ascent, the vortex tilting term also acquires a tilted structure aligned with the meridional gradient in vertical velocity. The vorticity budget clearly shows tiling is the dominant term that matches the tendency and leads to the vorticity anomalies, and the dominant term in tilting is the term associated with the meridional gradient of anomalous vertical velocity and vertical shear of zonal mean wind. The pronounced northward movement of vorticity anomalies over the Arabian Sea (more than over the Bay) further increases the tilt of the gyre associated with the convection, thus, winds over the northern Indian landmass, particularly above 20N become south-easterly. These anomalous winds tap into the north-westward moisture gradient that is present over much of the northern Indian landmass and helps move the convection further north over the northern part of the subcontinent. Thus, these two processes together pull the convection band northward over the South Asian monsoon region from the Indian Ocean and generate the NW-SE-oriented structure of the BSISO. We also show the vertical structures of the budget terms, which reveals that the moisture advection is primarily important in the middle to lower free troposphere.