Differences in Northward Propagation of Convection Over the Arabian Sea and Bay of Bengal During Boreal Summer

The governing dynamics that modulate the propagation characteristics of intraseasonal oscillations (ISO) during summer monsoon over the two ocean basins, Bay of Bengal (BoB) and Arabian Sea (AS), are investigated using observational analysis and high-resolution regional coupled ocean-atmosphere climate model simulations. ISO features are extracted over the Indian region using a data-adaptive spectral method called multichannel singular spectrum analysis. ISO exhibits stronger intensity over the BoB than over the AS. But ISO-filtered rainfall propagates at a faster rate (similar to 1.25 degrees/day) over AS as compared to BoB (similar to.74 degrees/day), giving rise to a northwest-southeast tilted band of rainfall anomalies. However, the composite diagrams of several atmospheric fields associated with northward propagation like vorticity, low-level convergence, and oceanic variables like sea surface temperature and mixed layer depth do not show this difference in propagation speed and all exhibit a speed of nearly 0.75 degrees/day in both the ocean basins. The difference in speed of ISO-filtered rainfall is explained through moisture flux convergence. Anomalous horizontal moisture advection plays a major role over AS in preconditioning the atmosphere and making it favorable for convection. Anomalous wind acting on climatological moisture gradient is the dominant term in the moisture advection equation. Easterly wind anomalies associated with a low-level anticyclone over India helps advect moisture from the eastern side of the domain. The northwest-southeast tilt of ISO is dictated by the atmospheric processes of moisture advection with the upper ocean playing a more passive role in causing the tilt.