Role of background moisture in dictating the Intraseasonal Rainfall over Bay of Bengal

The northward propagating intraseasonal Oscillation (ISO) is one of the dominant modes of tropical variability during Boreal summers. Several mechanisms have been proposed to explain northward propagation. Yet the factors that decide the ISO rainfall over a particular region remains elusive. in this  study we show that the ISO rainfall anomalies weaken across the south Bay of Bengal (SBoB) before they re-strengthen over the north Bay of Bengal (NBoB). We use the moisture budget to understand the reason for the same. We find that the horizontal moisture flux convergence predominantly controls the ISO rainfall anomalies over the two regions. Further analyses reveal that the convergence of background moisture by the ISO wind perturbations decides the ISO rainfall structure. We hypothesize that the weaker rainfall anomalies in the SBoB result from the weaker background column relative humidity and moisture, which do not allow the initial dynamic perturbations to grow as fast as they do in an environment with stronger background relative humidity and moisture (NBoB).