| Popis: |
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 remain elusive. This study shows 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 these weakening-strengthening cycles. We find that the horizontal moisture flux convergence predominantly controls the ISO rainfall anomalies over the two regions. The convergence of background moisture by the ISO wind perturbations decides the ISO rainfall structure. Since past literature suggests that the Planetary Boundary Layer (PBL) convergence is caused by barotropic vorticity, we further conduct the vorticity budget to understand the rainfall structure. We find that though vorticity tilting helps in the generation of the positive tendency of ISO vortex, it is the vorticity stretching that enhances the PBL convergence and hence the ISO rainfall over the NBoB. Further splitting of the stretching term helps us conclude that the convergence due to wind perturbations is the predominant term, representing feedback between dynamics and thermodynamics. 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). Our study helps us understand the factors that control the ISO rainfall over a particular region and could help improve the model simulations. |
