Downstream and In-situ: Two Perspectives on Initiation of Monsoon Low Pressure SysteMS over Bay of Bengal

Abstract

Monsoon low pressure systems (LPS) contribute to nearly half of the total summer monsoon rainfall over central India. As their mechanism is not well understood, their contribution is ill-represented in climate models, which has led to the underestimation of rainfall over central India in simulations. The initiation of these systems is commonly considered to be produced by atmospheric instabilities that grow in time and space. The influence of western Pacific typhoons in generating lows over Bay of Bengal has been overlooked in most of the studies. Their influence, termed as downstream amplification, contributes to 32% of the total, in comparison to the 68% by in-situ processes, obtained by examining 256 cases of LPS initiation from 1979 to 2017. This is in contrast to the results of some of the previous studies that attribute the majority of LPS initiation to downstream processes. Composite structures reveal that in-situ and downstream generated events essentially represent the same physical system, having similar dynamic and thermodynamic features, but differing in their mode of initiation. Lows initiated in-situ display greater randomness in the initiation date and location compared to downstream events that are preferentially initiated east of the head Bay, mainly in the months of August and September. The declining trend seen in the downstream generated cases suggests that the prediction of the initiation of LPS will become increasingly difficult in the future. Influence of vertically propagating Potential Vorticity (PV) anomalies and intensification of LPS over land are some of the factors that need to be explored.