Bay of Bengal branch of Indian summer monsoon and its association with spatial distribution of rainfall patterns over India

Abstract

Indian summer monsoon rainfall is characterized by a considerable variability on the spatial and temporal scale, though the standard deviation of rainfall on the temporal scale is much smaller (ca. 10% of annual long-term mean) compared with that of the spatial scale. It is known that the seasonal rainfall amplitude over India as well as the statistical properties of the subseasonal variability is strongly linked to the Pacific teleconnection on interannual timescales, such as El Nino-Southern Oscillation (ENSO) and La Nina, and the teleconnection is primarily arising from central and eastern Pacific Ocean. But, the west Pacific controls on the southwest monsoonal flow, and in turn, the modulation of rainfall variability in the subseasonal scale by this low-frequency seasonal to interannual background over India is poorly understood. Here, we examine the moisture pathways by which the spatial pattern of rainfall variability in the subseasonal scale is envisaged to be arising due to the interplay between the Bay of Bengal (BoB) summer monsoon circulation and the South China Sea (SCS) moisture dynamics under different background conditions provided by ENSO or the subseasonal movement of the Inter Tropical Convergence Zone (ITCZ). We observe that the south BoB branch of the summer monsoon could occasionally extend up to 120° E in the SCS and reverse back to Indian mainland. We propose a mechanism by which the south BoB branch interacts with the SCS (viz. western Pacific) atmospheric system, thereby facilitating a pathway of the west Pacific moisture intrusion into the Indian subcontinent. The study concludes that the changes in surface temperature/pressure spatial pattern between India and SCS region play an important role in the distribution of the rainfall. In this study, for the first time, the life cycle of south Bay monsoon flow and its interaction with the SCS are discussed, likely to warrant special attention for modelers trying to understand the spatial distribution of rainfall over India.