A contrast in biennial variability of rainfall between central India and the Western Ghats and its mechanisms

Abstract

The Western Ghats (WG) of Peninsular India, an integral part of the Indian summer monsoon rainfall, receives three times the average of all India rainfall. The averaged rainfall over WG is characterized by an intense tropospheric biennial oscillation (TBO) of 2–3 years of periodicity. The rainfall anomalies over WG are almost uncorrelated to rainfall over Central India (CI) in the TBO window. This study characterizes the WG and CI biennial rainfall variability and their governing mechanisms using observation and reanalysis datasets for 1980–2020. A zonally symmetric build-up of heat anomalies from the Iranian Plateau (IP) to the Tibetan Plateau (TP) extending from the surface to the mid-troposphere governs the TBO rainfall of CI. On the other hand, localized heating (cooling) over the IP and Pak-Afghanistan region (PA) and cooling (heating) over the TP governs the phases of TBO rainfall over WG. An increase (decrease) in anomalous heat build-up in the vertical column causes an increase (decrease) in anomalous moist static energy during positive (negative) WG TBO years extending over the IP (TP) region. Increased heating (cooling) over the IP and PA (TP) during positive WG TBO years can shift the center of near-surface cyclonic circulation, anchored over the Indian subcontinent and surrounding areas during the positive CI TBO years, to move westward. This shift in rainfall anomalies and the center of cyclonic circulation is because of the westward shift in anomalous moisture convergence from CI and significant moisture loss along southeastern Peninsular India. Considering the growing number of extreme rainfall events over the WG regions recently, the present work is an attempt to understand the mechanism through which TBO modulates WG and CI rainfall.