Unusual subseasonal variability of Indian summer monsoon rainfall in 2020

Above-normal all-India summer monsoon (ISM) rainfall was received in 2020 with 109% of its long-period average. According to the India Meteorological Department (IMD), ISM 2020 experienced robust month-to-month rainfall variations; in particular, the peak monsoon month of July 2020 witnessed deficit rainfall. The present study investigated the possible mechanisms that are responsible for the July 2020 deficit rainfall. Analysis revealed that the strong low-level moisture divergence caused by the westward-propagating atmospheric Rossby wave, induced by suppressed western North Pacific (WNP) convection, is primarily responsible for the July rainfall deficit. The WNP suppressed convection is linked to anomalous low-level anticyclonic circulation. It is noted that the intense WNP anticyclone is maintained by the tropical Indian Ocean (TIO) warming-induced atmospheric Kelvin waves and strong low-level convergence over the Meiyu-Baiu rainband. Unlike July, the strength of the WNP anticyclone and TIO warming are weaker in June and August. In addition to the observational analysis, the skill of the Climate Forecast System version-2 (CFSv2) model in predicting the July rainfall at a lead of about two months is examined. Strong positive rainfall anomalies over India are predicted by the model in July, which is in contrast to the observations. Despite predicting the TIO warming, the model failed to capture the westward extension of the WNP anticyclone, which impacted the July rainfall prediction. To understand the importance of the WNP anticyclone on ISM rainfall, a CFSv2 sensitivity experiment is carried out by imposing strong negative sea surface temperature (SST) anomalies over the WNP region to excite the anticyclone. Both the westward extension of the WNP anticyclone and suppressed rainfall over the core monsoon region in July are captured by the model when low-level circulation is imposed. The model sensitivity experiment strongly supports the role of the WNP anticyclone and its westward extension in inducing reduced rainfall in July 2020 over the monsoon trough region. The peak monsoon month of July 2020 displayed unexpectedly low precipitation over the Indian land region among all other summer months in spite of the presence of La Nina-like anomalies over equatorial central-eastern Pacific. Anomalous negative precipitation over the monsoon core region or monsoon trough region is mainly attributed to a strong anomalous anticyclone over the western North Pacific region that exerts a northwestward-moving Rossby wave due to suppressed convection over the head of the Bay of Bengal and the monsoon trough area. Figure: Schematic diagram illustrating the physical mechanisms associated with abnormal ISM precipitation during July 2020. Light red color in the Indian Ocean represents basin-wide warming. Brown and blue colors over India show negative and positive precipitation anomalies (mm center dot day-1) during July 2020. image