A curious case of the Indian Summer Monsoon 2020: The influence of Barotropic Rossby Waves and the monsoon depressions

TL;DR

This paper investigates the unusual monsoon rainfall in India during 2020, highlighting the roles of Rossby waves and monsoon depressions, and analyzing why certain low-pressure systems did not intensify into depressions.

Contribution

It identifies the influence of stationary Rossby waves and specific atmospheric conditions on the 2020 monsoon, providing insights into the dynamical factors behind the atypical rainfall pattern.

Findings

Warming over the Northern Arabian Sea boosted convection in August 2020.

Weak barotropic instability prevented LPAs from developing into monsoon depressions.

CESM-LE model struggled to accurately simulate monsoon rainfall and circulation patterns.

Abstract

Summer monsoon rainfall over India depends on synoptic-scale phenomena like lows and depressions. India's summer monsoon season 2020 saw above-average rainfall. North Indian Ocean had 12 LPAs in summer 2020 in JJAS season. This monsoon season has the highest all-India rainfall since 1976 in August 2020. 50% of seasonal LPAs emerged in August 2020, but none intensified into a monsoon depression (MDs). This study explores the potential factors behind LPAs not intensifying/concentrating as MDs. Warming over the Northern Parts of the Arabian Sea (NPAS) boosted convection in August 2020, resulting in substantial low-level convergence. Strong northwesterly winds from central Asia also combined with the cross-equatorial monsoon flow. This powerful flow across the Arabian Sea split into two branches: one extending to NW India following the monsoon trough, and another diverging into an anticyclone over the south Bay of Bengal (SBOB), reducing horizontal shear there (Barotropic Instability). Due to weak barotropic instability over the head BOB, LPAs cannot become MDs. In August 2020, the 200 hPa barotropic Rossby wave remains stationary over South Central Asia and retrogresses northeast to southwest. It affected all-India rainfall by increasing rainfall in the NW and Western Ghats (WG). Despite unfavourable dynamical conditions, the barotropic Rossby wave and low-level anticyclone over the WNP caused above-normal rainfall over India in August 2020. We also confirmed these mechanisms in Community Earth System Model Large Ensemble (CESM-LE) model simulations. Analysis showed that model has limited skill to mimic monsoon rainfall and associated circulation and failed to reflect mid-latitude circulation impact on ISM rainfall.