EAS Fall 2023 Seminar Series: Indian Summer Monsoon Onset and Intraseasonal Oscillations

Dr. Subrahmanyam Bulusu

University of South Carolina

The Indian monsoon is a complex, nonlinear phenomenon involving atmospheric, oceanic, and land-based interactions. Unfortunately, despite decades of research and the aid of new observational and modeling tools, monsoon forecasting remains challenging. Salinity is often neglected in climaterelated studies because it does not directly affect air-sea fluxes, though recent works have shown that regional salinity can play an integral role in ocean-atmosphere interactions in the Southeastern Arabian Sea (SEAS) region. The monsoon onset vortex (MoV), which is responsible for the initiation of the summer monsoon onset over southeast coast of India, is triggered by strong air-sea fluxes intensified by salinity-induced barrier layer formation in the SEAS region. Besides the monsoonal rainfall intensity is directly tied to the strength and northward propagation of intraseasonal oscillations (ISOs) with periodicities of 30-90 day (MJO events), 10-20 days (quasi-biweekly oscillation) and 3-7 day (synoptic events). These ISOs are responsible for variation from mean conditions of monsoonal rainfall and are central in modulating active (wet) and break (dry) monsoon conditions. The 3-7 day synoptic/weather signals are closely related to the active and break phases of monsoon trough movement.

The 2019 southwest monsoon season was the first strong monsoon since 1994 and occurred concurrently with a strong positive Indian Ocean Dipole (pIOD). The occurrence of high amplitude 3-7-day Synoptic Oscillations (SO) contributed heavy rainfall in 2019. Evaluation of coupled models such as the Climate Forecast System (CFS) version 2.0 reveals insufficient detection of ISOs,when compared with those detected by satellite-derived salinity from NASA’S Soil Moisture Active Passive (SMAP). Northward propagation of ISOs in SMAP sea surface salinity modulates the post-monsoon (October-November) air-sea interactions of the Bay in the northern Indian Ocean, which are a necessary component of determining upper-ocean mixing conditions in the Arabian Sea that then fuel monsoon onset for the next summer monsoon season. The usage of satellite-derived salinity in coupled atmospheric-ocean models will improve model simulations with respect to the influence of salinity on monsoonal processes.

As prediction of rainfall directly determines water management for agriculture, industry, and daily life in countries of this region that rely on monsoon rainfall, it is critical to better understand related dynamical processes for accurate rainfall prediction that impacts over a billion people and is thus important with respect to economic stability and national security.