Investigation of Precursor from Low Level Jet Stream for Indian Summer Monsoon from Mascarene High Near Madagascar Island

A major driver of the Indian Summer Monsoon (ISM) is the cross-equatorial low-level jet (LLJ), whose maximum intensity is usually found around 850 hPa, corresponding to an altitude of nearly 1.5 km. This monsoon system exhibits substantial intra-seasonal variability between June and September. At its onset, the LLJ gains strength from the Mascarene High near Madagascar, channeled via the Somali Current, which in turn triggers wind reversal over the Indian subcontinent. The present work is entirely data-driven, utilizing radiosonde and rawinsonde (RSRW) upper-air observations from the University of Wyoming. Atmospheric profiles at 925 hPa, 850 hPa, and 700 hPa were analyzed for temperature, humidity, and wind parameters during March of the years 2021–2025. North–south moisture flux components were calculated, revealing that a reversal occurs near Seychelles (station FSSS, WMO 63985; 4.66°S, 55.53°E), approximately three months before the ISM onset in India. The influence of El Niño on this variability was also detected. Findings suggest that moisture flux reversal over Seychelles can act as a precursor for long-range monsoon forecasts.