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

Abstract

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.

Introduction

India’s economy relies heavily on agriculture, which depends largely on the Indian Summer Monsoon (ISM) that provides about 75–80% of annual rainfall. Despite technological progress, agricultural productivity and development remain closely tied to monsoon behavior, making accurate monsoon prediction essential for planning and risk mitigation.

The southwest monsoon (June–September) is influenced by multiple large-scale atmospheric systems such as the Heat Low, Monsoon Trough, Mascarene High, Tibetan High, Tropical Easterly Jet (TEJ), and especially the Low-Level Jet (LLJ). The LLJ, a strong westerly wind around 850 hPa, plays a key role in transporting moisture from the equatorial Indian Ocean to India, thereby affecting the timing and distribution of rainfall.

This study analyzes five years (2021–2025) of atmospheric sounding data from the Seychelles (near Madagascar) to investigate the cross-equatorial LLJ and its moisture flux. Using wind direction and relative humidity data from the upper atmosphere (925–700 hPa levels), the research quantifies northward moisture transport crucial for the monsoon.

Results show a consistent onset of increased northward moisture flux at Seychelles in early March, approximately 80–90 days before the monsoon onset over India. This finding indicates that monitoring wind and humidity at Seychelles offers an early predictive signal for India’s monsoon onset.

Such early detection is vital for agricultural planning, drought preparedness, and improving food security in India, emphasizing the importance of atmospheric diagnostics in enhancing climate resilience.

Conclusion

Based on the analysis, several conclusions can be drawn: 1) The timing of the southwest monsoon onset over different parts of India can be anticipated by monitoring the preceding northward moisture flux at those locations. 2) The onset is not uniform; it varies from region to region across the subcontinent. 3) Tracking changes in moisture incursion also provides insights into the advancement and temporary stagnation of the monsoon. Since moisture transport signals appear well before the actual establishment of monsoon circulation, they can serve as a reliable precursor for long-range forecasts. Despite advancements in irrigation infrastructure, India’s agricultural output and economic stability remain highly sensitive to rainfall distribution. Therefore, using moisture flux variability as an early diagnostic tool can significantly strengthen seasonal forecasting and agricultural planning.

References

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Copyright

Copyright © 2025 Ramkrishna Datta, Saumitra Biswas, Sukriti Santra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.