Climate — UPSC Mains Model Answer

During winter, the Subtropical Westerly Jet Stream flows south of the Himalayas, over the Indian subcontinent. As summer approaches, intense heating of the Tibetan Plateau causes this jet to shift northward, beyond the Himalayas. This northward withdrawal removes the stabilising upper-air influence over the subcontinent, facilitating the northward advance of the Inter-Tropical Convergence Zone and triggering the southwest monsoon onset.

The Tropical Easterly Jet Stream develops during the summer monsoon season at approximately 9°N latitude at upper tropospheric levels near the 150 hPa pressure level. It is strongly correlated with active phases of the southwest monsoon, as it promotes divergence aloft over the Indian Ocean, sustaining the low-pressure systems and convective activity that drive monsoon rainfall inland.

The persistence of the Subtropical Westerly Jet south of the Himalayas during winter channels extratropical disturbances — known as Western Disturbances — eastward into northwest India. These disturbances bring critical winter rainfall and snowfall to states like Punjab, Haryana, and Himachal Pradesh, supporting rabi crop cultivation and replenishing Himalayan snowpack.

Accurate forecasting of jet stream behaviour directly informs the India Meteorological Department's monsoon onset predictions and agricultural advisories. Anomalies in jet stream positioning — linked to global warming and Arctic amplification — increasingly disrupt monsoon timing, posing food security and water management challenges.