Jet streams driving surge in western disturbances in northwest India

E-PaperSubscribeSubscribeEnjoy unlimited accessSubscribe Now! Get features like Thunderstorms and hailstorms battering northwest India this past week — unseasonal even by pre-summer standards — are the downstream effect of a disrupted polar jet stream and a shifting polar vortex that have driven an unusual surge in western disturbances, meteorologists said. The broader pattern reflects a structural shift in when WDs arrive. (ANI)March recorded eight western disturbances against a normal of five to six, and at least three more are expected through mid-April. The spike is made more striking by what preceded it: January and February saw far fewer WDs than usual, resulting in low snowfall across the Himalayas. The systems have intensified sharply since mid-March. Western disturbances are moisture-laden cyclonic systems that originate over the Mediterranean and travel eastward along the jet stream — the narrow, high-altitude band of fast-moving winds that circles the globe around 30,000 feet. Imagine the WD — typically travelling slightly lower in the atmosphere — as riding the coattails of the jet stream. When the jet stream wobbles, rather than flowing in a relatively straight path, it steers more of these systems into lower latitudes, including northwest India. “When the westerly jet stream is particularly wavy, we sometimes see an increase in the impact of the WDs in lower latitudes. The wavy jet stream is linked to rapid warming of the Arctic,” said Mahesh Palawat, vice president, climate and meteorology, Skymet Weather. A similar disruption to the polar jet stream drove snowstorms and extreme cold across parts of the United States this winter. The broader pattern, scientists say, reflects a structural shift in when WDs arrive — not just how many. “Normally WD activity peaks in winter and sharply reduces by spring. However, recent studies show that WD activity is getting shifted to the spring season, often with WDs occurring in April, May and June. It could be the influence of Arctic sea ice melting in recent years,” said M Rajeevan, climate scientist and former secretary, ministry of earth sciences. The trigger for this shift runs through a chain of interconnected atmospheric systems. The polar vortex — a mass of cold air that rotates around the Arctic — influences the jet stream, which in turn affects the North Atlantic Oscillation, a pattern of pressure differences over the Atlantic that determines how many WDs reach India and how far south they travel. “When the NAO is in negative phase, we do not see as many WDs, but now its phase is zero. As it moves to positive phase, we see more WDs in southern latitudes,” said OP Sreejith, head of the climate monitoring and prediction group at IMD. The immediate impact is forecast to intensify. IMD has warned of light to moderate rainfall over northwest India from April 2 to 5, with peak activity and hailstorms expected on April 3 and 4. Isolated heavy rainfall is likely over the Kashmir valley on April 3. Thunderstorms and lightning are likely over central India through April 6. Day temperatures across northwest India are expected to remain near-normal to below-normal through that period. On Tuesday, hailstorms struck Himachal Pradesh, Punjab, Haryana, Uttar Pradesh, east Rajasthan and west Madhya Pradesh. Thunderstorms with winds of 50-80 kmph were reported across the northeast, Uttarakhand, Gangetic West Bengal, Jharkhand and Delhi. “We can expect intense thunderstorms, lightning and hail in April. People should be cautious,” said M Mohapatra, director general, IMD. While the immediate effect has been relief from early heat, the unusual WD pattern poses risks to standing crops — and echoes the conditions that produced last May’s deadly storms. At least 59 people died across two states in a thunderstorm on May 23, driven by a similar convergence of western disturbances, cyclonic circulations and moisture from both seas. I write on the environment and climate crisis and I believe these are the most important stories of our times.