Decoding the monsoon onset: Signs & surprises

ON MAY 26, THE IMD DECLARED THE MONSOON'S ARRIVAL IN MUMBAI, MARKING THE EARLIEST ONSET IN OVER 75 YEARS

- Akshay Deoras

While torrential downpours across many regions in India, including Mumbai, in May, have captured national attention, behind the dramatic skies lie critical questions about the timing of the declaration of the monsoon's onset, its implications for key stakeholders, and how inaccurate weather alerts can undermine disaster preparedness.

The declaration of the monsoon's onset in India is one of the most awaited events of the year. More than simply a season of rain, it is, in fact, a massive seasonal shift in winds that transports vast amounts of moisture from the Indian Ocean toward the Indian subcontinent, ultimately leading to a large amount of rainfall.

At a basic level, the monsoon can be considered to be a giant land-sea breeze, arising from the differential heating between land and Indian Ocean. Since land warms faster than water, air pressure over South Asia is reduced compared to that over the Indian Ocean during summer months, drawing in moisture-laden southwesterly winds toward land. One could therefore argue that a hot summer season is essential for a timely onset of the monsoon and excess rainfall in the country.

And yet, the monsoon progressed rapidly in May this year despite most of the country witnessing a mild summer. This curious case demonstrates that there is no correlation between surface temperatures during the summer and the monsoon onset.