Dipak Kurmi
(The writer can be reached at dipakkurmiglpltd@gmail.com.)
In a development that has stirred meteorological interest and economic optimism, the India Meteorological Department (IMD) declared the onset of the southwest monsoon over Kerala on May 24, 2025—eight days ahead of its climatological schedule of June 1. Such an early arrival is significant not just for meteorologists but also for farmers, policymakers, and industries tied to the rhythm of the Indian monsoon, which delivers more than 70 per cent of the country’s annual rainfall and underpins the nation’s agrarian economy. The last time the monsoon arrived so early was in 2009, when it made landfall on May 23. The early onset this year was not just confined to Kerala; it extended to parts of the northeast, southern Karnataka, and even sections of the Bay of Bengal and the Arabian Sea.
Understanding the Declaration
of Monsoon Onset
Declaring the arrival of the monsoon is a rigorous scientific process. The IMD considers a combination of three critical meteorological parameters: rainfall, wind patterns, and outgoing longwave radiation (OLR). Firstly, 60 per cent of 14 designated meteorological stations across southern India—spanning locations such as Thiruvananthapuram, Kochi, Kozhikode, Mangalore, and others—must receive at least 2.5 mm of rainfall for two consecutive days. Secondly, westerly winds should be established up to a depth of 600 hPa, with wind speeds ranging from 15 to 20 knots at 925 hPa. Finally, OLR values, which indicate the amount of energy radiated from the Earth to space, must fall below 200 watts per square metre. A drop in OLR implies cloudier conditions, which are conducive to sustained monsoonal rainfall.
When these parameters are met over two consecutive days, the IMD officially declares the onset. This year, the criteria were not only fulfilled over Kerala but also across the entire Lakshadweep archipelago, Mahe in Puducherry, and several parts of the Arabian Sea and Bay of Bengal. The monsoon winds even reached sections of southern Karnataka and northeast India’s Mizoram, making this onset notably extensive.
Key Drivers behind the Early Onset
The 2025 monsoon onset was facilitated by a convergence of favourable atmospheric and oceanic phenomena. The monsoon arrived over the South Andaman Sea on May 13—eight days ahead of its normal schedule of May 21—hinting early on at a potentially accelerated advance. According to the IMD, conditions were “very favourable”, and a suite of global and regional factors played a catalytic role.
One of the most influential factors was the Madden-Julian Oscillation (MJO), a large-scale intraseasonal disturbance in the tropical atmosphere. The MJO travels eastward around the equator, bringing alternating wet and dry phases. When the MJO is in a favourable phase over the Indian Ocean, it enhances convection and boosts rainfall across the subcontinent. This year, its positive phase aligned with the pre-monsoon window, strengthening the monsoonal flow.
Another significant contributor was the Mascarene High—a semi-permanent high-pressure system over the southern Indian Ocean, near the Mascarene Islands. The high-pressure variations directly affect the cross-equatorial flow into the Indian subcontinent. This year, a strong Mascarene High intensified the pressure gradient, which in turn enhanced the inflow of moist winds towards India’s west coast.
Furthermore, increased convection—vertical transport of heat and moisture—also played a role. For instance, a convective system originating over Haryana drifted southeastward, bringing pre-monsoon showers to Delhi and signalling atmospheric readiness. Similarly, the Somali Jet, a powerful low-level jet stream originating near Mauritius and Madagascar, picked up strength. As it crossed into the Arabian Sea, it helped transport vast quantities of moisture towards India’s western coast, supporting the early onset.
Equally crucial was the formation of a heat low over Pakistan and adjoining northwestern India. This low-pressure area acts as a suction pump, pulling moist air from the Arabian Sea inland along the monsoon trough—a semi-permanent zone of low pressure stretching from the northwest of the country to the Bay of Bengal. A strong and well-established trough was instrumental in pulling the monsoon current deeper into the Indian landmass.
Lastly, the monsoon onset vortex, a cyclonic circulation over the Arabian Sea, contributed to the intensification and organization of monsoon winds. Its presence acted as a supplementary engine, further accelerating the inland progression of the monsoon.
Comparing Realized
and Normal Onset Dates
The accelerated onset this year led to the monsoon reaching multiple regions significantly earlier than usual. In Port Blair, the normal monsoon date is May 21, but it was recorded this year between May 15 and 16. Thiruvananthapuram and Kochi, which typically await the rains until June 1, experienced the onset on May 24. Bangalore, where the normal date is June 3, witnessed an early arrival on the same day. Similarly, Shimoga (June 4) and Karwar (June 5) saw the monsoon break on May 24. This uniform early spread suggests a well-organized and potent monsoon surge.
Geographical Spread on Onset Day
On May 24, the monsoon enveloped a vast geographical swathe, spanning the southwest and east-central Bay of Bengal, the Comorin region, the Maldives, and the central Arabian Sea. The rains soaked Kerala, Lakshadweep, and Mahe, and stretched into southern and coastal Karnataka, Tamil Nadu (barring northern areas), and northeastern states like Mizoram.
This spatial coverage is notably wider than in previous years, especially for Karnataka, where the monsoon typically sets in around June 5. The state experienced an advance of more than ten days—a rare feat. By the second day post-onset, May 25, the monsoon had moved further inland, reaching Goa, several parts of Maharashtra, and more regions in Mizoram, Manipur, and Nagaland.
The Northern Limit of Monsoon (NLM), an imaginary demarcation that charts the monsoon’s progression, now runs through Devgad, Belagavi, Haveri, Mandya, Dharmapuri, Chennai, Aizawl, and Kohima. The NLM’s advanced positioning indicates the monsoon’s rapid march northward, further fuelling hopes of timely rains in the core monsoon zone encompassing central and northern India.
Broader Implications
of the Early Onset
While the early arrival has brought cheer, especially to the agricultural sector preparing for sowing operations, it also brings with it certain caveats. An early onset does not guarantee uniformly good rainfall across the season. Much will depend on the monsoon’s intra-seasonal variability, potential breaks in rainfall, and the interplay of other climatic influencers like El Niño or La Niña, both of which can dramatically alter monsoon behaviour.
For now, however, the outlook is promising. The IMD’s long-range forecast had hinted at a normal to above-normal monsoon this year, and the early and widespread onset lends credibility to that prediction. In rural India, particularly across rain-fed farming regions, the early rains are likely to encourage timely sowing of key crops like paddy, pulses, and oilseeds. For hydropower projects and reservoirs, an early start helps with better water management and power generation. Urban centres, on the other hand, must brace for challenges including flooding, traffic disruptions, and public health issues linked to waterlogging and poor drainage.
The early onset of the 2025 southwest monsoon over Kerala and its rapid spread across southern and northeastern India marks an auspicious beginning to the four-month rainy season. Driven by favourable ocean-atmosphere dynamics, including a strong MJO, Somali jet, and Mascarene High, and supported by vigorous convection and a robust heat low, this year’s monsoon has defied typical timelines. Though the full impact will only be assessed over the coming months, the current indicators suggest a potentially fruitful season for agriculture, water resources, and economic momentum. As ever, the monsoon remains both a meteorological marvel and the economic heartbeat of India.