CHENNAI: El Nino is likely to persist through the remainder of the southwest monsoon, the northeast monsoon and into winter 2027, with the Indian National Centre for Ocean Information Services (INCOIS) forecasting up to 98% probability that the phenomenon will continue through March. The agency said El Nino conditions became established in June, while the World Meteorological Organization (WMO) has projected a rapid development into a strong El Nino during the July-Sept period.The latest outlook assumes significance for Chennai and Tamil Nadu, where a weak southwest monsoon has coincided with prolonged heatwave conditions over the interior regions and hot, humid weather along the coast. India Meteorological Department (IMD) has attributed the recent spell of above-normal temperatures to subdued southwest monsoon activity over the west coast.“Probability of ENSO phases shows El Nino as the dominant phase from July 2026 through March 2027, with probabilities ranging from 85% to 98%,” the INCOIS bulletin said.El Nino is the periodic warming of sea-surface temperatures over the central and eastern equatorial Pacific Ocean.It typically weakens the southwest monsoon by reducing moisture transport from the Arabian Sea, resulting in fewer rain-bearing systems and prolonged dry spells.In contrast, it has historically favoured a wetter northeast monsoon over Tamil Nadu by creating conditions more conducive for rain-bearing systems over the Bay of Bengal. Scientists, however, said the Indian Ocean Dipole (IOD), Madden-Julian Oscillation (MJO) and Bay of Bengal weather systems also play a key role in determining seasonal rainfall. El Nino also tends to favour warmer-than-normal winters over many parts of India, although winter weather remains strongly influenced by western disturbances.Historical rainfall data reflect this pattern in Chennai. Although the city lies in the rain-shadow region during the southwest monsoon, it has recorded below-normal June-September rainfall in eight of the nine El Nino years since 1987, against a seasonal average of 44cm. The deficits have often been offset during the northeast monsoon, which accounts for nearly 60% of Chennai’s annual rainfall and about 48% of Tamil Nadu’s yearly total.“Strong El Nino years such as 1997, 2015 and 2023 brought excess northeast monsoon rainfall over Chennai and much of Tamil Nadu,” said weather blogger Pradeep John, adding that the eventual outcome would still depend on Bay of Bengal weather systems and other climate drivers.Beyond rainfall, the INCOIS bulletin warned that persistent El Nino conditions could intensify marine heatwaves, trigger coral bleaching and affect fisheries by altering fish distribution, recruitment and marine ecosystems in the northern Indian Ocean.Meanwhile, the IMD has forecast light to moderate rain accompanied by thunderstorms over Chennai and neighbouring districts on Friday evening or night. Maximum temperatures may be around 37-38°C and minimum temperatures around 27°C, with rain expected to bring temporary relief before temperatures rise again between spells. Across Tamil Nadu, maximum temperatures may remain 2-4°C above normal at isolated places on Friday before easing over the weekend. There is also a possibility for moderate intensity rainfall over western ghat districts.“Day temperatures may drop the day after a rain spell. It will rise again before the next spell. We can expect some rainfall every other day,” an IMD official said.
