El Niño, a deficient monsoon, and the heat to come
The Southwest monsoon had entered the Andaman Sea on May 16, but its advance towards the mainland was slower than expected. The India Meteorological Department
The Southwest monsoon had entered the Andaman Sea on May 16, but its advance towards the mainland was slower than expected. The India Meteorological Department (IMD) had forecast its onset over Kerala by May 26, with a model error of plus or minus four days; the normal onset date is June 1. The monsoon had missed both markers, eventually setting in over Kerala only on June 4. What it brings with it, however, offers little comfort. projection According to projections presented by O.P. Sreejith, Scientist and Head, Climate Monitoring and Prediction Group, Climate Research and Services, IMD, Pune, the IMD’s updated long-range forecast, released on May 29, projects the season’s rainfall over the country as a whole at 90% of the Long Period Average (LPA), with a model error of plus or minus four percentage points. The accompanying five-category probability forecast puts the combined chance of deficient or below-normal rainfall nationally at 84% this year. Bharati Sabade, Scientist at the Cyclone Warning Centre, Visakhapatnam, said the underlying signal is already visible in the data. “The observed values of sea surface temperatures above the 0.5 threshold point towards an El Niño,” she said. For Andhra Pradesh, the numbers carry particular weight. Coastal Andhra Pradesh is forecast to receive below-normal rainfall, at less than 91% of its LPA, or roughly 547.4 mm against a seasonal normal of 601.3 mm. Rayalaseema’s outlook is starker: below 88% of LPA, or about 358.0 mm against a normal of 408.5 mm.
The State as a whole falls into the below-normal category against a four-month normal of 521.6 mm. June, which sets the tone for sowing, is expected to be the most deficient period, with rainfall projected at below 79% of LPA, or 86.9 mm against a normal of 94.1 mm, accompanied by above-normal maximum and minimum temperatures across most parts of the State. Behind these figures lies a transition underway in the equatorial Pacific. The IMD notes that neutral conditions are shifting towards El Niño, with models indicating that the phenomenon is likely to develop and persist through the season. The Indian Ocean Dipole, by contrast, remains neutral and is expected to stay that way. Professor C.V. Naidu, Head, Department of Meteorology and Oceanography, Andhra University, explained the Pacific mechanics driving this year’s outlook. The Southern Oscillation, he said, describes a seesaw in atmospheric pressure between Tahiti in the central Pacific Ocean and Darwin in northern Australia. “Tahiti pressure minus Darwin pressure is quantified as the Southern Oscillation Index. If it is positive and persistent, it is called La Niña; if it is negative and persistent, it is called El Niño,” he said. The result is a weakening of the trade winds that carry moisture towards India. “Why are we afraid of El Niño? Because the trade winds weaken,” he said, adding that the moisture-bearing Somali Jet, which crosses the equator off the Somali coast and feeds the monsoon, depends on a strong north-south temperature gradient that El Niño disrupts.
The BriefWire