Kolkata: The El Nino effect has started showing in Kolkata this monsoon, with heavy rain happening in clusters interspersed with dry and warm periods. Severe disruptions like the ones triggered by Thursday’s deluge could continue as the monsoon progresses, warned meteorologists.While Kolkata was lashed by a severe thunderstorm and 59 mm of rainfall on Tuesday, the city received 48.2 mm last Friday. On Thursday, Kolkata was lashed by another thunderstorm accompanied by incessant lightning strikes and 84 mm of rain.Weak monsoon currents and a simultaneous moisture incursion after monsoon onset have been triggering intense thunderstorms in Kolkata and south Bengal, said Regional Meteorological Centre weather scientists. “This is a typical feature of El Nino. It keeps monsoon currents weak, curbing the formation of rain-triggering systems like low pressures that drive monsoon rain. This has been leading to a temperature rise due to local heating, in the absence of rain. The result has been intense thunderstorms with lightning, since moisture incursion is high. We have received three so far in Kolkata and south Bengal in June. They have been occurring regularly over the last fortnight and will keep recurring till monsoon currents strengthen,” said RMC weather scientist Sourish Bandyopadhyay.The thunderstorms that struck Kolkata since last week were ‘local’, triggered entirely by local heating and moisture, added Bandyopadhyay. “They had very high wind speed and caused frequent lightning strikes, which are not consistent with monsoon rain. The latter is not accompanied by thunderstorms but takes the form of continuous drizzles that sometimes get heavy. Moisture incursion causes more lightning and tall cloud columns, which we saw this month. Thunderstorms that occur in April-May are intense with high wind speed but have fewer lightning strikes since moisture incursion remains low in the pre-monsoon period,” he explained.A ‘Long-Range Forecast for Southwest Monsoon’ issued by the IMD earlier this month said most regions, including east and south Bengal, could be impacted by El Nino. This will lead to maximum temperatures remaining above normal for most of June across most parts of India, including east India, while below-normal average monthly rainfall is expected during the month, it said.El Nino takes effect when the western part of the Pacific Ocean remains relatively cooler than the east. It makes wind travel downwards, which is not conducive to the formation of clouds. In contrast, the eastern Pacific, which is far away from the Bay of Bengal, experiences reverse conditions, making hot winds spiral up and leading to cloud formation.“Since the western Pacific is relatively closer to the Bay of Bengal, from where monsoon currents generate, the latter are likely to be weak during monsoon onset. This will not only reduce rain but also lead to more rainless days, pushing the temperature up, especially in June. So, we may see weak monsoon currents and infrequent rain after monsoon onset. This does not, however, mean that monsoon would be delayed. It has been progressing on schedule and could reach east India on time. But that doesn’t guarantee adequate rain due to the El Nino effect,” said a weather scientist.
