Mumbai: Despite the monsoon having arrived over Mumbai and adjoining areas on June 11- its normal onset time, the city has barely received any intense monsoon showers which are otherwise typical at this time of the year.
As of June 23, as per rainfall data available with the India Meteorological Department (IMD), the city’s rainfall deficit for the month so far stands at a significant 47%.
Since June 1, the IMD’s weather station in Santacruz — taken as representative of the city — has received 179.8mm of rain, as against the normal 342.1mm.
Experts and officials attributed these conditions to the absence of low-pressure systems in the Bay of Bengal and a ‘weak’ offshore trough in the Arabian Sea, close to the Konkan coast. When the offshore trough is strong, the moisture content in the atmosphere increases, resulting in more cloud cover and rainfall. Strong low-pressure systems in the Bay of Bengal then ‘pull’ this moisture from the sea and deposit it over the Konkan region, and play an important role in maintaining the strength of offshore troughs.
“Ideally, 12 low-pressure systems are expected to form in the four months of the southwest monsoon season, so you would get 3 systems in each month (i.e. one every 10 days or so). When these systems form, and if they are strong, they pull the monsoon winds over the Arabian Sea, which strengthens the offshore trough. Strong low-pressure systems might even travel to western India, causing direct rainfall over Mumbai-MMR. In such a situation, their presence can also trigger small vortices over the Arabian Sea near Mumbai, causing non-stop rain for a day or two that is generally heavy. We haven’t seen any low-pressure system or this action so far,” pointed out Akshay Deoras, an independent meteorologist and researcher at the University of Reading, UK, who closely tracks the southwest monsoon.
These low-pressure systems are not forming because meteorological conditions over the Bay of Bengal are not favourable for the same at present. “Our research has shown that the tropical intra-seasonal oscillations, including the Madden-Julian Oscillation (MJO), and the Boreal Summer Intraseasonal Oscillation (BSISO), modulates the formation of these low-pressure systems. But currently, there is no support by the MJO and BSISO since they are not active over the Bay of Bengal. As a result, the monsoon winds over entire Maharashtra are weak at present,” Deoras added.
MJO is a band of rain clouds that moves eastwards over the tropics and is responsible for most weather variations in the region – including the southwest and northeast monsoons. BSISO, one of the dominant phenomena over the Asian summer monsoon region, is characterised by northward/northeastward propagation over the Indian summer monsoon region.
Other experts agreed with this analysis. “There haven’t been any low-pressure systems across the Bay of Bengal this monsoon that could have favoured active monsoon conditions along the Konkan coast, particularly in Mumbai. There are three to four episodes of severe rainfall forecasted for the region between June and September, but it is difficult to predict when they will occur at the moment,” said Mahesh Palawat, vice-president (meteorology and climate change) at Skymet Weather, a private forecaster.
An official with the IMD’s regional forecasting centre in Mumbai, however, said that the city is likely to see intense downpours on June 25 and 26. “Though there is no low-pressure system in the east, there is an upper air cyclonic circulation forming over the Arabian Sea due to which several parts of the Maharashtra coast will receive intense showers. However, the intensity will rapidly reduce after June 26,” the official said.