Sea-surface temperatures over the East-Central Pacific may have cooled to the lowest level since 2017-18, the US National Oceanic and Atmospheric Administration (NOAA) said, in emerging evidence that it is gradually slipping into a La Nina event that is helpful for the Indian monsoon.
La Nina is the alter ego of the monsoon-killer El Nino phenomenon, a warming event in the same ocean.
La Nina or not, the monsoon has continued to hold strong over West, Central and East India on Tuesday, not least helped by a Madden-Julian Oscillation (MJO) wave over the West Indian Ocean (and adjoining South Arabian Sea), an analysis by the Australian Bureau of Meteorology (BoM) said.
MJO wave helps monsoon
The MJO wave travels periodically over the Indian Ocean from West to East and influences the weather pattern there as well as the Pacific Ocean to the East. It carries clouds, moisture and rain presides over the onset of regional monsoons and initiation of low-pressure areas, depressions and even cyclones.
The BoM said that monsoon conditions are currently prevalent across a broad region of Asia, extending from the Indian subcontinent to East China. A monsoon trough extended from North India across the North Bay of Bengal and reaching as far East-North-East as mainland China.
Watch for low retained
As a result, rainfall over parts of Central India and South China (which received tropical storm Nuri recently) has been well above the long-term average during the past week. The monsoon is progressing northwards through India at close to the long-term average rate, the BoM analysis said.
This expansive trough will now spawn a new low-pressure area over Bay in the next 3-4 days (around June 19), the India Meteorological Department (IMD) said in its update. This is expected to form over the North Bay, the ideal location for it to preside over the next productive phase of the monsoon.
Constellation of systems
On Tuesday, however, a remnant circulation from a predecessor low from the Bay lain anchored over South-East Uttar Pradesh, holding in place a larger monsoon shear zone higher up in the atmosphere, the presence of which itself is a surefire indicator of the strength of the overall monsoon system.
The shear zone is packed with opposing winds at heights of 3-6 km in the atmosphere, and represents the level at which the monsoon is most active. The shear zone also serves as the platform for rain-bearing systems (low-pressure areas/ depressions) to play around in.
Heavy rain forecast
Combined with the land-based trough from North-West Rajasthan extending to the plains of West Bengal (and further to mainland China), they would continue to cause fairly widespread to widespread rainfall over Maharashtra, Chhattisgarh, West Bengal, Sikkim, Odisha, Jharkhand and Bihar during the next two days.
Scattered heavy to very heavy falls and isolated extremely heavy falls are likely over Konkan & Goa, while it would be isolated heavy to very heavy falls over Interior Maharashtra, hills of West Bengal, Sikkim and Odisha. It would be isolated heavy over Central India, Bihar, Jharkhand and plains of West Bengal.
Widespread rainfall with isolated heavy falls is likely to continue over North-East India, while it would be scattered to fairly widespread with isolated heavy to very heavy falls over Gujarat during next four days. The brewing low in the Bay will trigger a fresh wave of rainfall for East and Central India.