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In what may be termed as an uncommon ocean phenomenon, the prevailing La Ninã conditions over the equatorial Pacific Ocean have entered the third consecutive year. The current La Ninã phase has been prevailing since September 2020.
Since the 1950s, La Ninã lasting for more than two years has been recorded only on six instances (see graph below), data by the India Meteorological Department (IMD) stated.
By mid-August, meteorologists at Australia’s Bureau of Meteorology (BoM), the US’s National Oceanic and Atmospheric Administration (NOAA), and India’s Monsoon Mission Climate Forecast System (MMCFS) confirmed that La Ninã conditions are here to stay till the end of 2022.
This was in contrast to the forecasts issued by these multiple agencies in April this year, which had said La Ninã would dissipate by August and conditions would turn neutral thereafter.
From remaining at the La Niña ‘watch’ status, two of BoM’s latest Climate Driver bulletins issued this month upgraded it to La Niña ‘alert’ status.
In June this year, the World Meteorological Organization had termed the persisting La Niña conditions as ‘stubborn’.
The El Niño Southern Oscillation (ENSO) is one of the key climate drivers for which the sea surface temperatures (SST) along the central and equatorial Pacific Ocean are constantly observed. It is important, as ENSO conditions can alter both the temperatures and rainfall globally, due to its strong interference on the global atmospheric circulations. It has three phases — El Niño, neutral and La Ninã.
El Niño is when the SSTs along the central and equatorial Pacific Ocean are abnormally warmer than normal. La Niña, on the other hand, is when cooler SSTs prevail over these regions.
In the Indian context, El Niño years have seen below normal monsoon rainfall and caused extreme heat, even though it may not be the single factor or have direct relations. In 2014, India received 12 per cent deficient rainfall during June to September.
La Niña years, on the other hand, are known to favour the Indian summer monsoon.
This year, India has received 740.3 mm of rainfall, which was quantitatively 7 per cent above the seasonal average till August 30. Out of the 36 states/Union Territories, 30 have received rainfall categorised as either ‘normal’, ‘excess’ or ‘large excess’.
Uttar Pradesh and Manipur (-44 per cent), and Bihar (-39 per cent) remain the worst affected states this season.
“The continuing La Niña is a good sign for the Indian monsoon. The monsoon rainfall, so far, has been good except Uttar Pradesh, Bihar and neighbouring areas,” M Rajeevan, former secretary, Ministry of Earth Sciences (MoES), told The Indian Express.
Roxy Mathew Koll, senior scientist at Pune-based Indian Institute of Tropical Meteorology (IITM), too, shared that La Niña could be one of the reasons for India receiving good spells of rainfall during the past three months. “Without La Niña, the monsoon currents could have been weaker,” Koll said.
Rajeevan termed the continuing La Niña as “abnormal”, and said, “It is surprising that it has continued for the last three years. It may be good for India but not for some other countries.”
The former MoES secretary also noted that “under climate change conditions, one must expect more such instances”. Climate change could be a factor driving such anomalous conditions. El Niño is also often also associated with heatwaves and extreme temperatures, as seen recently in parts of the US, China and Europe.
During past events of La Niña, India’s Northeast monsoon rainfall remained subdued. But the 2021 season remains an exception in recent years, pointed out Rajeevan.
Between October and December 2021, the southern Indian peninsular recorded a whopping 171 per cent surplus – the wettest winter monsoon ever recorded since 1901, data from the IMD stated.
However, the factors that are contributing in keeping the equatorial and central Pacific Ocean cooler, that too since September 2020, remain fully unclear. As SST values dipped to the lowest twice during the last two years, scientists are identifying the 2020 – 2021-2022 as the ‘double-dip’ La Niña. In this ongoing phase, the lowest SSTs were recorded around October-November in 2020 and April-May of 2021, the IMD data said.
La Niña years are infamous for frequent and intense hurricanes and cyclones in the Atlantic Ocean and the Bay of Bengal.
Over the North Indian Ocean too – covering the Arabian Sea and the Indian Ocean – chances of more cyclones are likely due to multiple aiding factors, including high relative moisture and relative low wind shear over the Bay of Bengal.
“La Niña favours a higher rate of cyclone formation in the Bay of Bengal,” Koll said.
Earlier this month, NOAA warned of higher than usual hurricanes over the Atlantic Ocean due to favourable atmospheric and ocean conditions. While August passed by without any hurricanes, NOAA does not rule out major hurricanes as the peak season of September and October approaches.
Over the North Indian Ocean, the post-monsoon months October to December are active months for cyclone development with November being the peak for cyclonic activity.
