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An El Nino event in the equatorial Pacific Ocean may be developing sooner than expected as many climate models have forecasted an El Nino as soon as in May 2023. The development of an El Nino in May or June may cause a possible weakening of the southwest monsoon season, which brings around 70 per cent of the total rainfall India receives and on which most of its farmers still depend.
It could also cause heatwaves and droughts in India and other regions around the world such as South Africa, Australia, Indonesia and the Pacific Islands.
It brings heavy rainfall and flooding to other regions such as California in the United States and could cause bleaching and death of coral reefs.
The El Nino is the warmer-than-normal phase of the El Nino Southern Oscillation (ENSO) phenomenon, during which there are generally warmer temperatures and less rainfall than normal in many regions of the world, including India.
During an El Nino event, the sea surface temperatures (SST) in the equatorial Pacific Ocean off the northern coast of South America become at least 0.5 degrees Celsius warmer than the long-term average (1980-2010).
In the case of a strong El Nino event as occurred in 2015-2016, anomalies can reach as high as 3°C, which is a record.
This unusual warming of the sea surface weakens the trade winds flowing above and could even cause their reversal. This decreases the amount of rainfall that these trade winds carry to most places around the world and heightens temperatures in general.
In the 2015-2016, there were widespread heatwaves in India that killed around 2,500 people in each of the years. Coral reefs around the world also suffered from bleaching and the sea level rose by 7 millimetres due to thermal expansion.
The El Nino, along with global warming, had made 2016 the warmest year on record. The El Nino in 2023 and going into 2024 may push the global average temperature towards 1.5 degree celsius warmer than the pre industrial average.
The warming of the oceans is one of the major impacts of an El Nino event. This is when ocean heat content is already at a record high, according to the World Meteorological Organization (WMO).
Many meteorological agencies, including the India Meteorological Department (IMD), track the evolution of the El Nino and incorporate it into their climate and weather models, forecasts and advisories. The IMD will take into account the possibility of an El Nino in its first long-range monsoon forecast it will issue around mid April.
Most of the models analysed by the World Climate Service, a private weather forecasting firm based in the United States, predicted SSTs in the range of 0.7-0.95°C above the long-term average between 1993-2016 for May 2023.
Forecasters and climate scientists were unsure of the development of an El Nino till now because of the spring predictability barrier, which is the weakening of the interaction between the ocean and the atmosphere during the northern hemisphere spring season. This makes it difficult for climate models to pick up the evidence for the development of an El Nino or a La Niña, which is the cooler-than-normal phase of ENSO.
A record three-year La Niña event ended in March 2023 and currently, the equatorial Pacific Ocean is at normal temperatures, known as the neutral phase.
“We are yet to come out of the spring predictability barrier, but that doesn't mean dynamical models are doing a bad job at predicting ENSO,” Akshay Deoras, research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, UK, told Down To Earth.
"The El Nino signal has been consistently strong in some models since early 2023, and its reason is now becoming clear. The equatorial Pacific continues to warm and the sea-surface temperature in its eastern part (off the coast of Peru) is well above the normal,” Deoras explained.
Such conditions are generally observed before the initiation of an El Nino event. Besides, there is a lot of heat available below the sea surface, which will continue to push the sea-surface temperature of the equatorial Pacific well above the El Nino threshold this summer, according to Deoras.
“A proper El Nino condition would occur only when the atmosphere responds to this change in the sea-surface temperature. Trade winds over the tropical Pacific must show a sustained weakening or a reversal in their direction and some models suggest this might happen as early as May, but we are yet to see a clear signal in other models,” said Deoras.
This suggests that whilst an El Nino event is extremely likely this summer, models are yet to agree on its magnitude and onset period, Deoras added.
“The El Nino dice is loaded this year because of the warm water that has accumulated during the three years of La Niña. So, the pacific is fully pregnant with the warm water that is needed to deliver an El Nino,” said Raghu Murtugudde, a climate scientist at the University of Maryland and the Indian Institute of Technology, Bombay.
“The side is likely to throw up an El Nino this year. El Nino peaks during Dec-Jan-Feb. So, the emergence can be weak or strong in the summer, depending on whether it will be a strong El Nino or a weak one,” Murtugudde added.
The southwest monsoon, which starts getting set up in the Bay of Bengal in May and makes an onset over Kerala in early June, is likely to get impacted by the El Nino.
“If the monsoon circulation is not affected much by other teleconnections, El Nino would weaken it, suppressing the summer monsoon season rainfall in India. Dry spells in such years become much more pronounced,” said Deoras.
Typically, the rainfall suppression is more in the second half of the season as the El Nino effect builds up, the expert noted. “However, it doesn’t mean that all four months of the season would be affected. Sub-seasonal factors such as the Madden-Julian Oscillation and monsoon low-pressure systems can temporarily enhance rainfall in some parts as we saw in June 2015.”
It is too early to determine how El Nino would affect the monsoon’s onset in Kerala and its further progression across India, Deoras concluded.