The National Oceanic and Atmospheric Administration (NOAA) of the United States recently announced the development of a weak El Niño in the equatorial Pacific Ocean that was expected to continue for a few months at least. The status of El Niño at this time of the year is usually the first indication of the kind of rainfall that is to be expected during the monsoon season later in the year.
El Niño is a phenomenon in which surface temperatures in the equatorial Pacific Ocean see an unusual rise. Over the years, it has been found to have a strong bearing on monsoon rainfall in India. While warmer temperatures are known to suppress monsoon rainfall, the opposite phenomenon of La Niña has been found to be helpful in bringing good rainfall.
In its announcement on February 14, NOAA said weak El Niño conditions had already built up in January and were likely to continue (with 55% probability) until the spring season in the northern hemisphere (mid-March to mid-June). It said that the probability of El Niño persisting into the summer was “50 per cent or less”. The Australian Bureau of Meteorology, meanwhile, said in a February 19 bulletin that the development of El Niño could continue until at least July.
More relevant to the Indian monsoon, the warming in the Niño 3.4 region of the Pacific Ocean, the region whose sea surface temperature is seen as be the best marker for the impact on India’s rainfall, has been forecast to remain in excess of 0.5°C above normal.
The likely impact
Dr Arindam Chakraborty of the Centre for Atmospheric and Oceanic Sciences at IISc Bangalore said data for the last 100 years showed that if the SST in Niño 3.4 was over 0.5°C above normal in the four-month monsoon season, rainfall over India gets affected.
“If El Niño 3.4 is greater than 0.5, it is likely to decrease rainfall. But we need to wait for a better prediction at this point because prediction through the northern spring season has higher degree of uncertainty,” he said. He also pointed to other evidence to suggest that the impact on the Indian monsoon might not be very large. “Even if we get El Niño in the monsoon months, its impact, statistically speaking, is not as high as when it is preceded by a La Niña in the winter. In this winter, sea surface temperatures were above normal, almost close to El Niño,” he said.
However, Raghu Murtugudde of University of Maryland, College Park, said there was a possibility that the El Niño could strengthen beyond spring. “I have myself announced on some groups that I expect the El Niño to grow into the summer which could mean that we may have a drought (in India). Some weather events like winds over the western tropical Pacific can finally determine whether El Niño will grow beyond spring,” he said.
El Niño events repeat themselves in a two- to seven-year cycle, with a strong El Niño expected every 10-15 years. However, since 2000, five El Niño events have already happened, and this year could witness a sixth one.
New scientific research is pointing to increased frequency of extreme El Niños due to climate change. A paper published in Nature Climate Change in July 2017 had suggested that such extreme events could happen twice as often as today if the average annual global temperatures reached 1.5°C above pre-industrial times.
Murtuggude, however, said that the increasing frequency could be because of other reasons as well. “This cannot yet be claimed to be in response to global warming with great confidence. It is related with the fact that trade winds got stronger and the eastern equatorial Pacific Ocean has remained colder since 1998. That makes El Niño more active. The stronger trade winds are not easily explained by global warming. So the story is much more complicated,” he said.