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The character of La Nina — an ocean-atmosphere event that usually brings down global temperatures — is changing, indicating a shift in natural climatic patterns in a warming world.
This is especially evidenced by La Nina’s strong continuation through the summer of 2022 and its involvement in the early, intense and extensive heatwaves in northwest and central India.
During a La Nina event, cooler-than-average sea surface temperatures prevail over the east and central Pacific Ocean, due to which the trade winds above the sea surface change in character because of a difference in the wind pressures.
This change in character of the trade winds is then carried all around the world affecting different regions in different ways. Many regions become colder and wetter, while many others become hotter and drier.
There are chances that the current La Nina could continue through the southwest monsoon season, winter of 2022 and even early 2023, and could lead to devastating and unpredictable consequences for India and many other regions around the world.
The only other instances of such long La Nina events were during 1998-2001 and 1973-1976. The 1973-1976 event was 37 months long and the longest since 1950, according to the National Oceanic and Atmospheric Administration (NOAA).
The longest La Nina on record was the one between 1998 and 2000 that lasted 24 months, according to a research paper published in the Journal of Meteorological Research in December 2018.
There is a 70 per cent chance that the current La Nina will continue from June-August 2022, according to the World Meteorological Organisation (WMO), and thus complete 24 months.
WMO predicted a 50-60 percent chance of it continuing from July-September. The weather agency also indicated chances of it continuing into the fall and winter seasons in the northern hemisphere but did not give specific probabilities for that happening.
On June 6, NOAA predicted that there was 58 per cent chance of La Nina for the August-October season and 61 per cent for the winter season.
More than the length of the ongoing La Nina, it is its changing character that is a cause of great concern. La Nina usually brings wet and cold winter and spring seasons for India, but this time large parts of India did not experience a spring season at all.
This happened as a north-south pressure pattern, which usually forms over India during the winter season and dissipates by spring, continued into March and April this year.
The pattern interacted with warm waves coming in from the rapidly warming Arctic region to initiate and prolong heatwaves in the country.
The characteristics of the La Nina in March-May also indicate a stronger La Nina than usual.
The month of May, for instance, was the second-strongest La Nina month on record, according to Emily Becker at the University of Miami, United States.
Becker, along with other climate experts, wrote a blog on the ENSO phenomenon for the climate.gov website of NOAA.
If the three months are taken as a season, then the temperature anomaly of the eastern and central Pacific Ocean was 1.1 degree Celsius below normal. This is the second-lowest since the 1950s when the temperature was 1.2°C below normal.
This was also only the second time since the 1950s that the La Nina became stronger (cooler) than the months of February-April.
The stubborn nature of the La Nina over the next many months can have devastating consequences for different regions of the world, including India.
India can experience a prolonged monsoon season the third year in a row, for instance. The monsoon seasons of 2020 and 2021 was also extended, leading to an increase in extreme rainfall events during the period and subsequent floods in many states. The extension had most likely happened because of the impact of La Nina, according to climate scientists.
The ongoing drought conditions in 40 per cent of the United States, the years-long drought conditions in the Horn of Africa and southern South America would also likely continue and become more intense due to the continuation of the La Nina, WMO said.
NOAA has also factored in the impact of La Nina and forecast an above-average hurricane season for the Atlantic Ocean, with a 65 per cent probability.
During a La Nina event, the vertical wind shear (change in intensity of vertical winds from near surface to higher parts of atmosphere) is low over the Atlantic Ocean and is conducive for the formation and intensification of hurricanes. A strong wind shear tears apart a hurricane from the top.
The continuation and changing character of the La Nina in an era of climate change has been highlighted by the WMO. Petteri Taalas, secretary-general of WMO, said in a press release on La Nina June 10:
Human induced climate change amplifies the impacts of naturally occurring events like La Niña and is increasingly influencing our weather patterns, in particular through more intense heat and drought and the associated risk of wildfires — as well as record-breaking deluges of rainfall and flooding.
Climate change scientists have also predicted an impact of global warming on the El Niño-Southern Oscillation (of which La Nina is a part) phenomenon itself, especially the extremes.
“Extreme El Niño and La Niña events may increase in frequency from about one every 20 years to one every 10 years by the end of the 21st century under aggressive greenhouse gas emission scenarios,” said Michael McPhaden, a senior scientist with NOAA’s Pacific Marine Environmental Laboratory, in an article on the NOAA website.
The strongest events may also become even stronger than they are today, he added.