Last month, the India Meteorological Department (IMD) forecast above-normal rain in the upcoming monsoon season in India, with “favourable” La Nina conditions expected to set in by August-September.
What are El Niño and La Nina?
Thus, climatologically, El Niño and La Nina are opposite phases of what is collectively called the El Niño Southern Oscillation (ENSO) cycle.
How could the incoming La Nina impact global weather?
Thereafter, La Nina conditions are expected to emerge and its impact could be effective from August.
Last month, the India Meteorological Department (IMD) forecast above-normal rain in the upcoming monsoon season in India, with “favourable” La Nina conditions expected to set in by August-September. Another recent bulletin from the IMD noted how the strength of El Niño conditions has weakened since the beginning of this year.
Why does the withdrawal and onset of conditions related to El Niño or La Nina matter? How do they impact different parts of the world? We explain.
What are El Niño and La Nina?
El Niño (meaning “little boy” in Spanish) and La Nina (meaning “little girl” in Spanish) are climate phenomena that are a result of ocean-atmosphere interactions, which impact the temperature of waters in the central and eastern tropical Pacific Ocean. These affect global weather.
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The Earth’s east-west rotation causes all winds blowing between 30 degrees to the north and south of the equator to slant in their trajectory. As a result, winds in the region flow towards a southwesterly direction in the northern hemisphere and a northwesterly direction in the southern hemisphere. This is known as the Coriolis Effect.
Due to this, winds in this belt (called trade winds) blow westwards on either side of the equator. Under normal ocean conditions, these trade winds travel westwards along the equator from South America towards Asia. Wind movement over the ocean results in a phenomenon called upwelling, where cold water beneath the ocean surface rises and displaces the warm surface waters.
At times, the weak trade winds get pushed back towards South America and there is no upwelling. Thus, warmer-than-usual sea surface temperatures are recorded along the equatorial Pacific Ocean, and this is known as the emergence of El Niño conditions.
A schematic diagram showing water and wind movements during El Nino and La Nina phases. (Image Credit: NOAA, US) A schematic diagram showing water and wind movements during El Nino and La Nina phases. (Image Credit: NOAA, US)
Conversely, during La Nina, strong trade winds push warm water towards Asia. Greater upwelling gives rise to cold and nutrient-rich water towards South America. Thus, climatologically, El Niño and La Nina are opposite phases of what is collectively called the El Niño Southern Oscillation (ENSO) cycle. It also includes a third neutral phase.
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El Niño events are far more frequent than La Nina ones. Once every two to seven years, neutral ENSO conditions get interrupted by either El Niño or La Nina. Recently, La Nina conditions prevailed between 2020 and 2023.
How could the incoming La Nina impact global weather?
Thanks to ENSO and the warming or cooling of ocean waters, the air circulation loop in this region also gets affected. That, in turn, affects the amount of precipitation over the neighbouring regions, and impacts the Indian monsoon as well.
The ongoing event of El Niño, which began in June last year, has weakened significantly. By June, neutral ENSO conditions are likely to be established. Thereafter, La Nina conditions are expected to emerge and its impact could be effective from August.
La Nina’s impact on India
With above normal rain forecast, the seasonal rainfall is expected to be 106 percent of the Long Period Average (LPA), which is 880mm (1971-2020 average).
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Except in east and northeast India, all remaining regions are expected to receive normal or above seasonal rainfall, the IMD said in April. Heavy rains could result in some regions witnessing riverine and urban flooding, mudslides, landslides and cloudbursts.
East and northeast India region, during La Nina years, receive below average seasonal rainfall. Therefore, there may be a shortfall in water reserves there this year. During La Nina years, incidents of thunderstorms generally increase.
“The east and northern India regions could experience thunderstorms accompanied by lightning. With increased farming activities undertaken during July and August rainy months, which coincides with the season’s enhanced lightning and thunderstorms, there is a high risk of fatalities over these regions,” said Mrutyunjay Mohapatra, Director General, IMD.
In addition to ENSO, there are other parameters which can impact the monsoon. “However, in a La Nina year, a deficit monsoon over India can be easily ruled out,” M Rajeevan, former secretary, Ministry of Earth Sciences, told The Indian Express.
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La Nina’s impact on the world
Similar to India, Indonesia, the Philippines, Malaysia and their neighbouring countries receive good rainfall during a La Nina year. This year, Indonesia has already witnessed floods.
On the other hand, droughts are common in southern regions of North America, where winters become warmer than usual. Canada and the northwestern coast of the United States see heavy rainfall and flooding. Southern Africa receives higher than usual rainfall, whereas eastern regions of the continent suffer below-average rainfall.
ENSO has a huge impact on hurricane activity over the Atlantic Ocean. During a La Nina year, the hurricane activity here increases. For instance, the Atlantic Ocean churned out a record 30 hurricanes during the La Nina year 2021.
Is climate change affecting ENSO?
Over India, El Niño is known to suppress the southwest monsoon rainfall and drive higher temperatures and intense heat waves, like the present summer season.
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In the past, monsoon seasons during years following an El Niño were 1982-1983 and 1987-1988, with both 1983 and 1988 recording bountiful rainfall. At present too, a similar situation could play out.
The 2020-2023 period witnessed the longest La Nina event of the century. Thereafter, ENSO neutral conditions developed, which soon gave way to El Niño by June 2023 that has been weakening since December last year. According to Rajeevan, “This swift shift to La Nina is a natural process. It has happened many years in the past.”
Scientists say that climate change is set to impact the ENSO cycle. Many studies suggest that global warming tends to change the mean oceanic conditions over the Pacific Ocean and trigger more El Niño events.
The World Meteorological Organization (WMO) has also said that climate change is likely to affect the intensity and frequency of extreme weather and climate events linked to El Niño and La Nina.
