Monsoon Is A ‘Monstrous’ Heat Source, El Niño Following La Niña Can Result In Rain Deficit: Expert
A climate expert said that an analysis of past monsoons has shown that an El Niño that follows a La Niña year tends to be the worst-case scenario in terms of monsoon deficits.
The Indian Meteorological Department (IMD) has predicted a "normal monsoon" for India in 2023. The IMD's announcement came a day after Skymet Weather Services said that India will witness a "below normal" monsoon season this year, due to the El Niño effect.
According to the IMD, the southwest monsoon seasonal rainfall in India is likely to be 96 per cent of the Long Average Period (LPA) of the seasonal rainfall over India, which is 87 centimetres for the 50-year-period from 1971 to 2020.
The southwest monsoon season in India is from June to September. When the rainfall range is 96 to 104 per cent of the LPA, it is considered to be normal, according to the IMD.
The El Niño effect is expected to impact India this year, and come after a three-year-long La Niña effect.
El Niño year following a La Niña usually results in monsoon deficits
A climate expert has said that an analysis of past monsoons has shown that an El Niño that follows a La Niña year tends to be the worst-case scenario in terms of monsoon deficits.
“We have just emerged from a record three-year La Niña and are likely heading into an El Niño. If the forecasts say the monsoon will still be normal, we need to wait and see what compensating factors may play a role,” said Raghu Murtugudde, Visiting Professor, Earth System Scientist at IIT Bombay and Emeritus Professor at University of Maryland.
What is La Niña?
The La Niña effect is believed to be one of the reasons behind the changes in seasonal weather patterns of India. The periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific is referred to as the La Niña effect. While the La Niña effect occurs every three to five years, occasionally, it can take place over successive years.
La Niña means “Little Girl” in Spanish, and is sometimes also called El Viejo, or simply a “cold event”.
According to the National Oceanic and Atmospheric Administration (NOAA), La Niña is the opposite of El Niño, the event in which warm water is pushed towards the west coast of the Americas.
Meanwhile, during La Niña events, trade winds, which are winds blowing steadily towards the equator from the north-east in the Northern Hemisphere or the south-east in the Southern Hemisphere, become stronger than usual. As a result, warm water is pushed toward Asia.
Off the west coast of the Americas, cold, nutrient-rich water is brought to the surface.
The La Niña event lasted till February 2023. According to the World Meteorological Organization (WMO), this was the first “triple-dip” La Niña of the 21st century, which means that the La Niña effect had been occurring for three consecutive years.
The triple-dip La Niña affected temperature and precipitation patterns in different parts of the world. While La Niña is associated with cooler than normal winters in the Northern Hemisphere, it increased global temperatures in 2022 because the effect has been taking place against a background of human-induced climate change. This made India’s weather more extreme, and affected seasonal weather patterns.
According to the WMO, more intense and longer monsoon rainfall in southeast Asia is associated with La Niña.
Widespread warmer than average sea surface temperatures in regions other than the equatorial central and East Pacific are believed to be responsible for the above-normal temperatures along the Arctic coast of Asia.
Snowfall accumulation over Eurasia during La Niña years
According to Dr Murtugudde, the most likely candidate playing a compensating role to the El Niño effect is the accumulation of snowfall over Eurasia during the La Niña years. He also said that seasonal rainfall totals have very little meaning now because during any monsoon season, erratic distributions of rainfall with extreme wet and dry spells are expected over different regions of the country.
“One also needs to keep in mind that seasonal rainfall totals now have very little meaning considering the erratic distributions with extreme wet and dry spells expected in any monsoon year - normal, deficit or excess. Floods, droughts, crop damages, and health impacts need to be dealt with as effectively as possible with short (days 1-3), medium (days 3-10) and extended (weeks 2-4) range forecasts. All states down to municipalities and panchayats have to pay attention to IMD warnings,” added Dr Murtugudde.
Eurasian precipitation being slightly below normal may offset El Niño impact
According to Dr Murtugudde, the Eurasian precipitation has been slightly below normal despite the triple-dip La Niña effect. He explained that this would favour a stronger monsoon in India and may offset the El Niño impact.
Dr Murtugudde said it is unclear if this is the reason behind the IMD’s forecast of “normal monsoon” in India.
What is the Indian Ocean Dipole?
Other factors influencing India's monsoon season include the Indian Ocean Dipole (IOD). The IOD is defined as sea surface temperature differences between two poles or dipoles: a western pole in the Arabian Sea, and an eastern pole in the eastern Indian Ocean south of Indonesia. The western pole is located in the western Indian Ocean. Due to the change in temperature gradients across the Indian Ocean, changes occur in the ascent and descent of moisture and air in certain regions.
When the IOD is sufficiently strong, it can negate the effects of El Niño, and steer the monsoon season in India. According to Skymet Weather Services, El Niño and IOD are likely to be 'out of phase' this year, because of which there could be extreme variability in the monthly rainfall distribution. The second half of the monsoon season is likely to be more abnormal.
Can a positive IOD offset the effects of El Niño this year?
Explaining if a positive IOD can offset the effects of El Niño, Dr Murtugudde said the prediction skills for an IOD remain low, and most importantly, the impact of IOD on monsoon is not very robust. He said that it is likely that the monsoon itself impacts the IOD.
Dr Murtugudde explained that since monsoon is a “monstrous” heat source, and IOD mostly occurs after the monsoon has nearly ended, it is important to focus on late season extremes in dry and wet spells.
He also said that planetary waves driven by Arctic warming and Arctic Sea ice anomalies are likely to result in late season anomalies.
“Such late season anomalies tend to come from outside the tropics as well, because of the planetary waves driven by Arctic warming and Arctic Sea ice anomalies. It is thus critical to understand what is driving the near-normal monsoon forecasts. As usual, it is important to hope for the best and prepare for the worst in terms of food, water, energy, health, transportation and other sectors,” said Dr Murtugudde.
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