Explainer: Why an El Niño doesn't always mean a bad monsoon for India

El Niño is commonly linked to droughts, but other factors like Indian Ocean Dipole, Equatorial Indian Ocean Oscillation and even snow cover in the Northern Hemisphere have a deep impact on the Indian monsoon too. (Photo credit: Bassant Meligy via Wikimedia Commons)

El Niño is officially here. This month, scientists at the National Oceanic and Atmospheric Administration (NOAA) announced that the ocean-warming event is now in its mature phase and could stay until 2024.

“Depending on its strength, El Niño can cause a range of impacts, such as increasing the risk of heavy rainfall and droughts in certain locations around the world,” said Michelle L'Heureux, a physical scientist at NOAA's Climate Prediction Center, in a statement.

The current El Niño is expected to be a moderate to strong event, with the potential to cause significant impacts on weather around the world. For instance, in India, El Niño events are connected with a weak summer monsoon and less rainfall. This could be devastating for the country, as it relies on the Indian summer monsoon (from May to September) for its drinking water and agricultural needs.

A NASA satellite spotted early signs of El Niño forming in March and April. Yet, in April, the IMD forecast that the country as a whole “is likely to receive 96 percent of the southwest monsoon and farmers need not worry about deficient rainfall”.

Let’s find out why.

Blow hot, blow cold

A study published in 2006 looked at rainfall trends in India over 132 years. It showed that severe droughts in India have always been in El Niño years, but the opposite is not true. This means that just the presence of an El Niño event is not enough to influence the monsoon. The study also found that its location matters. Warming in the central Pacific Ocean has a greater impact on the Indian monsoon than warming in the eastern Pacific Ocean.

There’s more.

While El Niño refers to the warming of sea surface temperatures, La Niña is the opposite condition, a cooling of sea surface temperatures (SSTs). The two phenomena occur cyclically to form the El Niño Southern Oscillation (ENSO). Research shows that ENSO is responsible for only about 30 percent of the yearly variability in India's summer monsoon rainfall.

The rest depends on factors such as IOD (Indian Ocean Dipole), EQUINOO (Equatorial Indian Ocean Oscillation) and even snow cover in the Northern Hemisphere, among others.

Indian monsoon

The IOD and EQUINOO, for instance, are climate patterns that affect the Indian Ocean. The IOD is caused by an oscillation of SSTs, while the EQUINOO refers to a see-saw in atmospheric conditions between the western and eastern parts of the Indian Ocean.

Both the IOD and EQUINOO have a deep impact on the Indian summer monsoon. Positive IOD and EQUINOO events increase moisture transport from the southeastern parts of the Indian Ocean. So they are associated with increased rainfall. They can also neutralize the effects of El Niño. The IMD has already forecast a positive IOD this year.

The monsoon is also affected by the variability of SSTs in the Atlantic Ocean and the Mascarene High. A warming of the Atlantic SST weakens the monsoon, while a cooling of the Atlantic SST strengthens it. The Mascarene High, on the other hand, is a high-pressure zone in the south Indian Ocean. It starts to develop in mid-April and is a major factor in the summer monsoon winds, which blow from the Indian Ocean towards the Indian subcontinent.

Factor this in

Aerosols, dust, and snow cover in the Northern Hemisphere can all affect the monsoon rains in India too. In a 2022 study, researchers at IIT Bhubaneswar found that dust transported from the Middle Eastern deserts to the Arabian Sea could increase rainfall in South Asia.

There is also a general inverse relationship between winter and spring snow cover extent and the southwest monsoon rainfall in the country. In other words, a lower snow cover in the Northern Hemisphere is often associated with more rain in India during the summer monsoon season. This year, as per the IMD, the amount of snow cover in the Northern Hemisphere and Eurasia during February and March 2023 was below normal.

A full understanding of all these factors is essential for better predicting the monsoon. It is also what makes the impact of El Niño on the Indian monsoon unpredictable and complex.