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Bengaluru: The temperatures over the Atlantic Ocean on the other side of the world are increasingly determining the intensity of the Indian monsoon, and it’s probably all because of the global warming crisis.

According to a study by researchers from the Centre for Prototype Climate Modelling at New York University Abu Dhabi (NYUAD), climate scientists need to begin factoring in Atlantic Ocean temperature variations if they hope to make accurate predictions about the Indian monsoon, which dictates the livelihood of lakhs of farmers and is known to have a big impact on the economy, in a warming world.

The study was published in the journal Geophysical Research Letters last month.

“We found that in recent decades (post-1975) equatorial Atlantic sea surface temperature has a seminal role in determining the strength of monsoon rainfall over central India,” the study’s authors, Dr R.S. Ajayamohan and Dr C.T. Sabeerali, told ThePrint in an emailed statement.

“The strengthening relationship is due to the increase in sea surface temperature (SST) over the equatorial Atlantic Ocean as part of global warming.”

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The Atlantic influence

India receives 80 per cent of its annual rainfall during the southwest monsoon, aka summer monsoon, between the months of June and September, though the intensity varies from year to year.

Several factors lie behind this shifting strength, such as El Niño Southern Oscillation (ENSO), where sea surface temperatures in the tropical Pacific Ocean rise to high levels for an extended period of time and then fall for long periods, and the Indian Ocean Dipole, where the Arabian Sea becomes alternately warmer and cooler than the Bay of Bengal.

Another factor is temperature variability over the surface of the Atlantic Ocean, a phenomenon known as the Atlantic Zonal Mode (AZM) or Atlantic Niño (“Niño” is Spanish for “boy”).

AZM and the Indian summer monsoon have been known to have an inverse relationship: When AZM is in a cold phase with cold temperatures, the Indian monsoon is strong, while a warmer AZM phase has resulted in weaker rains.

This physical connection between opposite sides of the world is called ‘telecommunication’.

AZM produces what is called equatorial Kelvin waves, which are a large-volume motion of water that affect the atmosphere and ocean. They propagate along the Equator towards the east and push winds and water inwards, into India, affecting the southwest monsoon.

“This alters the north-south gradient of upper-level temperature over monsoon domain and hence affect the strength of the monsoon,” the researchers noted in the email.

Future of monsoon modeling

It is now well-known that the effect of ENSO on ISMR has been decreasing for a while.

The NYUAD study shows that the inverse connection between AZM and ISMR has been steadily increasing to the point where it needs to be factored into existing climate models for the Indian monsoon.

“In order to further improve the forecast of Indian monsoon the model should correct the monsoon and equatorial AZM represented in model,” the researchers wrote in the email.

This, the authors added, was something they had suggested in an August 2018 study as well.

“We have reported in a previous paper… that if AZM variability is not simulated properly by a model, it will lead to a loss in the prediction skill of monsoon forecast,” they said to ThePrint.

It is as yet unclear if the effect of AZM on Indian summer monsoon has been increasing because of the decreasing influence of ENSO. The authors said further research was needed to determine this.

Also read: Indian summer monsoon amplified global warming 130,000 years ago

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