The Atlantic Nino effects

Synopsis: Monsoon predictions are a monumental challenge, especially when it comes to the spatial distribution and the northward migration of the monsoon trough.

Introduction

Recently, farmers from Madhya Pradesh threatened to take IMD to court for the inaccurate monsoon forecast this year. A question was also raised in Parliament about whether the Arctic warming had led to an erratic monsoon this year.

What is the issue?

Rainfall deficit: The onset of monsoon 2021 began on June 3, almost on time but subsequently, rainfall deficit of up to 30 per cent were seen in Kerala, Gujarat, Jammu and Kashmir, the Northeast and Odisha. The rest of the country is barely normal with deficit being less than 20 per cent.

El Niño in the Pacific: Instead, a return of the La Niña is forecasted by most models for later this year. Considering that 2020 was also a la Niña year, one would expect monsoon 2021 to be above normal.

Arctic affect: The Arctic can affect late-season rainfall and September has seen slightly above normal rain across India.

What is Atlantic Nino?

It is El Niño’s little cousin in the Atlantic, known as the Atlantic Niño, or the Atlantic Zonal Mode. Every few years, from June to August, there is a warming in the eastern equatorial Atlantic, which does not get as much attention as El Niño.

How Atlantic Nino was the cause for rainfall deficit in India?

Sea surface temperature: In 2021, due to Atlantic Niño, the Sea surface temperatures in the eastern Atlantic have remained more than a degree higher than normal this summer.

Number of low-pressure systems: Atlantic Niño’s impact on the monsoon has been known since 2014 when a study led by INCOIS showed that the number of low-pressure systems is greatly reduced by the Atlantic Niño, leading to deficit monsoons.

This year has seen a sharply lower number of low-pressure systems, which contribute up to 60 per cent of the seasonal total rainfall over the core monsoon zone.

The Atlantic and Indian Oceans are not directly connected in the tropics via the ocean: The Atlantic Niño affects the monsoon by producing atmospheric waves, which propagate into the Indian Ocean.

These waves affect air temperatures over the Indian Ocean and influence the land-ocean thermal contrast as well as Low Pressure Systems (LPSs). The biggest rainfall deficits from the Atlantic Niño tend to occur over the Western Ghats and the core monsoon zone. The deficit patterns are a sign of the Atlantic Niño’s influence.

Why monsoon prediction is a challenge in India?

Forecast models rely heavily on El Niño: But only about 50 per cent of the dry years are explained by El Niño. How can monsoons be predicted during non-El Niño years? Clearly, Atlantic Niño is a significant player in monsoon evolution and models and forecasters must pay attention to this Atlantic teleconnection.

Low-pressure systems or LPSs originate in the northern Bay of Bengal and are three-10 times more in number during the active period of the monsoon.

What is the way forward?

First, many of the Atlantic Niños occur during non-El Niño years and this offers a window of opportunity to increase forecast skills based on the accurate prediction of the Atlantic Niño. Indian scientists from INCOIS have argued that the Atlantic Niño is in fact predictable up to three months in advance.

Second, climate scientists are also aware of the monsoon prediction challenge and hence they should continue to try to improve monsoon forecasts.

Source: This post is based on the article “The Atlantic Nino effect” published in Indian Express on 29th Sep 2021.