Science & Technology

Did you know Indian and West African monsoons are linked?

A report by an IISc scientist says that El Nino oscillates between both regions and affects the intensity of their rainfall

By Akshit Sangomla
Published: Thursday 31 January 2019
Image: Getty Images Image: Getty Images

We all knew that monsoon in both India and West Africa is intimately linked to the regions’ agriculture economies, which is mostly either rain fed or irrigated by rivers that too depend on rains to maintain flow. But now scientists have found that rains in these regions might also be linked to each other through the El Nino Southern Oscillation (ENSO) phenomenon which affects both of them.  

A recent research paper published in the journal Climate Dynamics says ENSO’s relationship with Indian Summer Monsoon (ISM) has shifted towards the West African Summer Monsoon (WASM). For example, between 1911 and 1930 while the influence of ENSO on ISM was weak, the influence of ENSO on WASM was strong. It also says that this shift is part of a multi decadal cycle, which weakens ENSO’s relationship with ISM while strengthening its relationship with the WASM and vice versa.

And, between 1931 and 1980 these influences reversed in character. Then they flipped again between 1981 and 2015. The reason behind this see-saw relationship is a shift in one half of ENSO’s temperature gradient (cooling part) from the north west of the Indian Monsoon region to the northern part of West Africa.

But there is a catch. Current models do not agree and correctly predict that if this is going to shift again or how much time it will take to do so in a world reeling under climate change. The physics of Earth’s climate, especially regarding a large-scale phenomenon like El Nino, is too complicated to be simulated by computational models even ones that are currently using artificial intelligence. But, this might improve with better dynamic climate models which are closer to how clouds and turbulence actually behave in our atmosphere.

The warm phase of the ENSO or the El Nino refers to the unusual warming of the central and east-central equatorial Pacific Ocean which affects global weather including India and West Africa. The other half of this oscillation is formed by the cooling of the north western Indian monsoon region or the northern part of West Africa.

During El Nino, the warmer waters of the Pacific Ocean cause the winds in various regions, like the trade winds that come towards India, to reverse. This change of wind direction can lead to warmer winters and summers and a decrease in rainfall during the monsoon. Sometimes it also leads to drought.

In the cooling phase of the ENSO, known as the La Nina, the exact opposite of these events happen which might lead to increased rainfall inducing floods, flash floods and landslides.

In fact, the paper states, “During 1901-2015, the ISM recorded 22 droughts, of which nine were associated with El Niño and of the 18 floods recorded in that period, nine were associated with La Niña. On the other hand, for WASM, out of 21 droughts, nine were associated with El Niño, while of the 22 floods, seven were caused by La Niña, implying that 45 per cent of the ISM and 39 per cent of the WASM rainfall extremes are associated with ENSO.”

But how will this swinging of El Nino’s relationship affect the monsoon rainfall in India and West Africa? This and many such questions asked by Down To Earth were answered by Gaurav Srivastava, lead author of the research paper and climate change researcher at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science. Here are excerpts from the interview:

How is ENSO's relationship with the Indian monsoon going to shape up in the near future?

Coupled Model Intercomparison Project (CMIP5) models, which simulate short- and long-term impacts of climate change and are run under realistic anthropogenic emission of CO2 and other green house gases (GHGs) simulate a weakening of ENSO-ISM relationship. But to believe these results is not a good idea. There is an observed hiatus in global warming in the early 21st century. Also, the atmospheric models are yet not able to capture exactly either ENSO or Indian Monsoon, so it is really difficult to say that how would the ENSO-Indian Monsoon Relationship change in near future.

If the influence of the ENSO on the Indian monsoon decreases in the future then what other factors will come into play and how will this affect the intensity and distribution of the monsoon rainfall in the region?

If the impact of ENSO on Indian monsoon decreases, we can expect lesser frequency of inter-annual extremes (drought and flood). There are various local and remote phenomena which act together with ENSO and play significant role in modulating the Indian summer monsoon. But there is no analysis as yet which shows the phase coherence of ENSO with any other atmospheric phenomenon can significantly increase/decrease the monsoon.

This is really difficult to say what all factors might take over when the impact of ENSO weakens. But to me, ENSO is the largest source of inter-annual variability of Indian monsoon and nothing else can replace its impact, or can be as much powerful.

Are there examples of severe impact of ENSO on the West African monsoon?

The mega drought in the Sahel region of West Africa happened during 1970-1990 and killed almost a lakh people and displaced millions. There are studies which attribute the three-decade-long drought to changes in the ENSO dynamics. Nothing much can be said without a proper analysis but ENSO, for sure, has affected the monsoon over Africa.

If the influence of the ENSO on West Africa monsoon increases in the future, what other factors will come into play and how will this affect the intensity and distribution of rainfall?

There could be several local and remote phenomena which might become a key player in modulating the West African rainfall in that case. The largest one, to me, would be the Atlantic Nino and North Atlantic Oscillation. But again, I would say that the impact of ENSO is the strongest and distinct and not so easily replaceable.

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