‘The West African monsoon always had the potential of impacting the Indian monsoon’

The Indian Summer Monsoon (ISM) and the West African Monsoon (WAM) are of immense significance for around 1.7 billion people of India, northern and central Africa.

But both systems are undergoing adverse changes due to global warming. So much so that both these systems have been included under global climate tipping points.

Global tipping points are thresholds in large planetary systems that once crossed, would lead to irreversible changes and disruption of one climate tipping point could lead to disruption of other climate tipping points as well, forming a cascade.

In fact, new research suggests that the ISM and WAM are intricately related to one another and changes happening in the WAM can have immense impacts on the ISM.

Down To Earth spoke to Francesco SR Pausata, professor of climate science at the University of Quebec, Montreal, Canada, who is a well-known expert on the West African Monsoon. Edited excerpts:

Akshit Sangomla: Has the WAM always affected the ISM system or is it happening now due to climate change? If it is a new effect, why is it taking place?

Francesco Pausata: The WAM always has had the potential of impacting the Indian monsoon system. The circulation in the tropics is interconnected (walker circulation). Therefore, changes in circulation in one place is likely to affect circulation far afield due to an altered walker circulation.

AS: What is the exact influence of the WAM on the ISM?

FP: We proposed in a paper published in Climate of the Past last year that this remote influence is mediated by anomalies in Indian Ocean sea surface temperatures.

For example, changes in dust emission is a factor (reduced dust concentration will likely led to an increased sea surface temperature, hence higher Indian monsoon rainfall during the past).

Another reason is linked to the above-mentioned impact on the Walker Circulation. For example, intensification of WAM leads to higher convergence over the western Indian Ocean and lower convergence over the Amazon (decreased rainfall).

AS: How is the WAM going to impact the ISM in future climate scenarios?

FP: Future projections show a possible weak intensification of the WAM in the future. Therefore, other factors may play a stronger role in altering the ISM intensity.

On the other hand, the realisation of the Great Green Wall in the Sahel region may strengthen the intensity of the WAM significantly and this may have a cascade effect on the ISM.

However, more research is needed to understand the consequence of the Great Green Wall on climate locally and far afield.

AS: What do we know about the current tipping behaviour of the WAM system? Is it close enough to the African Humid Period (AHP) which lasted from 14,000 years to 5,000 years BCE?

FP: Even if the Great Green Wall project is terminated, we will be far from an AHP type of state.

Also, the AHP transition of the Sahara was regionally rapid. Hence, it was a tipping element with respect to changes in the driving force. So, it was slow with respect to time scales of individual humans and local ecosystems.

The WAM, thus, should not be regarded as a tipping element of the climate system, able to dramatically impact local ecosystems as its changes will be gradual enough for the local ecosystems to adapt.