Monsoons can go for a toss. What will that mean to us?
Context: Monsoon in India and West Africa — the most significant monsoon rainfall systems — may be in for changes due to greenhouse gases, new research has warned. The changes may be rapid or gradual in the present as well as near future.
This is the reason why scientists have included both, the Indian Summer Monsoon (ISM) and West African Monsoon (WAM) under climate-tipping points. These are critical thresholds in massive Earth ecosystems such as the Amazon rainforest or the Greenland Ice Sheet that, when crossed, can lead to abrupt and irreversible changes in the systems.
The systems are also interrelated, which means that a collapse of one can lead to a cascading effect on the others.
Indian Summer Monsoon (ISM) and West African Monsoon (WAM)
Both ISM and WAM have remained relatively stable for thousands of years, with intermittent abrupt and gradual changes in the intensity and distribution of rainfall due to natural climatic causes such as changes in solar radiation.
WAM shifted northwards between 14,500 and 5,000 years ago, which increased rainfall in the northern and western parts of Africa.
One of the prominent results of this increased rainfall was the increase in vegetation in the Sahara desert, the evidence of which can be observed in the region’s paleoclimate records.
The northward shift of the WAM had also caused significant changes in the social and cultural life of human settlements at the time, especially along the Nile river.
Similarly, the ISM has undergone changes at various times in the past few thousand years. The destiny of the Indus Valley Civilisation was closely intertwined with that of the changing monsoon patterns.
Paleoclimate records in the form of sediments in the ocean or lake, limestone formations in caves, fossilised tree rings and ice cores give an estimate of how the climate behaved hundreds, thousands or even millions of years ago.
Cataclysmic changes
Recent research has indicated that previously unknown factors are influencing both, the ISM and the WAM. A research paper showed that marine heat waves (MHWs) in the Indian Ocean region are impacting the ISM.
Such heat waves are caused by an increase in the heat content of oceans, especially in the upper layers.
The MHWs in the North Bay of Bengal and the western Indian Ocean reduce monsoon rainfall over central India, the study established.
The occurrence in the North Bay of Bengal increases rainfall over the southern peninsular area.
MHWs are huge patches of warm water and they change the way the atmospheric circulation works. The availability of more heat and moisture during an MHW makes the air move upwards which is known as ‘convection’.
To compensate the rise of convection with warm moist air, there is a subsidence of rainfall in other regions. The rising convection creates a low pressure below which pulls in the moisture-laden winds from other areas.
When there are MHWs in the western Indian Ocean region, they pull the moisture-laden monsoon winds towards that region, not letting them move towards the Indian subcontinent. This weakens the monsoon system leading to dry conditions, mainly over central India. In the case of MHWs in the North Bay of Bengal, because of the location, more rainfall occurs over southern peninsular India while central and northern India remain dry.
Worldwide, MHWs are one of the major results of human-induced global warming. But research on their impacts started only about a decade ago.
During an MHW, the average temperatures of the ocean surface (up to a depth of 300 feet) goes 5-7 degrees Celsius above normal. Around 90 per cent of the warming caused by greenhouse gas emissions is absorbed by the oceans.
The year 2021 broke all previous records for ocean heat
The increase in marine heat waves was due to rapid warming in the Indian Ocean and strong El Nino events.
The West African Monsoon, on the other hand, is getting affected by a host of inter-linked factors such as dust emissions from the Sahara desert, evaporation from the lakes of the region and moisture feedbacks from vegetation.
Another important factor in the case of WAM is a climate tipping point called the Atlantic Meridional Overturning Circulation (AMOC). The Gulf Stream ocean current usually carries warmer water from the tropics to higher latitudes and brings back colder water. But now, evidence suggest that the Gulf Stream is slowing down, which will lead to changes.
The collapse of the Atlantic Meridional Overturning Circulation (AMOC) might change the wind and rainfall patterns of the WAM which could lead to disruptions in the lives of 300 million, mostly agricultural people of west and central Africa who depend on the rainfall.
The West African Monsoon is powered by the temperature difference between the cooler tropical Atlantic Ocean and the warmer African continent. It has three distinct seasons with onset between March and May, high rainfall between June and August and southward shift from September to October.
The balance in temperatures on land and in the ocean which drives rainfall during these seasons may get disturbed by the slowing down of the AMOC as the heat transfer from northern hemisphere to the southern hemisphere becomes inefficient and warms up the tropical Atlantic Ocean.
The greening of the Sahara due to intensification of the WAM can lead to impacts on El Nino, tropical cyclone activity and even the Indian Summer Monsoon rainfall.
Can you answer the following questions?
The destiny of the Indus Valley Civilisation was closely intertwined with that of the changing monsoon patterns. Discuss.
