On land, the Indian summer is not as consistently hot as it used to be, but the Indian Ocean is getting hotter. The reduced difference between land and ocean temperature means the monsoon is weakening, at least since 1990, as researchers have now proved
The South Asian summer monsoon is weakening “significantly” due to increasing temperatures in the Indian Ocean and relatively subdued warming over land in the central Indian sub-continent, according to a new study published in Nature Communications, a peer-reviewed journal from the group that also publishes Nature magazine.
Six authors, led by Roxy Mathew Koll of the Centre for Climate Change Research at the Indian Institute of Tropical Meteorology (IITM) in Pune, analysed data between 1901-2012 to establish this long-term trend.
They found that there was a 10-20% decrease in the mean rainfall in the Indian subcontinent. The monsoon was decreasing over central South Asia – from south of Pakistan through India to Bangladesh.
Rainfall over the central-east and northern regions of India, along the Ganga-Brahmaputra-Meghna basins and the Himalayan foothills, had declined.
The decline is crucial because in these regions agriculture is still largely rain-fed. The South Asian monsoon brings sustenance to around two billion people.
“The main thing is that unlike other oceans, which are open to the north and south, the Indian monsoon region is landlocked,” Koll told indiaclimatedialogue.net.
The intense heating during late spring and summer around the equator, with the earth rotating to the right, leads to south-westerly winds under normal conditions.
Due to the higher ocean temperatures in the Indian Ocean, the normal movement of cooler air from the sea to hotter land, with clouds bearing rain, is weakening. This lower contrast between sea and land temperatures is responsible for the declining monsoon over South Asia.
As the authors state, “The variability of the monsoon makes the region one of the most susceptible areas around the world to the impacts of climate-related natural disasters such as droughts and floods.”
Andrew Turner of the University of Reading in Britain told indiaclimatedialogue.net: “The study shows that rapid warming of the equatorial Indian Ocean is a cause for declining rainfall in parts of India over the last century. As they show with their own model experiment, imposing surface warming over the equator will lead to ascending motions there and suppress ascending air over India, leading to declining rainfall over the Indian region. Such a result is not surprising.”
Raghu Murtugudde of the University of Maryland, a co-author, told indiaclimatedialogue.net, “The monsoon pattern is quite different because of the unique land-ocean configuration with the continent coming so close to the equator with a warm ocean to supply the moisture. But this also means that there is a competition for rain between the land and ocean.”
“The circulation associated with warming over the western Arabian Sea is resulting in more moisture being picked up from south of the equator, but as the winds make a turn to the northeast to reach India, the moisture is being dumped as rain over the ocean and thus less is reaching the Indian subcontinent,” he added. “Such warming and such land-ocean competition is not happening elsewhere to the best of our knowledge. The strong warming is a combination of the increasing oceanic precipitation and the wind changes which are reinforcing each other. If you have rainfall over the ocean, the ocean gets more stratified with freshwater, making it difficult for the colder water to mix into the surface even if winds get stronger and thus the heating of the surface layer gets stronger and the surface gets even warmer.”
Previous studies had shown that the monsoon “drivers” – the difference between land and sea temperatures – in the era of global warming should get stronger. Land in the summer in the northern hemisphere was warming faster than the oceans, which would imply that the monsoon would also be stronger.
What is more, the rising ocean temperatures could raise the moisture in the atmosphere over the sea due to higher evaporation and induce a stronger monsoon, but that is not the case for India.
“The monsoon over South Asia is characterized by seasonal reversal of the strong winds and totally sticks to the definition. Nowadays, the monsoon also refers to regions where there is a clear alternation between winter dry and summer rainy seasons, which is why there are other monsoons like in West Africa, Australia and even America – centred over the Panama canal between North and South America,” Koll told indiaclimatedialogue.net.
The western Indian Ocean has been warming at a faster rate than any other tropical ocean in the world – dubbed a “curious case” – according to an Indo-French study in 2014, also led by Koll.
The western Indian Ocean and north-western Arabian Sea have warmed by more than 1-1.5 degrees Celsius in the month ending October 2014.
The latest study questions whether manmade carbon dioxide emissions have played a role in weakening monsoon circulation in recent decades. It is possible that weakened air currents amplify the sea surface temperatures, due to lower evaporation.
In a 2012 review article on climate change and the South Asian summer monsoon, Turner and H. Annamalai of the University of Hawaii also cite the “Asian Brown Cloud” caused by increasing particulate matter due to inefficient cook stoves altering monsoon patterns.
“We must pay attention to emerging ideas about complex dynamical feedbacks from within the tropical Indo-Pacific region to be sure that the South Asian monsoon will remain stable in the future,” the authors state.
Other oceans, same effect
While the latest study examines the role of Indian Ocean warming, it doesn’t rule out other similar impacts due to changes in sea surface temperatures in the tropical Pacific and Atlantic.
These are El Niño events – the global phenomenon associated with a band of warm ocean water temperatures that periodically develops off the Pacific coast of South America and sets in motion factors which impact the South Asian monsoon, as at present.
“Though I attributed the current warming to increasing El Niño events, it does not diminish the role of increasing greenhouse gases. In fact the warming is due to both direct effect of greenhouse gases and the increased El Niño events,” Koll clarified.
“We know that rising greenhouse gas concentrations are responsible for warming generally. The real question is then what is causing this enhanced warming over the equator,” Turner told indiaclimatedialogue.net. “Our paper already published suggests that rising aerosol emissions are the cause. We have shown that over the last several decades, rising anthropogenic [man-made] aerosol emissions (such as sulphate pollutants) over the northern hemisphere have led to relatively less warming over the northern hemisphere land and a decline in land-sea temperature contrast.”
“In addition, very strong aerosol emissions over India also lead to important local effects that act to reduce rainfall there. Importantly, it is only when models include the full range of aerosol processes (“indirect effects”) that they are able to reproduce the observed reductions in monsoon rainfall,” Turner added. “There are no strong emissions of polluting aerosol over the equatorial Indian Ocean that compare with the strong pollution from northern India and that may be one factor contributing to the large warming over the equator.”
Subimal Ghosh of IIT-Bombay told indiaclimatedialogue.net: “The monsoon has a weakening trend specifically from 1950, but unfortunately the reasons are not yet understood properly. The new generation climate models fail to simulate this trend and this raises a major question on the reliability of climate models. This is probably due to poor understanding of the reasons behind the weakening trend.”
“Against this background, the study by Dr Koll and others is revolutionary: it clarifies the hidden reason behind the weakening of monsoon. They show that the western Indian Ocean has a higher warming trend over last 112 years, which is more than the so-called warm pool (central East Indian Ocean) of Indian Ocean.”
Murtugudde told indiaclimatedialogue.net, “We need to have longer-term data to estimate the heat content in this region and get the models to capture this warming and weakening of the monsoon. Most models are failing to capture the trends, as reported by Subimal Ghosh for the past 50 years and for future decades by scientists from IITM. So we need to understand the coupled ocean-atmosphere processes in the Indian Ocean better. Every bit helps with data from satellites and models.”
Turner and Annamalai concur. “Understanding how the monsoon will change in the face of global warming is a challenge for climate science, not least because our state-of-the-art general circulation models still have difficulty simulating the regional distribution of monsoon rainfall. However, we are beginning to understand more about processes driving the monsoon, its seasonal cycle and modes of variability. This gives us the hope that we can build better models and ultimately reduce the uncertainty in our projections of future monsoon rainfall.”