The northeast and southwest monsoons have become more intense over the last 60 years, making it important to understand how and why it happens, and what it means for the future.

BY G B S N P VARMA

Monsoons have become more extreme over central India in the last 60 years. Intensified rainfall events have become more frequent, causing floods, flashfloods, and landslides. Groundwater-replenishing moderate rainfall events have receded into the background in this context.

The aftermath of rain and floods in Chennai and the rest of Tamil Nadu is a reminder of what a changing monsoon circulation can do. It stands etched in both physical and psychic realms.

At the end of the day, any circulation, including the monsoon, is about the movement of energy. All the energy the earth receives comes from the sun. The terrestrial system is influenced and altered by the sun’s energy, the earth’s shape, and the tilt of its axis and its rotation around the sun and its physical, geographical, topographical features, and the composition of atmosphere and ocean.

To remain in balance—although absolute balance is unattainable—incoming and outgoing energy should be equal.

If the incoming energy is more than the outgoing energy, it leads to heating of the atmosphere. While this naturally happens over timescales of millions of years, the present heating of the earth and the atmosphere is due to fossil fuel emissions from human activities, which create particle pollution and carbon dioxide which acts as a greenhouse gas. Heat is trapped in the atmosphere which is similar to when you cover yourself with a blanket to keep you warm.

Fountain Ink talks with two experts in the field to make sense of what exactly happened in the case of the changed monsoon circulation in Tamil Nadu.

Deepti Singh, a postdoctoral fellow at Lamont-Doherty Earth observatory, Columbia University, New York, works—in addition to climate change related research—on changing monsoon patterns in the context of climate change and global warming. A study prepared along with her colleagues, published in Nature Climate Change, shows intense flooding and more dry spells happening between 1951 and 2011.

Dr Kevin Trenberth is a distinguished senior scientist in the Climate Analysis Section, National Center for Atmospheric Research, Boulder, US. He was a lead author of the 1995, 2001 and 2007 Scientific Assessment of Climate Change reports from the Intergovernmental Panel on Climate Change (IPCC), and shared the 2007 Nobel Peace Prize which went to the IPCC. He talks about circulations perspective of energy and climate change.

What are your thoughts on Chennai and Tamil Nadu rains? What happened with the northeast monsoon that resulted in so much rain?

Deepti Singh: It was a combination of anomalous factors that resulted in
extreme precipitation. The temperatures in the Indian Ocean have been much warmer than normal, related to the El Niño that’s brewing in the Pacific. This El Niño  has crossed some previous records of temperatures. Anomalously warm conditions associated partly with the El Niño result in excess moisture availability so when it rains, it rains much heavier.

The flooding was particularly bad in Chennai not because of a single event but a because of a sequence of heavy rainfall events that had a cumulatively bad impact. The region was hit by at least three heavy rain events associated with low pressure systems moving through in the month of November. Another issue is that sea level rise that has occurred in the region leads to additional coastal flooding with these storms.

(Note: The three weather systems were a deep depression over the Bay of Bengal between November 8 and 10; a well-marked low pressure area over southwest Bay of Bengal between November 12 and 18; and low pressure between November 28 and December 4.)

What is the historical perspective on these monsoon circulations (both southwest and northeast)?

There are many studies which show that extremely heavy rainfall events such as this one have increased over many regions in India, despite an overall weakening of the southwest monsoon. A recent study has shown that this could be related to increasing greenhouse gas emissions. Both these circulations experience substantial influences from the Pacific and Indian oceans, both of which have warmed in recent years.

How does the moisture travel in these circulations? How were they historically and how are they faring now, and why?

During the summer monsoon, the main moisture source is the Indian Ocean. The Somali jet/monsoon winds carry moisture from the Indian Ocean to land. There are additional contributions from the Bay of Bengal and Arabian Sea as well as from local precipitation recycling. The contributions of these sources varies by region but the peninsular part gets most of its rain from the
Indian Ocean.

During the northeast monsoon, the main source for the peninsular region, most of the moisture is from the Bay of Bengal. In fact, the eastern coast gets more rain with the northeast than the southwest monsoon.

In what ways are the southwest and northeast monsoons changing, and when did this start? Do we know why?

The monsoons have been evolving over many timescales. There are decadal scale changes in the monsoon that are purely natural variability. However, there is strong evidence to suggest that aerosol and greenhouse gas emissions associated with human activities have changed the monsoon in at least the last five decades. The weakening of the southwest monsoon is primarily attributed to anthropogenic aerosol emissions.  Additionally, the increasing intensity of extreme rain events interspersed with longer dry spells are a direct result of greenhouse gas warming.

The timing of the southwest monsoon has also changed with more rainfall in April and May and an earlier onset. This has also been attributed to aerosol emissions.

Are torrential rain and floods going to be the new normal?

In addition to what I’ve stated above, yes this is a new normal.  We expect to see heavier extreme rainfall events with enhanced global warming because a warmer atmosphere can hold more moisture. Therefore, as I said before, when it does rain it will rain much more heavily. Studies show that some recent events like the Uttarakhand flooding were worsened by the contribution of anthropogenic global warming.

What do you expect to see in 2016; what does your research suggest in the coming year?

I think it will be interesting to see how this large El Niño will continue to impact the Indian subcontinent. There is a possibility of a much stronger impact on the 2016 summer monsoon too since the impact of an El Niño that is typically strongest in winter is felt in the following summer season. And this El Niño is still to peak. Therefore, we could expect a drier monsoon and drought conditions.

How does the climate system store, move, and release energy?

Dr Kevin Trenberth: The climate system has several internal interactive components. The atmosphere and ocean currents move the heat around, while sea, land, ice, ecosystems, vegetation help store energy and maintain energy balance. Clouds too have this blanketing effect but they also reflect the radiation back out into space, cooling the surface of the earth. Heating patterns from the sun determine atmospheric instabilities.

Clouds, rain, cyclones (low-pressure systems), anticyclones (high-pressure systems), cold and warm winds arise out of temperature differences and distribution of heating. What happens with these systems?

These weather systems move about, grow, mature, and decay over a period of time. Through the release of energy, the climate system tries to get back to equilibrium.All atmospheric flows are connected to movement of energy in some way, usually to reduce gradients (differences).

How do humans induce energy imbalance?

The main way in which human activities can compete with nature is if they somehow interfere with the natural flow of energy from the sun, through the climate system, and back out to space. These energy imbalances are influencing the Indian monsoon. A significant complication is air pollution which blocks out the sun.

How do greenhouses gases contribute to energy imbalance or excessive energy in the system? What happens to energy in the system when greenhouses gases are increasing?

Heat-trapping greenhouse gases, such as carbon dioxide, act as a blanket and warm the climate system. Most (over 90 per cent) of the extra trapped energy goes into the ocean, where it warms the ocean and increases sea level through expansion. It also melts sea ice and land ice, and the latter adds more water to the ocean, also increasing sea level. Of course it also warms the land and the atmosphere. The warming is not uniform especially because of the land-ocean distribution and hence the atmospheric circulation, including monsoons, changes.

How does the atmosphere let the radiation back into space? What’s trapping it?

Most of the atmosphere consists of nitrogen and oxygen (99 per cent of dry air), which let the radiation back into space. It is the water vapour, which varies in amount from zero to approximately three per cent, carbon dioxide, and some other minor gases present in the atmosphere in much smaller quantities that absorb some of the thermal radiation leaving the surface and reemit radiation from much higher and colder levels out to space.

Could you please explain monsoons from energy perspective?

Monsoons are direct overturning cells that move energy from land to ocean. They are greatly influenced by the earth’s rotation but otherwise respond to the heating of land relative to the ocean in summer.

You say that there is a global monsoon circulation, although there is broad categorisation into regional Asian-Australian, African, and American monsoons. Some studies subdivide the Asian monsoon into three: the South Asian, the East Asian, and the Southeast Asian. How does the energy move around in the circulations?

In all these circulations, much of the movement of energy occurs through large-scale overturning of the atmosphere.

(Note: Large-scale overturning of atmosphere has a three-dimensional character: Warm air is lighter and buoyant and hence rises, so that tends to leave a vacuum behind. Cooler air rushes in, which, in turn, tries to create a hole where the cooler air came from. At the same time, the rising warm air pushes the air aloft away from where it is rising to, and that air subsides and tends to fill in the gap vacated by the cooler air. So, an overturning circulation is created—warm air rising, cooler air sinking.)

What else do you think we can expect?

The increase in heat causes climate change and if that occurs too rapidly, as is happening now, it is extremely disruptive. So the effects are mostly adverse. In India there is a greater risk of drought and wild fires, as well as very heavy rains, such as seen recently in Chennai.

(G B S N P Varma is a freelance journalist based in Andhra Pradesh.)

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