The word ‘monsoon’ is synonymous with the rainy season for most of us on the Indian subcontinent. However, the Indian monsoons are not ordinary rains. But what are they?
Aditya Vijaykumar, a second year PhD scholar at ICTS, speaks to three scientists of the ICTS monsoon initiative- Prof. Amit Apte, Prof. Rama Govindrajan and Prof. Vishal Vasan, to understand why the monsoons caught their interest.
From left to right: Prof. Amit Apte, Prof. Rama Govindrajan and Prof. Vishal Vasan.
Photo © ICTS-TIFR.
Prof. Amit Apte (A): A PhD in physics didn’t stop Amit from choosing research in applied mathematics as his career. At ICTS, he works on dynamical systems, both on theoretical aspects and applications, mainly in earth sciences. His main research interests are in relating data assimilation problems to dynamical systems theory.
Prof. Rama Govindrajan (R): Rama is a chemical engineer by undergraduate training and did her PhD in fluid dynamics. She works on flow instabilities, and her research interests span a wide range of applications in fluid dynamics, from matter and flows in the sea, to air flows around aircraft wings and formation of clouds.
Prof. Vishal Vasan (V): Like many youngsters of his generation, Vishal trained as a mechanical engineer before switching over to a career in mathematical research. His main research interests include partial differential equations, their solutions and their applications in a wide variety of physical problems.
How did this group come into existence?
V: There is a story there.
I started off in Mech engineering, and the entry into math was through fluid mechanics. Specifically, through geophysical fluid dynamics. When I joined ICTS, Amit floated the monsoon idea, “it seems like a difficult problem, do you want to work on it?” he said. Now you can’t get out! (laughing).
A: Before Vishal had joined, in 2013, there was a worldwide series of events called Mathematics of Planet Earth (MPE) in which ICTS was a partner. One of the ICTS events was the Ramanujan Lectures by Prof. Andrew J. Majda on Clouds, Climate, and Tropical Meteorology. That is when I got talking to Rama, she had previously tried to get me interested in nonlinear dynamics of clouds and failed. But MPE was very useful. Theories of monsoon are descriptive. We started to think about questions such as ‘What are specific problems we could get stuck at? How do we formulate problems? We are still stuck but we got a good start with the help of one of our postdocs, Raj.
R: There was a project to study the dynamics of the Bay of Bengal. Debashis Sengupta was putting together an Indo-US group and he invited me to be a part of that project. That has been a very useful program. There is no direct relation between monsoon and Bay of Bengal, but one day it will all come together. We also have the first measurement of turbulence in the Bay (Ritabrata Thakur’s work). That’s how I became interested in studying the monsoon.
Why should one study the Indian monsoon? How different is it from the cloud formation theory and water cycle that we study in our school textbooks?
R: I’ll start with clouds. Why should clouds form? By all simple logic clouds should not form! Imagine water evaporating from the ocean surface and rising up as water vapour. On the surface you will have a blob of wet air, that is, air containing water vapour. This air is slightly warmer than its surrounding air and is lighter than the surrounding air, and hence it starts rising. It is easy to say that it reaches a height of a few kilometres where the amount of water vapour it was carrying is enough to make it supersaturated, because as you go up it is colder. Now consider a blob of smoke coming out of a chimney. Near the chimney one can see dark black smoke, but at a certain height above the chimney, the smoke vanishes! This happens because the smoke mixes with the air surrounding it. But then how does water rise so high up when smoke cannot? From the point of view of fluid mechanics, both scenarios are similar. The only way to explain why water vapour goes so far up is by working backwards and saying that the water vapour did not mix with the air surrounding it. How this process works is still an open question, and so is why and how clouds form. Also, just because a cloud has formed, why should there be monsoon? Maybe Vishal can address some that.
V: Yes. One thing that we learned from our work in the past three years is that what we learnt in school was wrong, with regard to how the Monsoon works! For the purpose of this question, let’s define monsoon to be the ‘rainy season’. A caveat is that this is not the definition that atmospheric scientists use. India gets hot in the summer and then there is a giant sea breeze that brings water vapour over the Indian landmass and that’s what causes the rain. Three years ago, we learned from Prof. Sulochana Gadgil, an expert in the field, that this is not true! One can agree with this by just looking at the rainfall data gathered over the years. For example, we would expect that it would rain more near the coasts, and yes, it does! But then, why does it then rain so much in Central India? If hotter places get most rainfall, because they would be drawing in the air, by that logic, Rajasthan should get a lot of rainfall, but clearly it does not!
R: Yes, and the hottest time of the year is not when it rains the most!
V: Yes, and when it rains, we see that it gets cooler. So why then does it continue to rain for almost four months? So, the school level understanding is misleading. This is not to say that the heating of the land does not play a role in the monsoon; just that the simple version of the story is not true! There is a lot more that is going on. Most of the questions I mentioned are open questions. One thing we know for sure about the monsoon is that it happens every year- year after year! But how the monsoon varies in time and space, is a very important open question.
R: Also, to be noted is that Sahara and Saudi Arabia are very hot places around the same latitude as us, but they do not have a monsoon!
V: We don’t know the full story of why our school knowledge is wrong. What we do know is that the monsoon is part of global weather patterns- what happens here in India, is a consequence of things happening in the equatorial regions of the Indian ocean, Indonesia and Tahiti.
What questions are being addressed by your research?
V: The main question still is ‘what is the monsoon’?
A: As a mathematical question. Depending on who you ask you shall get different answers.
V: What I would like is a first order mathematical model to describe the monsoon. There is no such model yet. For example, we have a hierarchical set of models to describe El Nino. But for monsoon - none.
A: The main mathematical question is - what is a simple model for monsoon.
R: But we also have realized that coming up with a model is going to be pretty hard. Meanwhile, we are playing with the data.
V: Based on your choice of definition of monsoon, you will describe a different aspect. For example, you may have a model that describes the wind, but how does that describe the rain? Maybe the equations of wind must mention of rain! And vice versa. Of course, you could try to build a grand model, but then that is literally asking ‘What is the relationship?’.
The Automatic Weather Station (AWS) installed at ICTS campus. The AWS measures and records temperature, pressure, precipitation, humidity, solar radiation and speed of winds and their directions, every 15 minutes.
Photograph © ICTS-TIFR
How is the approach of the ICTS Monsoon Group different from that of other groups?
A: We bring a slightly different background and hence it is different in principle. Building models specifically for the Indian monsoon is not a major focus of any research group in India or even outside India. Mathematical tropical meteorology is not a big field. It is barely becoming a field. We are after a zeroth order model and not for prediction like other people. This will eventually feed forward to prediction but that is not our primary focus.
V: In 2018 we wrote a paper where we did an analysis of pure rainfall measurements. Our attempt was to use techniques in computer science to investigate whether there are spatial and temporal patterns in the data. There are spatial patterns. When it rains in certain areas it is very likely to rain in some other areas. And these patterns stick around for a day or two. Existence of these patterns seem consistent across years. Why did we need these hi-fi CS techniques? Because the daily rainfall is a noisy manifestation of the process.
R: There are so many aspects to this! And that is why we need so many young people in it!
Modelling Monsoon is a very hard problem with so many open questions. It is ripe for people to walk in. And nobody else is doing it for the Indian monsoon! It is a difficult problem to simulate, and we need to do it because it directly affects us!
The subject is not restricted by field or preference. And it cannot be done by three people!
What makes the Indian monsoon different from other monsoon systems?
V: Here is where your colloquial definition of monsoon as rainy season comes into play. Atmospheric scientists use the word monsoon to mean a/the ‘seasonal change in wind’. No statement about rain. By this definition (seasonal reversal of wind), there are about 9 weather events across the planet called monsoons. The Indian monsoon is the biggest amongst them in terms of area of impact. It’s also perhaps the one we understand the least. The mechanisms of the different monsoons might be different too. Some are in fact land-sea breezes whereas the Bay of Bengal seems to play a much more complicated role in the Indian monsoon. The Australian monsoon (occurring between June-October in Australia and South-east Asia) is perhaps similar in the dynamical sense. All of these monsoon systems have one thing in common - seasonal change in wind.
What about climate change? Can it affect the Indian monsoon?
V: Yes, but in what way? That is a harder question. There are some studies that suggest annual volume of rain is changing. Like a lot of phenomena that is affected by climate change, it is really the distribution of events that changes. We don’t have enough data to say one way or the other. Most people do believe that the mean rainfall will go down and the extremes will get worse.
R: India actually gets a lot of water from rainfall and we get more than we need. But the distribution is highly non-uniform. And we need to get rain at the right time for farming purposes.
A: Uncertainty for climate prediction is huge, especially for tropical regions. Most phenomena regarding clouds are uncertain. Naturally then, studying the effect of climate change on the Indian monsoon is also plagued by many uncertainties.
V: None of this is to say climate change isn’t real!
R: Earlier in the interview, we talked about the water cycle, a term which gives us the impression that all the processes are periodic in time with some modifications here and there. The problem is that we are introducing irregularities in this ‘cycle’. For example, we are using too much groundwater. So, climate change or no climate change, these things are taking us away from the ideal situation. When the groundwater in sucked dry, the rivers are sucked dry as rivers are what feed the groundwater reserves; and when the rivers are sucked dry, the water cycle is disturbed.
What are the avenues that upcoming researchers can contribute in?
R: I talked about the complexity of the monsoon - there are so many things we haven’t even looked at! A lot of people are actually working on problems related to the monsoon, but what we really need is an army of people working on it. Besides, it is a problem that impacts hundreds of millions of lives! What more does one need in a problem?
V: The monsoon is well-known to be a difficult problem to simulate for global weather models, but the effort of simulating and understanding it well is not being made by other countries because it doesn’t affect them as much. Their weather is well-simulated by their models; if we want models to explain our weather, it is up to us to step up and look at it. By ‘us’, I don’t mean just the group at ICTS - for instance it would be great to have a biologist and an atmospheric scientist to figure out how the forest cover impacts the monsoon. People across different research fields can contribute.
R: If a person likes writing down equations on paper, or doing experiments in the lab, or flying through clouds to take data, or even doing computer simulations - there are many avenues they can contribute to.
A: Needless to say, this cannot be done by just the three of us at ICTS.
V: We are more than happy to share our research problems with others!
A look at the advance and retreat on 2018 monsoons over the Indian sub-continent.
Animation courtesy: MD Madhusudhan