- Monsoon varies on different scales of time. Most of this variation is caused by natural phenomena such as the Indian Ocean Dipole and remote events such as El Niño and Arctic climate variability.
- This natural variability, especially in long-term patterns and changes in the frequency of heavy rainfall events, can be impacted by anthropogenic climate change, leading to more pronounced and unusual events.
- We need more rigorous studies to understand these changes and their outcomes, says monsoon expert Madhavan Nair Rajeevan in this interview.
Monsoon often involves variable and extreme rainfall, influenced by natural atmospheric and ocean phenomena as far as in the Arctic and the Pacific oceans. Climate change makes these changes more pronounced, says one of India’s leading monsoon experts.
Madhavan Nair Rajeevan is the Ministry of Earth Sciences (MOES) Distinguished Scientist at the National Centre for Earth Science Studies, Thiruvananthapuram. He served the India Meteorological Department for over two decades, focusing on weather forecasting and climate services, including long-range monsoon forecasting, development of prediction tools and climate data sets. Then, he moved on to the Indian Space Research Organization, after which he worked as the Secretary of the Ministry of Earth Sciences from 2015 to 2021. He is famous for his contributions to monsoon variability and prediction, climate change, extreme weather events and prediction of mesoscale systems.
Mongabay: Why do rainfall patterns vary, sometimes dramatically?
M.N. Rajeevan: Monsoon varies at different timescales. It can be diurnal. It rains at specific times. It has its own reasons. On the foothills of the Himalayas, it is early in the morning. Over central India, it is in the afternoon. On the west coast, also it is early morning.
Then there is variability in terms of a four to five days’ timescale. We call it synoptic timescale, when these low-pressure systems move and bring in rain. Then we have intra-seasonal variability of active and break spells. For a few days, we may have good monsoon rains. After a few days we will enter into a phase of weaker rain. There is also seasonal variability. It can vary from year to year. Then monsoon has a multi-decadal variation involving 30-year cycles or 40-year cycles.
Most of these variabilities are due to natural causes. Studies have shown that anthropogenic effects also influence this variability. This natural variability can be influenced or modified by anthropogenic effects. Especially in long-term changes, and changes in the frequency of heavy rainfall events. There we find a lot of influence coming from global warming.
Mongabay: So, it is not necessarily due to climate change?
M.N. Rajeevan: Whenever you find some kind of anomalous situation, we cannot attribute those things to climate change. But we tend do that. That is the easy way of doing it. Climate change is a basket. Anything can be put inside it. For example, the late onset this year. In an El Niño year late onset is possible. The Indian Ocean is warm; the rain does not move northward. Then there’s also the cyclone that came to Gujarat. That must have also caused some kind of (delay in onset). This kind of delay is possible. It is very common. It cannot be attributed to climate change.
Now monsoon has picked up. But once monsoon has picked up, it is not necessary that active monsoon situation will continue for days. It will continue for a few days and then it will go into a weak spell. Then again it will come up. This is the situation we have right now. The monsoon is active and it may go into the next phase.
Mongabay: What are the factors that influence monsoon variability?
M.N. Rajeevan: Monsoon varies in different scales. That is mostly influenced by El Niño. To some extent Indian Ocean and Atlantic Ocean also play some role. El Niño is the most predominant factor influencing the monsoon. Indian Ocean Dipole (IOD) also influences it, but not that strongly. (IOD is defined by the difference in sea surface temperature between two areas – a western pole in the Arabian Sea and an eastern pole in the eastern Indian Ocean south of Indonesia. The IOD affects the climate around the Indian Ocean Basin and is a significant contributor to rainfall variability in this region.)
In some years it works, in some years it does not. Prediction of IOD from the models is not perfect. But the model is very good in predicting El Niño. Especially very close to May-June. We can trust El Niño prediction, but not IOD that much. IOD’s effect is not very prominent in all the years. This year IMD has given a normal (monsoon) forecast assuming that in spite of El Niño, there is indication of a positive IOD. A positive IOD will help nullify the adverse effect of El Niño, (as IMD notes).
Mongabay: What is the reason behind the rainfall deficit this season in many parts of India?
M.N. Rajeevan: The reason behind rainfall deficit in the early monsoon phase that the monsoon came late. The monsoon has a way of progressing. The Arabian Sea will help the monsoon onset to progress along the west coast. Then the Bay of Bengal becomes very active, with lows forming. The east coast and central India, northwest India, Gujarat and (interior) parts of Maharashtra will get rain only when the Bay of Bengal is active.
Mongabay: What causes a trend of more extremely severe cyclones in the Arabian Sea?
M.N. Rajeevan: In recent years, all oceans have been warming up. The Indian Ocean is warming the fastest. Especially the Arabian Sea. Arabian Sea temperatures are going up. The intensity of the cyclone does not depend on sea surface temperature, but the volume of warm water in the ocean. The more the thermal energy, the more the intensity (of the cyclone). In recent years, the Arabian Sea has been becoming very warm. It has more warm water. That is why when a cyclone forms, it intensifies into a very severe cyclone.
Mongabay: May is becoming more active…
M.N. Rajeevan: Active in the Arabian Sea. October-November also. We were taught in the training school that once a good monsoon comes; the Arabian Sea becomes very cool. With strong winds and stirring of the ocean water, the ocean will become very cool. Nowadays this is not happening. Even after a good monsoon, the Arabian Sea is still keeping its warmth. Even the Bay of Bengal to some extent. The Bay of Bengal I can understand, but the Arabian Sea is (different), especially the northern Arabian Sea. We always used to think the northern Arabian Sea cools off. It does cools off, still the water is warm enough to enable a cyclone to form and sustain it. That is very critical.
Without any doubt, these changes are attributable to global warming. Maybe natural cycle also is playing a role.
Mongabay: What about the influence of changes in the poles?
M.N. Rajeevan: Polar influence is very strong, still more studies are required. We have some evidence that Arctic amplification (rapid Arctic warming) and sea ice melting are playing important roles. I was not working in this field, but recently I just reviewed all the papers for a talk. I can say there is growing evidence of Arctic warming and amplification, and the sea ice is melting.
There are many ways it can influence the monsoon. One is through mid-latitude westerly weather system. Deep amplitude waves can be formed. They can disturb the monsoon. There is this North Atlantic meridional circulation. In 100s of years, deep ocean water flows from the Pacific across Indian Ocean and goes to the North Atlantic and comes up as a cold current. Then it goes back. It involves a huge exchange of ocean water. That is going to get influenced and slow down because of the Arctic sea warming. That can have an implication on the monsoon.
To some extent Arctic sea ice melting can also influence the characteristics of El Niño. The El Niño effect is well known. Any change in the El Niño can be there. There are three or four ways in which Arctic amplification and Arctic sea ice melting can influence the monsoon. People might say it is far away. But there is growing evidence. We need to do more model studies, data collection and understand how it really works.
Mongabay: Can there be short-term influence of changes in the poles?
M.N. Rajeevan: Quite possible. Many things we see now could be attributed to Arctic amplification. We may not know immediately, (but we can) try to find out what exactly they are through a very detailed model study. Otherwise, we will need data from a huge number of years to find out whether there is a statistical relationship. My gut feeling is Arctic amplification may be playing an important role (in monsoon). Not only influencing seasonal rainfall, but also heavy precipitation events even in pre-monsoon season.
Mongabay: What causes localised intense rain events?
M.N. Rajeevan: That can happen in (in context of) global warming. Theory says large scale moisture convergence is happening. There is more water vapour, so that there will be large-scale convergence. And more precipitation. When we get huge rainfall at individual locations like Thiruvananthapuram, that cannot be attributed to global warming. However, if the whole region like the whole central India is getting a lot of rains heavy rains that could be. Local events can be due to local factors, local topography and synoptic systems prevailing over there.
Mongabay: What are the challenges in forecasting at different timescales?
M.N. Rajeevan: Challenges are plenty. Till monsoon comes, till June, models are pretty good. When the monsoon comes, the predictability comes down sharply. So many complexities. Models have not understood these complexities. Still, we are able to predict up to four to five days. There is useful skill in prediction for farmers for two weeks. We produce four-week forecasts, but last two weeks are not very good. First two weeks we can believe more. You can take action based on that. Then seasonal forecasting has its own challenge. Some years it works, some years it does not work. I am writing a book on the Indian monsoon.
Monsoon forecasting is really a challenging job. It is complex. We have made a lot of efforts to improve our forecasts. We have better understanding about monsoon now. We also have better skill in predicting monsoon. That skill we can improve upon. Models are improving. We are on the right track.
Banner image: In recent years, all oceans have been warming up. The intensity of a cyclone depends on the volume of warm water in the ocean. The more the thermal energy, the more the intensity of the cyclone. Photo by Sayantan Mitra/Wikimedia Commons.