- The frequency and duration of monsoon weather systems have changed.
- We have the best weather forecasting models; still often users do not fully understand what the forecasters speak.
- Data, essentially a form of knowledge, should be shared widely to maximise its benefits for society, says climate scientist M.N. Rajeevan in this interview with Mongabay India.
More flash floods, landslides, untimely rain, and extended seasons — monsoon weather systems are changing in a warming globe. A new book, South Asian Summer Monsoon: Processes, Prediction and Societal Impacts by monsoon experts Madhavan Nair Rajeevan, Parthasarathi Mukhopadhyay and Arindam Chakraborty, explains the structure and dynamics of monsoon weather systems, rainfall variability within a day to many decades, and human impacts on monsoon. The book, to be published by Elsevier next month, on February 1, 2025 also discusses monsoon forecasting for farming, water resources, drought and flood control, disaster management, public health and energy.
The lead author Rajeevan, one of the world’s leading climate scientists and Vice Chancellor of Atria University, Bengaluru, tells Mongabay India about the story of this dream project from his monsoon forecasting days at the India Meteorological Department (IMD), in this interview.
Contributions from co-authors Mukhopadhyay, a numerical weather prediction expert (NWP) at the Indian Institute of Tropical Meteorology (IITM), and Chakraborty, a monsoon dynamics specialist at the Centre for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science, make it a “comprehensive reference book”, he said.
Mongabay India: Writing this book has been your long-cherished wish. How did it all begin?
M.N. Rajeevan: Yes, writing this book was a long-cherished wish of mine. As you may know, after Y.P. Rao’s famous book published in the mid-1970s, we did not have a comprehensive book on the Indian monsoon. I was planning to start the work around 2005-2006. But I could not start, in 2008 I moved to Indian Space Research Organisation (ISRO) and then to the Ministry of Earth Sciences (MoES). In 2015, I became the secretary. I am glad that I could finish the book now, 20 years after I had originally planned. It kept me busy for almost two years, but worth spending two years.
Mongabay India: Could you share your personal tale of “chasing the monsoon”. You come from a place where the monsoon enters mainland India.
M.N. Rajeevan: I was always amazed and fascinated by the monsoon and the beautiful rains. As a child, I always wondered why the reopening of school in the first week of June coincided with the first showers of the season in Kerala. It was only in high school that I learnt that the rains were actually the southwest monsoon, which arrives in Kerala in the first week of June. I learnt more about the vagaries of the monsoon only when I joined the India Meteorological Department (IMD) in 1985. The IMD’s long-range forecast for the 1994 monsoon rains turned out to be a complete failure. After this setback, I was asked to join the Long-Range Forecasting (LRF) department to improve IMD’s monsoon forecasts. This marked the beginning of my in-depth exploration of the monsoon and set me on a long-term research journey into its variability, dynamics and teleconnections. During this time, I really started to chase monsoon — a beautiful but complex natural phenomenon.
Mongabay India: And that led to the compilation of the “Rajeevan data set,” if you like…
M.N. Rajeevan: Previously, IMD faced criticism for not making its data readily accessible to researchers. Encouraged my seniors, I explored the possibility of creating a daily gridded dataset that could be shared openly with users, addressing concerns over the challenges of sharing raw station data in bulk. This was no small task, as the dataset involved extensive quality control to address missing data and errors. The resulting precipitation dataset, so called “Rajeevan dataset” in 2006 garnered worldwide recognition and became a vital resource for climate research, earning more than 1,000 citations — a remarkable achievement that remains a source of great personal satisfaction. I have always believed that data — essentially a form of knowledge — should be shared widely to maximise its benefits for society. These datasets exemplify that philosophy, enabling researchers worldwide to make advances in monsoon and climate research.
Mongabay India: Your upcoming book South Asian Summer Monsoon: Processes, Prediction and Societal Impacts explains the structure and dynamics of monsoon weather systems — are they dramatically, significantly changing in a warming globe?
M.N. Rajeevan: Yes, the monsoon weather systems are changing in a warming globe. Even though the dynamics or physical process has not changed, the frequency and duration of these weather systems have changed. We are now experiencing more monsoon weather systems (known as rainstorms) with longer durations that cause heavy rainfall and flooding. In recent years, there is a tendency for the monsoon weather systems to interact with mid-latitude weather systems, causing abundant rainfall over the northwestern Himalayas. There is also a tendency for the monsoon low pressure areas (the main rainfall producing systems) to form later in the season.
Mongabay India: Monsoon onset, withdrawal, and the monsoon map seem to be changing, aren’t they? To what extent are these changes due to climate variability and climate change?
M.N. Rajeevan: Yes, monsoon onset and withdrawal have changed. Even though the onset over Kerala has not changed, there is a tendency of monsoon onset to occur little later (almost one week) over central parts of India. There is also a tendency of monsoon withdrawal from northwest India. The future climate change scenarios suggest that these changes will continue, suggesting a monsoon season with a longer duration.
Mongabay India: What are the most significant teleconnections (remote influences) relevant to monsoon?
M.N. Rajeevan: The most important teleconnections influencing the monsoon are, of course, the El Nino/Southern Oscillation (ENSO), which is accompanied by changes in the Sea Surface Temperature (SST) over the equatorial Pacific. An El Nino event is considered to cause a poor monsoon, while a La Nina event is favourable for a good monsoon. However, there is no one-to-one relationship between ENSO and monsoon. Only 50% of El Nino years are associated with monsoon droughts and we can experience droughts even without El Nino. In addition to ENSO, SST fluctuations over the equatorial Indian Ocean and the Atlantic Ocean also play an important role in modulating the Indian monsoon.
Mongabay India: Going by global standards, how good is India’s forecasting at different scales of time and space?
M.N. Rajeevan: We have the best weather forecasting models (as good as those used in the USA and the UK). We also assimilate global observations as they do. Therefore, we have a state-of-the-art weather forecasting system. We also have a state-of-the-art system for extended range forecasts (10–30 days) and seasonal forecasts (up to one season ahead) based on statistical and dynamic methods.
But the expertise of the monsoon forecasts cannot be compared one-to-one with the skill of the forecasts produced in the USA and European countries. There are more weather systems in the mid-latitudes or at the poles, which are more predictable. Their weather systems are better organised than the monsoon weather systems, which are more transient and chaotic.
Despite this limitation, our weather forecasts are good and useful up to 4–5 days in advance. Our accurate forecasts for tropical cyclones are a good example of this. Our extended range monsoon forecasts are good up to 2–3 weeks in advance. Seasonal forecasts are generally less predictable. However, we have a reliable seasonal forecasting system with quite good accuracy. Just two examples: We were able to predict the drought of 2015 and the excessive monsoon of 2024.
Mongabay India: What are the most urgent needs of monsoon forecasting — in terms of science, tech and user interface?
M.N. Rajeevan: A better understanding of physical processes will allow more accurate predictions on different temporal and spatial scales. Two good examples are land-atmosphere interaction (land surface processes) and convective parameterisation (How different clouds are treated in weather prediction models).
Three-dimensional observations of the Earth system are essential components for an NWP system. The current global observation system is diverse, robust and rapidly evolving. More intensive observational campaigns are required to understand these physical processes better and include in the models. In this effort, during my tenure, I had initiated a research testbed at Bhopal to understand monsoon clouds better. IITM is developing this centre. There are challenges in assimilating these diversified data into the models.
For extended range and seasonal forecasts, we need to improve our coupled climate models in representing physical processes, especially the coupled (atmosphere-ocean) processes.
We need to improve the last mile connection. There is still a gap between the forecasters and the users. At many times, users do not fully understand what the forecasters speak. Also, the forecasters are not fully aware the scope of their requirements.
Mongabay India: Briefly, what role did each co-author play in preparing this book?
M.N. Rajeevan: We worked as a team to complete this book. Even though, I took the main responsibility, contributed to most of writing, Arindam (monsoon dynamics) and Partha (weather prediction) have used their expertise and shared information to complete the book. I must put on recorded their valuable contributions in this book.
Mongabay India: What more would you like to tell your readers?
M.N. Rajeevan: This book is recommended as a comprehensive reference book on the South Asian summer monsoon for undergraduate, postgraduate and Ph.D. students. It can also serve as a professional reference book for monsoon researchers and non-specialists such as policy makers and monsoon enthusiasts.
Read more: Ocean processes influence monsoon, but it’s not always climate change: M.N. Rajeevan
Banner image: Representative image of rainfall in Kerala, captured in 2009. Image by Dhruvaraj S. via Wikimedia Commons (CC BY 2.0)
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