We are in the midst of a raging monsoon that appears to have taken a short break from its routine duties. This year, several states have faced floods and extreme rainfall. Floods are not unusual during this season, but quite unusually the number of extreme rainfall events has been increasing in the last two decades. In Mumbai and Pune, century old records for July rains were broken this year even as Mahabaleshwar, the popular hill station in Maharashtra, received more rains than wettest Cherrapunji. Across India, more than 1000 extreme rainfall events were observed in August. For the science of forecasting monsoon, they add another layer of complexity to a problem that has eluded solutions for more than a century now.
Nearly four decades ago, the celebrated author and journalist Khushwant Singh commented that, “for others to know India and her people, they have to know the monsoon”. He was referring to the monsoon as an all pervading influence on life, culture and society. Yet, as a scientific pursuit, knowing and understanding the monsoon is more easily said than done. Ancient Indians did know that monsoon is related to the wind directions. It is indeed the reversal of wind directions caused by unequal heating of land mass and sea during summer months. This alone would not have produced copious life-giving rains without the mighty Himalayas to trap the moisture laden air coming from the Indian ocean. Such simplistic description hides the immense complexity of the process itself.
It is this complexity that confronted Henry Blanford, India Met Department’s first director, when he issued the first monsoon forecast in 1882. It was based on just one indicator, the Himalayan snow cover. His successor Sir Gilbert Walker, a statistician in his own right, realised that monsoon is a global climate phenomenon and not a regional storm in a South Asian tea cup. In a first of sorts, he systematically analysed huge amounts of weather data to reveal the nature of its complexity. The monsoon’s progress in the Indian subcontinent is correlated with the changes in the atmospheric pressure thousands of kilometers away in the Pacific ocean and southern Indian ocean. Together with the sea surface temperature variations in the Pacific, they constitute the El Nino Southern Oscillations, a major driver of world’s climate. A century of research since Walker’s time has reinforced the viewpoint that the Indian monsoon is part of a global climatic pattern. It is a grand spectacle of nature, global in scale and reach, and cannot be viewed in isolation. The operational monsoon forecasts issued by the Indian Meteorological Department since 1909, and overhauled in 1988, were based on statistical techniques incorporating these global scale correlations discovered by Walker.
If the weather and monsoon are complex physical processes, then the laws of physics, rather than statistics, should be the basis for weather prediction. The potential of physics-based weather prediction was highlighted by a turning point in WW II—the Normandy landings. It took place on 6 June 1944. The commander of the Allied forces Dwight Eisenhower was given two opposing weather advisories based on two different approaches to weather prediction. One, based on physics of the problem, suggested putting off the invasion by a day to June 6, and the other based on correlating with the past weather events advised him to stick to June 5 schedule planned earlier. As it turned out, June 5 had storms and rough seas, while June 6 had fair weather. Eisenhower’s crucial decision to delay the invasion by a day not only turned the tide against the German forces but unwittingly demonstrated that physical processes must be taken seriously for prediction.
The English meteorologist Lewis Richardson proposed in 1920s that the mathematical equations representing the physical processes of weather can be solved to predict weather, if its current state is known. This technique called numerical weather prediction (NWP) involves laborious calculations and is humanly impossible without a computer. Much before the advent of computers, Richardson manually attempted this exercise to predict the weather in Paris and failed miserably. When the first computer ENIAC was commissioned in 1945 at the University of Pennysylvania in the United States, NWP was one of its main goals. First of such prediction was released in 1950. As computers have become more powerful, major climatic features such as the Indian monsoon and even climate change are now routinely estimated and predicted using refined global climate models.
Even as we are still far from understanding the monsoon, the extreme rainfall events witnessed this year and over the last decade have ramped up the difficulty level of the prediction problem. According to the 5th Synthesis Report of the Intergovernment Panel on Climate Change (2015), India might witness an increasing number of extreme rainfall events during monsoon due to climate change. The scientific goalposts have shifted and are probably continuously shifting due to climate change. The only unchanging feature is India’s enduring romance with rains, expressed eloquently in Tamil sangam poetry Kurunthogai, “but in love our hearts are as red earth and pouring rain; mingled beyond parting.”
M S Santhanam
Physicist and a professor at the Indian Institute of Science
Education and Research, Pune