District-wise rainfall to date (blue means excess, red means deficient)
Dr A K Shrivastava, head of climate research division at the Pune office of India Meteorological Department (IMD), observed the rainfall and subsequent phenomena closely. He said the primary reason for such a huge amount of rainfall was a low pressure area forming in the upper air, three to four km above the ground surface (lower troposphere), in the North Maharashtra-South Gujarat (N-Mah S-Guj) region.
“While this localised low-pressure area attracted moisture-filled winds toward itself, the pressure gradient and strong winds from the Arabian Sea aided the rapid incursion of moisture to the western coast, and to interiors of Maharashtra,” he said.
Water-heavy monsoon winds gather more moisture as they cross the Arabian Sea. Further, greater the surface temperature of seawater, the greater the quantum of moisture added in the path. Though extensive research needs to be done reach a conclusion, scientists feel that this time, the Arabian Sea let in more moisture than normal, and the N-Mah S-Guj low pressure circulation accentuated the flow, or speed, of the winds.
Scientists at the Indian Institute of Tropical Meteorology (IITM) said that while moisture incursion is a very regular phenomenon, it is still unknown why the winds gave more rains beyond the Western Ghats, instead of yielding most of the heavy rainfall to the coastal Konkan strip.
Oceanographic data from various agencies, including NASA, shows that the surface temperature of the Arabian Sea has been increasing rapidly over years, especially after the 1990s. Scientists all over the globe say this is an effect of global warming.
“As global temperatures rise, the moisture-handling capacity of the atmosphere increases. Now this moisture ultimately results in stronger convective currents in the monsoon season on India’s landmass,” Mrutyunjay Mohapatra, director general of IMD, had told Business Standard in an interview last month.
Incidentally, the forecast issued by the IMD on July 25 did not capture occurrence of heavy rainfall in Madhya Maharashtra in the last week of July and first week of August, but had predicted the possibility in northern plains. It is only in the August 1 forecast that IMD predicted “fairly widespread to widespread” rainfall in the flooded region.
IMD’s own assessment of its forecasts shows that the 'miss rate'—or the chance of missing an extreme weather event—increases with time. While there is a 29 per cent chance (miss rate of 0.29) of missing a rainfall event one day in advance, the chance of missing rises to 55 per cent when it comes to predicting an extreme event that is five days away. Preparation for such events easily takes a week.
Data from the IMD shows the extent to which this rainfall broke all records. The sub-division of Madhya Maharashtra, which includes Pune, Satara, Kolhapur and Sangli—the epicentres of flood—received 170 per cent more rainfall than normal in the week ending July 31. In the next week, it received 320 per cent more (over four times) than the normal rains in that week, which was followed by a week that saw twice the normal rainfall.
It is this sheer amount of rainfall that caused the floods, most experts said. But it needs to be accurately determined what portion of the flood was a rain flood, and what proportion a river flood, said D S Pai, head of climate research and services at IMD Pune.
“The rains in July had already filled up the river basins and dams in the western Maharashtra (IMD’s Madhya Maharashtra sub-division) region. The extremely heavy August rains did not leave any room for storage, and dam water had to be released in a controlled manner, it primarily seems,” he said.
Urban areas such as the cities of Kolhapur and Sangli, however, got choked due to improper town planning, scientists said.
Pulak Guhathakurta, who heads the Climate Application and User Interface Group at IMD Pune, said that had it been a normal July, floods would not have been this deadly. “Continuous rain when all dams are full resulted into a situation which could not have been controlled. It was the highest rainfall the region has seen in this period since rainfall measurement started,” he told Business Standard.
When it was raining cats and dogs in western Maharashtra, a depression was formed in the Bay of Bengal. It moved westwards, crossing Madhya Pradesh and reached Rajasthan/Punjab region, delivering extremely heavy rains and causing floods there too.
Due to a strong Arabian Sea branch and a weak Bay of Bengal branch, western India has seen heavier falls, and more frequent and more devastating floods this time around, than eastern states such as Bihar, Assam and Jharkhand.
India lost $80 billion in absolute terms to extreme natural phenomena in two decades to 2018, a report by United Nations Office for Disaster Risk Reduction has shown. In comparison, the US lost nearly a trillion dollars in two decades, followed by China, at nearly half a trillion. Economic losses in Germany, France and Italy were less than those in India. The report measured losses due to extreme phenomena such as earthquakes, extreme temperature, flood, drought storm and tsunami. It also noted that malnutrition and stunted growth are high in areas of repeated flooding in India.