New study finds Mumbai rains intensifying; city needs to strengthen rain resilience measures

These findings have been published in the latest edition of a top peer-reviewed international journal, Theoretical and Applied Climatology.

Mumbai: The intensity of monsoon rainfall -- between June to September -- in Mumbai city and suburbs is intensifying at the rate of approximately 5.18mm per year while the intensity of the overall annual rainfall the city and its suburbs receive (this includes pre-monsoons and unseasonal rains) has gone up by 22 mm per year.

The findings indicate two important inflection points in Mumbai’s recent weather data: the frequency of heavy (more than 120 mm/day) and extreme heavy rainfall events (more than 250 mm/day) increased over Santacruz after 1994, while in Colaba the increased has been noted after 2005, the year when Mumbai suffered a historical deluge receiving 944mm of rain in a single day. (Bhushan Koyande/HT Photo)

These findings have been published in the latest edition of a top peer-reviewed international journal, Theoretical and Applied Climatology.

The findings indicate two important inflection points in Mumbai’s recent weather data: the frequency of heavy (more than 120 mm/day) and extreme heavy rainfall events (more than 250 mm/day) increased over Santacruz after 1994, while in Colaba the increased has been noted after 2005, the year when Mumbai suffered a historical deluge receiving 944mm of rain in a single day.

“Examining data from both stations together, along with BMC data, we found that the period between 2001 to 2005 saw a significant change in the way Mumbai experiences rainfall, with general patterns becoming more erratic and individual events becoming more severe during and after this period. There are abrupt change points throughout the data, but the largest group is between these years,” said Rohit Mann, research scholar at the University of Kurukshetra’s geography dept. and lead author of the analysis -- ‘Observed trends and coherent changes in daily rainfall extremes over Greater Mumbai’.

The analysis was carried out for the period 1985–2020 for two stations of the India Meteorological Department (IMD) -- in Santacruz and Colaba -- and 23 stations were selected for the period 2006–2020, from the dense rain gauge network of Municipal Corporation of Greater Mumbai (MCGM), which was established after the deluge of July 2005. A subsequent extreme rainfall event in 2011 saw 1058mm of rain in a single day over Colaba.

The analysis of the full dataset, which was provided by the IMD and BMC shows Santacruz as being the most rain-prone location in the city, with a mean annual rainfall of 2306mm between 1985 and 2020. BMC’s Gowalia station was found to be the least rain-prone, with a mean annual rainfall of 1717.55mm.

“Furthermore, the year 2020 has recorded the highest rainfall (3721.99 mm) and the least in 2015 (1005.46 mm). It was observed that the years 2007, 2011, and 2020 are the years of maximum rainfall and 1986, 2002, 2015, and 2018 were the years of minimum rainfall. It is clearly seen from the analysis that the rainfall deficit years coincide with the El Nino years,” the study notes.

“Essentially, the July 2005 floods are not an isolated incident. When viewed in the context of a larger data set, we can see that it is reflective of broader changes in the Indian Summer Monsoon Rainfall, due to increasing easterly wind anomalies from the equatorial Pacific and enhanced cross-equatorial flow in the Indian Ocean, which are examples of climate change,” said Mann.

His findings are similar to those which have been recently highlighted in the BMC’s Mumbai Climate Action Plan (MCAP) and which warn of the need to urgently improve the city’s flood resilience to combat changing patterns in rainfall.

Based on daily rainfall timeseries data from 37 stations over the last 10 years, Mumbai experiences, on average, six heavy (64.5 – 115.5mm), five very heavy (115.6 – 204.4 mm), and four extremely heavy (> 204.5 mm) rain events per year. Each year, around 9.5%, 8.7%, and 5.8% of all monsoon rainfall is classified as heavy, very heavy, and extremely heavy, respectively, the MCAP states.

More recently, “the four-year period between 2017 and 2020 has seen a steady increase in the extremely heavy rainfall events. Spatially, most ERE tend to occur as localized clusters in central and western areas like Worli, Dadar, Kurla and Andheri,” the MCAP cautions.

An official with the BMC’s environment department, responsible for implementing the MCAP, said, “It is no secret that heavy rainfall days in Mumbai are increasing, and that there is more spatial variation as well. Actions recommended to make the city more resilient to flooding in the MCAP will be implemented after the BMC’s climate cell is formed soon.”

DS Pai, director of the Institute of Climate Change Studies and former climate scientist at IMD Pune, said, “I have not examined this study myself, but the broad observations are consistent with the kind of climate change impacts we are seeing across India. There will of course be similar changes in local rainfall data from cities as well.”