City got 212% more rain than normal this week
Alternating periods of intense and subdued rainfall is a pattern that is projected to grow more prominent in coming years due to climate change, experts warn
Mumbai: Between July 1 and July 6 this year, the city received 634.3mm of rain. This number is significant for two reasons. Firstly, it is more than twice the normal rainfall amount for those six days, which stands at 203.6mm, per India Meteorological Department (IMD) data. That’s 212% more rain received by Mumbai this week than it normally should.
Secondly, 634.3mm of rain is a large chunk of the total seasonal rainfall received by the city this monsoon. It’s 68.5% of the total 926.1mm of rain recorded since June 1 -- which itself is 25% more than the normal measure of 740.7mm that Mumbai should have recorded, at the IMD’s base weather station in Santacruz.
But a few weeks ago, on June 23, Mumbai was staring at a rainfall deficit of -47%, with several citizens and forecasters either wondering or trying to explain the lack of any “proper rainfall”, though the monsoon had officially been declared over Mumbai on June 11.
This irregularity, what experts call “intraseasonal variability”, has become a prominent characteristic of the monsoon not just over the city, but across the country, and bears a prominent climate change signature. Last year too, the city experienced something similar, with 20% of the total seasonal rainfall (between June and September) being recorded on three separate days.
Alternating periods of intense and subdued rainfall is a pattern that is projected to grow more prominent in coming years due to climate change, experts warn.
“This will have a lot of implications for agriculture and flood risk mitigation. The real cause behind this variability is hard to pin down with a single answer. One explanation is, of course, warming in temperature. More heat means more energy which means more instability in atmosphere. Since a warming atmosphere can hold more water, the presence of more moisture than usual could increase the likelihood of extreme precipitation events.
But regionally for Mumbai and India, modes of the tropical intraseasonal oscillation i.e., the Madden Julian Oscillation and the Boreal Summer Intraseasonal Oscillation also play an important role in modulating rainfall patterns, and climate change could be affecting them in a complex way,”
said Akshay Deoras, meteorologist and researcher at the University of Reading, UK.
JR Kulkarni, a former senior scientist at IITM in Pune, said that despite receiving excess rains for three years in a row, monsoon patterns over the Konkan (as with the rest of India) are now showing intermittent dry periods with short spells of heavy rains within the monsoon season, and that the number of extreme rainfall events have increased by at least three times over the region.
Based on data from 37 stations over the last 10 years, the BMC’s Climate Action Plan for Mumbai states that the city experiences, on average, six heavy (64.5 – 115.5 mm), five very heavy (115.6 – 204.4 mm), and four extremely heavy (> 204.5 mm) rain events per year. “The four-year period between 2017 and 2020 has seen a steady increase in the extremely heavy rainfall events. Spatially, most of these tend to occur as localised clusters in central and western areas like Worli-Dadar, Kurla and Andheri,” the MCAP cautions.
“It would be wrong to attribute individual extreme rainfall events to climate change without carrying out attribution studies, but one thing is certain: intraseasonal variability as a phenomenon will become more pronounced. We need more local level data to understand how this might impact a city like Mumbai,” Kulkarni said.
A recent analysis of data from Mumbai’s Mesonet rain gauge system -- published in the Elsevier journal Urban Climate -- has recently shone a light on intraseasonal and diurnal (daily) variability of rainfall in the city.
The analysis, led by researchers from the India Meteorological Department (IMD) had four main findings. One, Mumbai exhibits a gradient in the rainfall between the north and the south, with the north receiving more rain typically. Two, “heavy to very heavy rainfall events showed a propensity to occur at sub-daily hours” while “events with moderate rainfall seem to occur throughout the day.”
“This is true even for recent rains in the city, this week. Most precipitation in a given 24 period tends to happen at night or early morning,” said Jayanta Sarka, who heads the IMD’s regional centre in Mumbai.
The analysis found that most of the very heavy to extreme rainfall events (in excess of 120 mm) occur around midnight to early morning hours, with rainfall intensifying in the late evening along coastal regions, and reaching inland regions at midnight.