City receives 90% of June’s average rain in just five days: IMD

Mumbai: The city has received 90% of June’s average rainfall in just the last five days, as per the data shared by the India Meteorological Department (IMD).

With 485mm of rain, as against the seasonal normal of 537mm, the city’s base weather station in Santacruz recorded 90% of the month’s average rainfall between June 24 and June 29, according to the IMD.

Similarly, the department’s coastal station in Colaba received 371.4mm of rain in the same duration, against the monthly normal of 542.3mm.

The IMD’s yellow alert for the city, indicating moderate showers at isolated places, remains in place for June 30, with calmer conditions expected to prevail on the weekend. However, cloudy skies and light showers will continue till at least June 4, per official forecasts.

This phenomenon -- of large rainfall deficits being made up in days -- conforms to changing trends in city’s recent weather data.

The data shows a sharp increase in the frequency of heavy (more than 120 mm/day) and extreme heavy rainfall events (more than 250 mm/day) amid intermittent dry spells, instead of moderately intense showers spread out uniformly across the monsoon.

The deluge of July 2005, when Mumbai received 944mm of rain in a day, is an extreme manifestation of this trend. A subsequent extreme rainfall event in 2011 saw 1,058mm of rain in a single day over Colaba.

“This larger trend does have to do with climate change, but it would not be accurate to cite climate change as the underlying cause of Mumbai’s current wet spell without a proper rapid attribution study first,” said Sushma Nair, meteorologist with the IMD’s regional centre in the city.

“Usually, the onset period of the monsoon is very vigorous. But it is certainly the case that Mumbai has been seeing more extreme rain events in recent years,” added Nair.

Akshay Deoras, a research scientist with the University of Reading’s department of meteorology, in the UK said, “It is hard to say with confidence that the active rainfall phase in Mumbai-MMR during the monsoon’s progression is linked to climate change.”

“The offshore trough is generally vigorous when the monsoon reaches Mumbai, so it is not surprising to see heavy rainfall during a couple of days in this phase. The means number of rainy days in the region in June and September is around 14, but the region gets a lot more rainfall in June than September, indicating a skewed rainfall pattern,” Deoras added.

The intensity of monsoon rainfall — between June to September — in Mumbai city and suburbs is also intensifying at the rate of approximately 5.18mm per year, according to a recent paper -- ‘Observed trends and coherent changes in daily rainfall extremes over Greater Mumbai’ -- published in the journal Theoretical and Applied Climatology.

The paper adds that at the same time the intensity of the overall annual rainfall the city and its suburbs receive (this includes pre-monsoon and unseasonal rains) has gone up by 22 mm per year.

Rohit Mann, a researcher with the University of Kurukshetra’s geography department, said, “General patterns are becoming erratic and individual events are becoming severe. This 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 clear examples of climate change.”

These changing patterns are also highlighted in the BMC’s Mumbai climate action plan (MCAP), which warns 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.

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 localised clusters in central and western areas like Worli, Dadar, Kurla and Andheri,” the MCAP cautions.

Graphic:

HL: Rain Check

Intro: Despite its inordinately delayed arrival, the monsoon has managed to make a significant dent in the city’s rainfall deficit in just a matter of days. Here are a few numbers indicating a change in rainfall pattern.

Rain received (June 24-29) Monthly average

Santacruz 485mm 537mm

Colaba 371.4mm 542.3mm

5.188mm per year

The rate at which the monsoon rainfall is intensifying between June to September in Mumbai city and suburbs

22 mm per year.

The rate at which the intensity of the overall annual rainfall the city and its suburbs receive (this includes pre-monsoon and unseasonal rains) has gone up.

City experiences:

6

Number of heavy rain events (64.5 – 115.5mm) on an average

5

Number of very heavy rain events (115.6 – 204.4 mm) on an average

4

Number of extremely heavy rain events (> 204.5 mm) on an average

(* Based on daily rainfall time series data from 37 stations over the last 10 years.)

Each year in city:

9.5%

Of all monsoon rainfall is classified as heavy

8.7%

Of all monsoon rainfall is classified as very heavy

5.8%

Of all monsoon rainfall is classified as extremely heavy.