Why rain has swept Mumbai early this year and how it will influence monsoon onset

The ongoing spell of showers has made for an unusual May for the region, which is typically associated with scorching temperatures and heatwave days. This year, Mumbai started receiving intermittent spells of heavy showers coupled with thunderstorms and gusty winds as early as May 6. In what has made it the city’s wettest May month since 2021, the Colaba coastal observatory has already recorded 85.2 mm of rainfall, and the Santacruz station has logged 47.4 mm until Monday morning.

This also spurred a dip in temperatures, with the city even waking up to its coldest May day since 1951 on May 8 when the minimum temperatures touched 22.2 degree Celsius at the Colaba coastal station.

After receiving heavy rain between May 6 and May 11, the city witnessed a break from showers even as it continued to receive a drizzle during the early hours amid a yellow alert. Meanwhile, other regions of Maharashtra continued to experience heavy rains, thundershowers and squally winds.

Why is Mumbai receiving early showers this year?

Mumbai is no stranger to unseasonal showers during the summer months — a phenomenon which scientists typically attribute to the interaction of winds. What makes this year an anomaly, however, is the fact that the showers, which set in very early, have been prolonged, albeit sparse, and continued throughout the past two weeks.

Shubhangi Bhute, Director, IMD Mumbai, said the region experienced early showers between May 6 to May 8 owing to an upper-level trough, formed owing to the presence of a western disturbance system. However, Bhute said the actual pre-monsoon showers only kicked off after May 13.

“The showers which we witnessed in the second week of May were system-induced. Technically, the showers which the region has been experiencing since May 13 can be attributed as pre-monsoon showers, as that was when the south-west monsoon advanced into parts of the South Bay of Bengal, Andaman Sea, and the Nicobar islands,” said Bhute.

This is stark contrast to the general pre-monsoon showers which set off in the city towards the fag end of May.

Bhute attributed the favourable monsoon conditions to neutral El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). “Presently, neutral El Nino as well as neutral IOD conditions are present over the Indian Ocean. These neutral conditions may contribute to good monsoon spells and also are likely to result in favourable conditions of early monsoon onset,” said Bhute.

ENSO is an ocean-atmospheric phenomenon that is considered one of the most important recurring natural events. It influences the Indian monsoon between June and September.

Meanwhile, weather experts, including Athreya Shetty, also pointed to the early heatwaves this year, which set off as early as February.

“Whenever the heat spikes early, we tend to see early pre-monsoon in those years. Besides this, Mumbai generally receives dry weather owing to anticyclone systems, which create a high-pressure area and lead to an influx of hot winds from the deserts in the Northwestern region. However, this year, the strong western disturbance system, early in May, weakened the anticyclone, further killing its effect on warm conditions,” said Shetty.

What does this mean for monsoon onset in the Mumbai region?

With the pre-monsoon showers setting in early, the IMD Mumbai director told The Indian Express that the Mumbai region may experience an early monsoon onset this year. Typically, the official date of monsoon in Mumbai is June 11.

As per current projections, the Southwest Monsoon is expected to enter Kerala by May 28, which is much before the normal. After this, it generally takes between 8 to 10 days for the Southwest Monsoon to reach and surpass Maharashtra.

According to Shetty, Mumbai may see the onset of monsoon as early as the first week of June, this season.

Meanwhile, the IMD said in its long-range forecast that this year, the country is expected to experience above normal monsoon, which is forecast to be “105% of the Long Period Average (LPA) with a model error of ± 5%”. The Met Department has pointed out that the neutral El Nino and IOD conditions are slated to prevail during the monsoon, which will contribute to a bountiful season.