Vortex: A tiny weather system that interfered with Mumbai monsoon
After a dry spell at the beginning of the week, rainfall activity intensified in Mumbai on Wednesday morning and moderate showers continued through the day. The Santacruz station of the India Meteorological Department (IMD) — which is indicative of the city — received 5.5mm rainfall between 8.30am and 5.30pm on Wednesday while the Colaba station, representative of the island city, recorded 37.6mm rainfall in the same period.
A small low pressure system called a vortex formed towards the north of the city’s coast made conditions more conducive for rainfall on Wednesday. It amplified the southwesterly winds, resulting in more rain.
The city clocked in a total 89.6mm of rainfall in the 24 hours ending 8:30am on Wednesday. Mumbai has so far seen 1,259.3mm rain since June 1, which is about 48% in excess of the seasonal normal for the period. However, in the first 12 days of July, the city recorded 148.3mm of rain, which is -55% of the normal (332.9mm).
IMD had anticipated as much as 200mm of rainfall in about 24 hours between Sunday and Tuesday. However, on Monday, the city received only 2.8mm of rainfall as most of the rainfall had been deposited in the sea. Incidentally, a similar vortex had moved clouds away from the city earlier this week, resulting in little rainfall inland.
So, what is this weather phenomenon that interrupted or meddled with the southwest monsoon rains?
Usually, during the monsoon season, southwesterly winds in the lower part of the atmosphere push clouds and rain from the Arabian Sea towards land. While this is the ideal condition for monsoon activities, some low-pressure activities near the coast can interfere with the wind movement.
Akshay Deoras, an independent meteorologist and PhD student at the University of Reading, England, said, “Imagine mixing coffee in a cup of milk with a spoon. You can see the coffee swirl in the cup. This is what a vortex would look like in the atmosphere if we were able to see it straightaway. It is a small low-pressure circulation that is powerful enough to interfere with southwesterly winds.”
The location of these vortices and their subsequent interaction with southwesterly winds can have an impact on how much rain the city gets.
A vortex forms when there is a significant upward motion of the air over a region, for example hot air rising from the sea. In such a situation, the air from surrounding regions rushes to this region and the effect of earth’s rotation adds a swirling effect.
The most common reason for the formation of a vortex is low pressure along the west coast. Usually, low pressures occur when hot air over the sea rises and wind starts blowing from the land into the sea. This creates a vortex of air swirling along the coast.
On Sunday, even as clouds formed along the coast, a vortex disturbed the wind pattern, blowing clouds away from the city, said Shubhangi Bhute, scientist at IMD Mumbai.
“Incidentally, Raigad received a decent amount of rainfall as the clouds moved south on Sunday and Monday,” Bhute added.
However, on Wednesday, as the vortex moved towards the north-end of Mumbai’s coast, it amplified the southwesterly winds, thereby increasing rainfall activity.
Estimating where the vortex can form can help predict heavy rain spells in Mumbai, said Deoras. “One can well predict such heavy rains in Mumbai by estimating where a vortex can form. However, numerical weather prediction models, which can well predict large-scale features of the monsoon, are often not able to well simulate the location and intensity of small vortices, resulting in forecasting failures,” he added.