Opinion Why have rains subsided in Mumbai after a historic early onset?

However, since then, monsoon activity has receded considerably. Data from the Indian Meteorological Department (IMD) show that the Santacruz station received only 9.2 mm of rain in the first three days of June. The Colaba station registered only 8.2 mm rainfall, nearly 10 mm below the normal for the period.

Typically, Mumbai receives an average of 537 millimetres of rainfall in June, the month it experiences the onset of monsoon.

Why is there a lull in monsoon showers?

Heavy spells of rain during the monsoon in Mumbai are generally influenced by factors such as strong westerly winds, which usher in moisture and build convection. Weather systems like the monsoon trough or the Somali Jet also play a role.

For the record, the monsoon trough is an elongated low-pressure area which extends from the heat low over Pakistan to the Bay of Bengal. It is a semi-permanent feature of monsoon circulation, and its southward movement results in heavy downpours.

The Somali jet is the low-level, inter-hemispheric and cross-equatorial flow of air, which moves further into eastern Africa by May, followed by arrival into the Arabian Sea. It reaches the west coast of India in June. The strengthening of the Somali Jet is another factor that contributes to heavy rains.

However, currently, none of the systems that trigger monsoon currents are active in the region. “Presently, there is no active system like strong westerlies, offshore troughs or even a strong Somali Jet, which can lead to heavy rains. Therefore, the city is only experiencing on-and-off showers for the past few days owing to the presence of moisture and onset of monsoon,” said an IMD scientist.

Mumbai-based Rajesh Kapadia, who runs the Vagaries of Weather blog, said that the low-pressure area (LPA), which had resulted in heavy showers between May 24 and May 27, has also fizzled out. “Since this LPA is no longer persisting and we have no other supporting system such as a monsoon trough, we are only passing showers at present,” added Kapadia.

According to meteorologists from the weather bureau, the southwest monsoon is characterised by dry and wet spells, which are phases influenced by the presence of active systems. Last year, too, the city experienced a lull in showers in June after an early monsoon onset on June 9.

When can Mumbai expect a resurgence in rains?

Indicating no immediate revival of heavy showers, the IMD has stated that the city and its neighbouring districts will continue to experience only light showers for the next five days. No alerts have been sounded for the region until at least June 7. “There is no major system at present which indicates that we will receive heavy showers immediately. We are closely observing our models to forecast the intensifying rains,” said an IMD official.

The region could see a revival of active monsoon after June 7. “The break in monsoon conditions may prevail till at least June 9. However, starting June 7, we can expect some thundershowers in the interiors of Maharashtra. Meanwhile, monsoon may gain strength from June 7 or June 8 onwards as well,” said Kapadia.

He added that an unseasonal ‘ridge’ is prevailing currently, which is the opposite of a trough that is characterised by a low pressure area. However, a trough is expected to develop along the west coast by June 8.

The IMD’s long-range forecast bulletin has said that the city, as well as most parts of the country, will continue to receive “above normal” rainfall in June. It will also usher in a dip in temperatures, which will likely remain in the normal to below-normal range.