Kochi: The onset of the southwest monsoon over India remains uncertain as two critical parameters for its declaration have not been met, even after the initial predicted date of May 26 has passed. The India Meteorological Department (IMD) relies on sustained westerly winds and specific rainfall thresholds at 14 designated manual rain gauge stations to officially announce the monsoon's arrival.
Westerly Winds Hindered by Cyclonic Circulation
The first key factor is the establishment of sustained westerly winds, which transport moisture from Madagascar and drive the southwest monsoon toward India. However, these winds have been disrupted by an upper air cyclonic circulation persisting over the southeast Arabian Sea near the Lakshadweep region. This circulation causes frequent shifts in wind direction, alternating between westerly, easterly, and other directions, making it difficult to confirm a stable westerly flow. An IMD official explained that the winds must be persistent and well-marked, and clarity will emerge only after the circulation pattern dissipates.
Rainfall Distribution Criteria Unfulfilled
The second criterion involves rainfall distribution across 14 stations located in Lakshadweep, Kerala, and Mangalore. According to IMD parameters, at least eight to nine of these stations must record 2.5 millimeters or more of rainfall for two consecutive days after May 10. Currently, about 60 percent of the stations have not yet met this threshold. The stations include Amini and Minicoy in Lakshadweep; Thiruvananthapuram, Punalur, Kollam, Kottayam, Alappuzha, Kochi, Vellanikkara, Kozhikode, Thalassery, Kannur, and Kudlu in Kasargod, Kerala; and Mangaluru in Karnataka.
Historical Context and Future Adjustments
Earlier, rainfall was the sole criterion for declaring the monsoon onset. The inclusion of westerly wind criteria came with the advent of satellite-based weather forecasting, which improved accuracy. However, the official noted that climate change may necessitate further revisions to these criteria. Although the normal onset date for the southwest monsoon is June 1, a standard deviation of seven days is considered normal, meaning the delay is not unprecedented.
As the conditions remain unresolved, the official declaration of the monsoon onset continues to be in limbo, with meteorologists closely monitoring the evolving weather patterns.
