Poor winds, rainfall impact declaration of southwest monsoon onset

Kochi: Amid the confusion over the onset of southwest (SW) monsoon, and the passing of the initial prediction date of May 26, two parameters used to determine the arrival of the monsoon over India remain unmet.

Sustained westerly winds and the requisite rainfall recorded in 14 designated manual rain gauge stations determine the onset of the SW monsoon in India.

The first key factor is the establishment of sustained westerly winds, which carry vast amounts of moisture from Madagascar and drive the SW monsoon to the country. However, these winds have been hindered by an upper air cyclonic circulation persisting over the southeast Arabian Sea adjoining the Lakshadweep area.

The second criterion for the official declaration of the monsoon onset relates to rainfall distribution. As per IMD parameters, at least eight to nine out of 14 stations must record 2.5mm rainfall and above consecutively for two days after May 10. With both the wind and rainfall parameters yet to be fully satisfied, the official declaration of the onset of the SW monsoon remains in limbo.

The 14 stations used to assess the rainfall distribution are located in Lakshadweep, Kerala and Mangalore. According to meteorologists, the presence of sustained westerly winds is a crucial requirement for declaring the onset. “The westerly winds have to be persistent and well-marked. However, the prevailing upper air cyclonic circulation is causing frequent shifts in the directions, with winds alternating between westerly, easterly and other directions. This makes it difficult to ascertain whether a stable westerly wind has been established or not. It will be clear once the circulation pattern is over,” said an IMD official.

The rain gauge stations are located in Amini and Minicoy in Lakshadweep; Thiruvananthapuram, Punalur, Kollam, Kottayam, Alappuzha, Kochi, Vellanikkara, Kozhikode, Thalassery, Kannur, Kudlu in Kasargod; and Mangaluru in Karnataka. “Around 60% of the 14 stations are yet to record 2.5mm rainfall and above consecutively for two days. Earlier, rainfall was the only criterion for declaring the SW monsoon onset. Later, westerly wind was included in the criteria once satellites were used for weather forecasts which increased accuracy. With climate change, we will have to change the criteria for weather forecasts,” said the official.

Though the normal SW monsoon onset is June 1, it has standard deviations of seven days.