Why Southwest Monsoon hit India sooner than usual?

Hyderabad: Summer season in Telangana comes to an end with rains giving a much needed respite from the heat. The early arrival of the Southwest Monsoon can be termed responsible for this break.

The Southwest Monsoon usually sets in Kerala on June 1, and it advances over most parts of Northeast India by June 5. However, this year monsoon has set in over Kerala eight days before the usual date and in Mizoram, 12 days before the usual date.

The Indian Meteorological Department (IMD) has announced an early onset of monsoon in Kerala on May 24, first in 16 years. Last time the Southwest Monsoon set in this early was in 2009 on May 23.

Favourable atmospheric and oceanic conditions

The IMD stated that the early onset of the Southwest monsoon was driven by favourable atmospheric and oceanic conditions. A low-pressure area which developed over the Arabian Sea and a trough line stretching across Vidarbha enhanced moisture flow and atmospheric convection, helping the monsoon advance faster.

According to the Indian Express, fast-moving Madden-Julian Oscillation (MJO) wind bands and changes in the intensity of the high-pressure area around the Mascarene Islands in the South Indian Ocean have contributed to heavy rainfall along India’s west coast.

The onset of monsoon over Kerala and its further advance over the country was declared following some guidelines.

a) Rainfall

After May 10, 60 per cent of the available 14 stations report rainfall of 2.5 mm or more for two consecutive days. These 14 stations are: Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore.

The onset over Kerala can be declared on the second day if the following two conditions are also met.

b) Wind field

The depth of westerlies, winds which blow from west to east in the 30° to 60° latitudes of both hemispheres, should extend up to 600 hectopascals or hPa (a measure of atmospheric pressure).

Additionally, wind speeds at 925 hPa should range between 15–20 knots (27–37 km/h) in the region between 5°N–10°N latitude and 70°E–80°E longitude.

c) Outgoing Longwave Radiation (OLR)

The Sun heats the Earth, and the Earth, in turn, releases some of that heat back into the atmosphere and space. OLR is the terrestrial radiation released into space. Cloudy, rainy, or stormy areas emit less OLR, because clouds block the heat from escaping. Lower OLR values indicate dense cloud cover and active convection, both of which are signs of monsoon conditions.

OLR values must be below 200 wm-2 in the region between 5ºN - 10ºN Latitudes and 70ºE - 75ºE Longitudes for the Indian Southwest Monsoon.

On May 22 and 23, clouds increased over the Southeast Arabian Sea and adjoining peninsular India, with OLR being less than 200w/m on May 24. The depth of westerlies over the South Arabian Sea extended up to 4.5 km above mean sea level. The strength of the Westerlies in the lower levels is about 20-30 knots.

There has been widespread rainfall in Kerala on May 23 and May 24, satisfying the criterion for the declaration of the onset of Southwest monsoon over the State.