It is almost certain that the onset of the 2016 monsoon can be declared today (29th May). Based on rainfall alone, the criteria for “onset” are fulfilled. However the formal Monsoon Onset over Kerala (MOK) is still a subjective call. Attempts are ongoing to establish objective criteria.
The date of the MOK is declared by IMD every year based on subjective estimates prepared by operational forecasters. …….
‘‘Although the onset of monsoon is associated with changes in the atmospheric circulation features in the lower and upper troposphere, a sustained increase in the rainfall at the observatory stations of Kerala and the island stations over the south-east Arabian sea is an essential feature of the monsoon onset. It is difficult to quantify these precisely and so the experience of the forecaster plays a key role in declaring the date of monsoon onset in individual years.’’
The IMD forecaster, while declaring the date of the MOK, has been taking into consideration subjectively the following features
- the rainfall should be widespread over Kerala and adjacent areas, with large rainfall amounts at individual stations;
- this rainfall persists over several days;
- the lower-tropospheric westerlies in and around Kerala should be strong and deep; and
- the air should be rich in moisture (high relative humidity) up to at least 500 hPa. However, there is no widely accepted objective definition of the MOK.
There also have been attempts to derive objective methods for the MOK. …..
at least 60% of the 14 weather stations across Kerala and coastal Karnataka should record 2.5 mm rainfall or more for two consecutive days. ….. Simultaneously, the depth of the westerly winds should be up to 600 hPa (or 12000 ft high), from the equator to 10°N Latitude, and between Longitude 55°E and 80°E. The zonal wind speed over the area bounded by Latitude 5-10°N and Longitude 70-80°E should be around 25 to 35 kmph in the lower levels. The OLR value should also be less than 200 Wm-2 in the box confined by Latitude 5-10°N and Longitude 70-75°E.
The rainfall criteria have been met today. Skymet writes:
…… we watch out for more than 2.5 mm of rainfall for consecutive two days in at least 60% of the stipulated 14 weather stations across Kerala, coastal Karnataka and Lakshadweep. These stations are Minicoy, Aminidivi, Kollam, Thiruvananthapuram, Kannur, Punalur, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kasargode and Mangalore.
As of now, more than 60% of the available 14 stations have already received more than 2.5 mm of rain for the last 2 days. As for the masses, Monsoon-like rainfall activity has already commenced in the region. The following rainfall figures (in millimeters) recorded over the last 3 days suffice for the above statement.
So far, so good.
It seems highly probable that the formal declaration of the onset of the monsoon over Kerala (MOK) will be declared for today or tomorrow. The signs are encouraging that 2016 monsoon rainfall will be classified as “good” (upto 10% above the long-term average) and may even be declared to be in “excess” (>10% above long term average).