Incessant rains have been lashing the West Coast including Kerala. For me, Monsoon has well arrived and it’s just a formal declaration that needs to be notarized.
The Monsoon buzz is going louder and larger with every passing moment. As and when we approach the onset phase of Southwest Monsoon, it often becomes the most debatable topic at the high tea. More so, after two back-to-back droughts.
This year seems nothing contrasting. This compels me to come to a settlement about the onset of Monsoon 2016. So, for me it looks like Monsoon is here and has entered the Kerala coast. Skymet had predicted the onset of Monsoon by May 30 this year. Yet, it has not been declared by the governing authority till now. This has raised a ‘hush hush’ noise in the industry. I would like to say that the most important manifestation of Monsoon arrival, i.e. rainfall, is being observed in sufficient amounts. In fact, the rainfall criterion has been met or rather ‘over-met’!
Onset of Monsoon is functional with some parameters and should be declared only after conforming to these standards. Sometimes compliance with all the parameters simultaneously to the copybook effect is difficult to attain. Onset parameters are a set of objective values but mostly seen getting applied in a subjective manner. A similar situation is being observed this time around.
The table below will clear our doubts about rainfall in millimeters:
The rainfall criterion mentions that 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. Rainfall figures exceed our requirement in the weather stations of Minicoy, Amini, Kollam, Thiruvananthapuram, Kannur, Punalur, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kudlu and Mangalore.
The second requirement is the depth of the westerly winds, which 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. Yes, available data suggests that this benchmark has been met.
The zonal wind speed over the area should be in the range of 15 to 20 knots in the lower levels. Take a look at the wind speed since May 24:
Now coming to the outgoing longwave radiation (OLR), which should be less than 200 Wm-2 in the box confined by Latitude 5-10°N and Longitude 70-75°E.
Rains don’t happen without clouds. Rather they (rains) are a by-product of atmospheric dynamics governed by equatorial winds.
A look at the OLR values since a week will conform the last bet as well.
The OLR value is more susceptible to changes and is likely to be more variable than the other factors. This is because no system can cause a densely overcast sky for a long duration during the onset phase, so to keep the outgoing radiation below the 200 Wm-2 mark. These values have differed the onset criterion in the past as well. Nevertheless, it’s worth noting that the OLR value has remained well around the threshold.
There have been views expressed in the media that the cross-equatorial flow of westerly winds is essential for the establishment of Monsoon. Last year, during the onset of Monsoon on June 5 the cross-equatorial flow was not seen throughout the ITCZ. The 'C' was formed but the magnitude was not that strong.
This year as well, the 'C' has been formed though the magnitude is not very strong. In any case, the equatorial flow is not consistent and continuous all along the Inter Tropical Convergence Zone (ITCZ). In fact, this trough of ITCZ is a difficult nut to crack even for hard core meteorologists across the world.
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Please Note: Any information picked from here must be attributed to skymetweather.com
