This year the appearance of the depression in the Bay of Bengal has set the base for Monsoon 2016. It’s now knocking at the Bay Islands, a little before time, proceeding further towards the Kerala coast. Furthermore, promising signs of another weather system in the Arabian Sea will enhance rainfall activities along the West Coast in next few days. Once again, a good indicator for timely arrival of Monsoon 2016.
Nevertheless, the national weather forecasting agency, India Meteorological Department (IMD) recently released a report that onset of Monsoon 2016 will be delayed by about a week. This situation obliges me to put down my thoughts on the timely arrival of Monsoon this year.
The onset of Monsoon, just like everything else related to it, is a complex phenomenon involving research for weeks or even months. Before declaring the onset of Monsoon in Kerala we should follow certain guidelines, which revolve around changes in rainfall and wind pattern, humidity and outgoing longwave radiation (OLR).
First and foremost, as we approach the normal onset date 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. These stations are Minicoy, Amini, Kollam, Thiruvananthapuram, Kannur, Punalur, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kudlu and Mangalore.
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.
As soon as the above-mentioned criteria are met at a stipulated time of the year, we should technically declare the onset of Monsoon. As per Skymet Weather, all these factors will fall in place between the May 28 and 30. We at Skymet Weather, have sufficient reasons to substantiate this statement. In fact, the rainfall and wind pattern graphs available with Skymet Weather also seem to indicate the timely arrival of Southwest Monsoon.
As per the rainfall models, there will be two peak rainfall stages in the coming days: one around May 28-30 and the next one between June 5 and 6. The rainfall during the first peak will definitely be a shade lower than the second one. Nevertheless, the first peak will bring sufficient rains to meet the Monsoon onset criteria. The grey portion of the graph is the average rainfall forecast (in millimeters) from May 12 to June 11 over the region falling under the box of Longitude 75-77.5°E and Latitude 8-10°N. The bluish part below indicates the departure of rainfall figures from the mean. This clearly explains that the rainfall criteria will be easily met.
Wind pattern will also meet the desire requirements during the first peak phase. And in the presence of clouds and rain, the OLR will definitely be less than 200 Wm-2. IMD is probably confiding in the second rainfall peak around June 5-6 for likely onset of Monsoon 2016.
Now, we come to Skymet Weather’s 15-day rainfall forecast. The animated graphics below show that the ongoing spell of heavy showers in Tamil and Kerala will be closely followed by good rains along the West Coast.
The weather system in the Arabian Sea seems to be persistently enhancing the rainfall activity along the West Coast as we approach the normal onset date of Monsoon in Kerala. Even if we start observing the rains from May 26 itself, the onset criteria will be met by May 28. Giving some consolation to weather models (generally having limitations), Skymet Weather believes that Monsoon will lash Kerala by the predicted dates between May 28 and 30.
The spoilsport El Niño is also declining. During the last fortnight, Nino 3.4 (region dominating Monsoon in India) witnessed a drop of 0.5°C, from 1.1°C on April 25 to 0.6°C to the present date.
Also read, El Niño draws closer to threshold neutral
There are high chances that the onset of Southwest Monsoon in mainland of India will coincide with El Niño reaching the threshold neutral stage. The in-built complex characteristics of Southwest Monsoon are also influenced by external oceanic-atmospheric phenomena like Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO). IOD will remain neutral for now and MJO will also traverse through the favorable zones of eastern Indian Ocean. Therefore, I think that the onset of Monsoon will not be hampered by El Niño, IOD or MJO.
Featured image credit - indianexpress.com