Understanding monsoon onset date

Although expected to hit Kerala coast over the next few days, each day's changing weather conditions over the Arabian sea can have an impact over onset

The normal date for monsoon onset over Kerala is June 1, determined by IMD based on monsoon onset data during 1961-2019. But historically monsoon onset dates have had very large variation also, with some arrivals as early as mid-May and some as delayed as past mid-June.

Forecasting monsoon onset is one of the most challenging aspects of IMD’s work. (AFP)

Overall, the most delayed monsoon onset was on June 19, 1972, and the earliest on May 18 in 1990, IMD officials said.

Among the 16 El Nino years since 1950 for instance, the earliest monsoon onset took place on May 18 in 2004 while the most delayed was on June 19 in 1972, according to data with India Meteorological Department (IMD).

Monsoon onset date is typically forecast by the IMD around mid-May, and helps the farming community determine their sowing cycles for the Kharif crop. But forecasting monsoon onset is one of the most challenging aspects of IMD’s work.

The challenge is being felt this year too.

A June 4 prediction

On May 16, the IMD predicted that the monsoon was likely to make onset over Kerala on June 4 with a model error of +/-4 days. On June 4, the department issued a statement: “Conditions are becoming favourable with an increase in westerly winds over south Arabian Sea. Also, the depth of westerly winds is gradually increasing and today, the 04th June, the depth of westerlies has reached up to 2.1 km above mean sea level. The cloud mass over Southeast Arabian Sea is also increasing. We expect that these favourable conditions for monsoon onset over Kerala will further improve during the next 3-4 days. It is being monitored continuously and further updates will be provided tomorrow, the 05th June.”

Simply put, the increase in westerly winds and cloud mass were good signs signalling an onset. However, the situation had changed on June 5.

In its update on Monday, the IMD said: “At present, westerly winds over the south Arabian Sea continue to prevail. However, in view of a cyclonic circulation formed over the southeast Arabian Sea, the cloud mass is now more organized and concentrated over the same area and there has been some reduction of clouds off Kerala coasts in the last 24 hours. Also, under the influence of this cyclonic circulation, a low-pressure area is very likely to form over the same region during the next 24 hours. It is likely to move nearly northwards and intensify into a depression over southeast and adjoining east central Arabian Sea during subsequent 48 hours. In view formation and intensification of this system and its near north-ward movement likely to influence critically, advance of southwest monsoon towards Kerala coast, so IMD is monitoring all these features and will issue daily updates of both the monsoon and new system.”

In short, a depression over the Southeast Arabian Sea has led to a reduction of cloud cover—a key condition for the onset—off the Kerala coast, from where the monsoon descends into the subcontinent.

On Tuesday, June 6, the depression had intensified, and would likely lead to a cyclonic formation that could intensify into a very severe category, the IMD noted.

"Depression has formed over Southeast Arabian Sea. Latest observations indicate that a depression has formed over southeast Arabian Sea and about 920 km west-southwest of Goa, 1120 km south-southwest of Mumbai, 1160 km south of Porbander and 1520 km south of Karachi. It is likely to move nearly northwards and intensify into a cyclonic storm during next 24 hours over Eastcentral Arabian Sea & adjoining southeast Arabian Sea," IMD issued a statement on Tuesday morning.

Once the cyclone forms, it will be named Biparjoy, is expected to intensify into a very severe cyclone and is likely to move northwards towards the Oman coast according to some models.

Cyclones normally do not form during monsoon season but if they do, they can have an impact on the monsoon current depending on their location and intensity. For example, the depression has pulled all the convection and moisture towards it so the cloud cover has reduced over parts of Kerala according to IMD.

“If the cyclone heads towards Oman/Yemen as shown currently by global models, the monsoon progress may get hampered and monsoon will stall for a few days. However, if the cyclone heads towards Pakistan coast as currently shown by some models, then monsoon progress will not be much impacted,” explained Vineet Kumar Singh, researcher, Typhoon Research Center, Jeju National University, South Korea.

“The problem is that the cloud cover over Kerala has reduced due to the development of this depression over the Arabian Sea. It is one of the criteria that should be met. We are hoping that the onset criteria are met during the next 2 days. When the developing cyclone moves away from the Indian coast it can pull the monsoon current along with it leading to a very weak progression of monsoon over the Indian mainland. That can impact the sowing of crops. We are carefully watching the system now,” said Mahesh Palawat, vice president, of climate and meteorology at Skymet Weather, a private weather forecaster.

How the onset is determined

The monsoon normally sets in over Kerala around June 1. It advances northwards, usually in surges, and covers the entire country around July 15. But how does the IMD declare monsoon onset? There are three criteria:

1. If after May 10, 60% of the 13 stations in Kerala, situated in Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore in Karnataka report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala will be declared on the second day

2. Depth of westerlies should be maintained at a specified level

3. Satellite-derived Outgoing Longwave Radiation (OLR) is low. OLR represents the total radiation going to space emitted by the atmosphere or the extent of cloudiness.

“The developing cyclone can impact the monsoon currents in different ways. It will pull the monsoon current northward along its track so it could mean good progress of monsoon along the west coast. So, it could bring good rains over Kerala and Karnataka. The conditions for monsoon onset have not been met yet over Kerala. As per IMD’s long-range forecast, we can expect onset to be around June 8 to 10. As of today, conditions remain dry over many parts of Kerala,” said M Rajeevan, former secretary, ministry of earth sciences.

A large variation

‘Bogus’ monsoon onsets are not new to the Indian subcontinent. These can happen due to peak pre-monsoon showers that are mistaken for the monsoon and tropical disturbances that are unrelated to the monsoon.

Over the last 150 years, the date of monsoon onset over Kerala (DMOK) has varied widely.

According to a 2017 paper published in the journal, Indian Academy of Sciences, the earliest onset was on May 11, 1918, while the most delayed was on June 18, 1972. The paper based its findings on data sets used in previously published peer-reviewed papers.

J. Fasullo and P.J Webster published a paper titled 'A hydrological definition of Indian monsoon onset and withdrawal' in the journal American Meteorological Society in 2003 and argued that rainfall over Kerala may be susceptible to ‘false’ or ‘bogus’ monsoon onsets, which are associated with propagating tropical intra-seasonal disturbances unrelated to the monsoon onset.

The disturbances are characterised by an enhancement of convection and westerly surface winds similar to the MOK but occur over a smaller scale and last for a smaller duration (a week or less). Often bogus onsets are followed immediately by extended periods of weak winds and clear skies that result in heat waves and droughts in India, according to a paper by M Rajeevan, former secretary, ministry of earth sciences and DS Pai, IMD’s Environment Monitoring and Research Centre (EMRC) head.

“A delayed onset doesn’t determine total seasonal monsoon rainfall. This is because monsoon has no memory of when it made onset. We know that monsoon season begins on June 1 and lasts till September 30 but this memory is not with monsoon. Monsoon quantum is determined by other largescale oceanic and atmospheric features such as El Niño-Southern Oscillation, state of Atlantic Ocean and Indian Ocean. In 1997 which was an El Nino year, there was good monsoon rainfall. It was a unique year and experts then had said monsoon turned to be normal in 1997 because of a positive Indian Ocean Dipole (IOD). But every El Nino year will not be like 1997. This year being an El Nino year, we can expect monsoon will be subdued,” Rajeevan said.