Is it time for the rain?
Declaring monsoon onset is careful calculation of three specific criteria
In less than a fortnight, monsoon— the lifeblood for India’s farming sector — is expected to make an onset over Kerala. But this year monsoon could play truant as an El Nino is evolving which is known to hurt the Indian summer monsoon.
The Indian Meteorological Department (IMD) declares monsoon onset over Kerala based on three features:
- If after May 10, 60% of the 13 stations in Kerala, situated in Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudlu 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
- Depth of westerlies should be maintained at a specified level
- Satellite derived Outgoing Longwave Radiation (OLR) is low. OLR represents the total radiation going to space emitted by the atmosphere or extent of cloudiness.
On May 16, the IMD forecast that the monsoon was likely to make an onset over Kerala on June 4 with model error of +/-4 days. IMD forecast a “normal” monsoon at 96% (with an error margin of +/-5%) of the long period average. The LPA for the monsoon season between June to September is 87 cm which is calculated for the period of 1971 to 2020. Private weather forecaster, Skymet Weather predicted “below normal” rainfall during the monsoon season to the tune of 94% (with an error margin of +/-5%).
IMD follows a well laid out process of identifying onset parameters before announcing monsoon onset. “The first thing that is considered is rainfall. At least 60% of stations should report more than 2.5 mm rainfall for two consecutive days. Before monsoon sets in, thunderstorms normally start. When at least 60% of these stations record rainfall, it means there is large-scale rainfall activity in Kerala and surrounding regions. But if we have only rainfall as a criterion then there is a high chance of a bogus monsoon onset,” explained M Rajeevan, former secretary, ministry of earth sciences.
“This happened in 1995 and again, in 2015. So, two other criteria are also critical. These are: a cross-equatorial flow of winds or a westerly flow at both lower and higher levels and whether there is adequate clouding over the region which is also a sign of rainfall approaching,” he elaborated.
“At present, the conditions are not good for the monsoon. There is a very large cyclone developing over the west Pacific which will most likely impede monsoon’s development because all the moisture will be concentrated there. The Somali Current also strengthens around this time as an indication of monsoon approaching but that has not happened either as yet,” added Rajeevan.
The Somali Current (SC)/Undercurrent is the western boundary current system of the Arabian Sea. Before the onset of the summer monsoon (March–May), the SC flows northward as an extension of the East African Coastal Current, according to a paper in ScienceDirect. Eventually the water from the Somali Current enters the Southwest Monsoon Current.
On May 18, private forecaster Skymet Weather also warned that cyclone development over the West Pacific during the last week of May could impede monsoon onset over India. “Such storms impact the wind pattern over hundreds of kilometres which invariably depletes moisture over India. Monsoon in India largely depends on low-level moisture transport from the Indian Ocean towards the Indian landmass. The sapping of moisture can, in turn, weaken the monsoon stream. Cyclogenesis over the Northwest and West-Central Pacific Ocean is stronger and more frequent during the weak monsoon surge over the Indian subcontinent. These typhoons also have a tendency to move northward and recurve. The overall pattern becomes detrimental to streamlining of the monsoon surge,” Skymet Weather had said in a May 18 statement.
A powerful typhoon is headed towards Guam, which could be the strongest tropical cyclone to impact the island in decades. Typhoon Mawar passed just north of Guam on May 24, bringing lightning, heavy rain, strong winds, and widespread power outages in the U.S. Pacific territory, as per a report in the New York Times.
“Typhoon Mawar is a very big system. It could move towards the Philippines. It is presently in open waters and a system of this size can pull the winds and moisture towards its core. If that happens then the cross-equatorial flow of winds is impeded. That can delay onset but we cannot say with certainty immediately,” said Mahesh Palawat, vice president, climate and meteorology at Skymet Weather.
M Rajeevan, and D S Pai, the director of Institute of Climate Change Studies, Government of Kerala, in their 2009 paper titled: ‘Summer monsoon onset over Kerala: New definition and prediction’ published in Indian Academy of Sciences Journal said: “The first rains of monsoon occur over Burma and Thailand in mid-May and subsequently extend to the northwest. The monsoon sets over Kerala around 1 June with a standard deviation of about 8 days. In association with the monsoon onset, heavy rains lash the south peninsula after the cross-equatorial low-level jet is established across the Somali coast into the near-equatorial Arabian Sea. This phenomenon is usually accompanied by the formation of a mid-troposphere shear zone across the Bay of Bengal to the south-east Arabian Sea in which a cyclonic vortex may be embedded.”
India is not new to so called ‘bogus’ monsoon onsets. These can happen due to peak pre-monsoon showers mistaken for monsoon and tropical disturbances that are unrelated to monsoon. Over the last 150 years, the date of monsoon onset over Kerala (DMOK) has varied widely, the earliest being 11 May, 1918 and the most delayed being 18 June, 1972, according to a 2017 paper in Indian Academy of Sciences.
Fasullo and Webster (2003) proposed a hydrological definition of Indian monsoon onset and the withdrawal to address this. They 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 monsoon onset over Kerala (MOK) but occurring over a smaller scale and lasting 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 Rajeevan and Pai’s paper.
In 2006, India Meteorological Department adopted new criteria for declaring MOK operationally. They then started using the three parameters.
Another interesting feature that may not be a good sign for monsoon onset is active western disturbances impacting northwest India in May end. On Wednesday, IMD said a cyclonic circulation is lying over north Pakistan in lower tropospheric levels. A trough runs from northwest Uttar Pradesh to West Bengal coast in lower tropospheric levels. These systems are likely to move across northwest India during May 23 to 26. It will be accompanied by moisture supply from Arabian Sea to northwest India during the same period and bring rainfall to the entire region.
Palawat said this was an active western disturbance, very unusual for May end which is impacting the region. This implies that the monsoon wind flow has not been established yet. “During the monsoon season, western disturbances normally do not impact the Indian region except during monsoon breaks when they can interact with other systems. Western disturbances move to northern latitudes and a southwesterly wind pattern is established during monsoon season,” said Rajeevan.
Western disturbances are extratropical storms that originate in the Mediterranean region which bring sudden winter rain to the north-western parts of the Indian sub-continent. It is a non-monsoonal precipitation pattern which is driven by the westerlies, as per the IMD.
IMD on Wednesday also said that the northern limit of southwest monsoon continues to pass through Nancowry in Nicobar. Conditions are favourable for further advance of southwest monsoon into some more parts of south Bay of Bengal, Andaman Sea and Andaman and Nicobar Islands during next two days. Next few days will tell if the careful choreography of various atmospheric and oceanic features will welcome monsoon rains over India.
The National Oceanic and Atmospheric Administration of the United States, forecast on May 11 that there is an 80% chance of El Nino establishing in May, June and July and 90% chance in June, July and August. El Nino is characterised by an unusual warming of waters in the eastern equatorial Pacific, which has a high correlation with warmer summers and weaker monsoon rains in India.
According to India’s agriculture ministry, 51% of India’s farmed area, accounting for 40% of production, is rain-fed, making the monsoon critical. With 47% of the country’s population dependent on agriculture for their livelihood (as per to this year’s Economic Survey), a bountiful monsoon has a direct correlation with a healthy rural economy.