The onset of the monsoon over Kerala was predicted to unfold on 26 May, with a model error of +/- three days. However, there was no sign of the seasonal weather system approaching the coast as of Thursday, say experts.
After a scorching summer marked by never-before-seen heatwaves and deficit rainfall, reports of the early onset of the southwest monsoon had come as a relief to the southern state. Experts had hoped that an early onset would likely rectify what global warming has unleashed in the recent years – delayed monsoons.
The Quint spoke to experts to understand what repercussions this delay could possibly have on weather patterns.
Onset of Monsoon over Kerala & Karnataka May Be Delayed Until First Week of June
The India Meteorological Department (IMD) considers a range of factors, such as rainfall, wind field, and Outgoing Longwave Radiation (OLR), before declaring the onset of monsoon.
First, cyclones that occur just before the monsoon can impact normal circulation and may actually push the onset back, say experts. However, cyclones occurring just a few days before the scheduled arrival of the monsoon, like cyclone Asani, could facilitate the drawing of moisture, aiding in the early onset.
On 12 May, the IMD predicted the "advance of the Southwest Monsoon into the south Andaman Sea and adjoining southeast Bay of Bengal around 15 May 2022." On 19 May, the IMD said that the onset over Kerala was possible by 25 May.
For the monsoon onset to be declared, at least 60% of the designated 14 stations enlisted in Kerala must report rainfall of 2.5 mm or more for two consecutive days. The 14 stations are Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu, and Mangaluru. As of 26 May, only 33% of the stations in Kerala reported significant rainfall.
Over the last 50 years, the date of onset in Kerala has ranged from 19 May 1990 to 18 June 1972. This variance is largely attributed to the state of the ENSO – El Niño–Southern Oscillation – in the preceding winter. Over the past decade, this onset of monsoon in 'God's Own Country' has occurred on or around 1 June.
"We had predicted that the impact of El Nino would have favoured an early onset. But there is no delay as well. There is no reason for concern," said Raghu Murtugudde, a professor of CMNS-Atmospheric & Oceanic Science, University of Maryland, and also a visiting professor at IIT Bombay.
Due to the prevailing weak La Niña conditions, there is a chance that the onset and subsequent progress of the monsoon over Kerala and Karnataka may be delayed until the first week of June.
The yellow alert for various districts till May 29 has been withdrawn by the IMD. However, there is a possibility of heavy rains in a few pockets in Kerala.
Lack of Constant Stream of the Westerlies Across the Arabian Sea Failed To Aid in Onset
A key factor that influences the southwest monsoon is the Madden-Julian Oscillation (MJO) – a tropical wave that travels around the equator every 30-40 days – that modulates the precipitation patterns in India. This cross-equatorial flow of monsoon winds is currently quite strong over the Arabian Sea but did not intensify further north which could've aided the monsoon progression over land. So, most rainfall activity seems to be concentrated over the Arabian Sea and not over the land areas, experts opined.
"The cyclone Asani had expedited the cross-equatorial flow. It was rapid and had strengthened over a few days and so we saw unusual behaviour of heavy rainfall around 16 May. This system was expected to move but stayed in the same place. We need to have a constant stream of the westerlies across the Arabian sea that will aid in the onset."GP Sharma, President, Meteorology and Climate at Skymet Weather
He also explained that the Indian Ocean Dipole (IOD) index which has become more negative over the past few days seems to have been responsible for weakening the monsoon even when it was expected to advance.
"This means that the western part of the Arabian sea that is closer to Kerala is cooler than the side towards Indonesia. A cool sea surface is not good for rains and so the rainfall will be a little suppressed this year, especially in South India,” he added.
Summer Saw Soaring Temperatures, Deficit Rainfall
One reason for this is the ongoing westerly desert winds coming in from the Arabian Peninsula that usually causes heatwaves during the summer in India, said Raghu Murtugudde. Since 11 March, 16 Indian states and Union territories have suffered from heatwaves. Gurugram recorded a temperature of 48.1 degrees celsius on 15 May, the highest since 1966.
March 2022 was the warmest March month in the last 122 years and April 2022 was the fourth-warmest April month in the same time period.
Soaring temperatures, combined with a complete lack of rainfall, led to a 30 percent increase in forest fires in April in Uttarakhand as compared to the same period last year. The cumulative deficit in rainfall for northwest and central India between 1 March and 28 April was 87 percent and 70 percent respectively.
This deficit in rainfall is due to the lack of rain-bearing systems, especially western disturbances, which usually bring rainfall in the pre-monsoon season, said Mrutunjay Mohapatra, director-general of the IMD, during a press conference on 30 April.
"Monsoons have become extreme in the past few years because of the way the Arabian sea has warmed up. This year's pre monsoon cyclone activity has also been quite low, which made the heatwave much worse. Heatwaves were due to a mixture of La Nina pressure patterns, winds blowing from the Middle East towards India and a vapour pressure deficit. This led to crop yields going down by over 30%."Raghu Murtugudde, a professor of CMNS-Atmospheric & Oceanic Science, University of Maryland
"So a good rainfall will be a blessing and also the prayer is that the rainfall should be uniform and shouldn't wreak havoc in just one area," he added.
However, it is to be noted that it was certainly much wetter than normal for Kerala during the pre-monsoon season. Between 1 March to 25 May, the state had collectively recorded 615.1 mm precipitation, which is a 108% 'large excess' compared to its long-term average for this period (295.4 mm).
Now, this raises the question – will Kerala face the wrath of torrential rains leading to a flood-like situation like in the past few years? Experts have indicated that the monsoon's performance this year is likely to be normal.