The arrival of monsoon over the Kerala coast will likely be delayed by a few days, the India Meteorological Department said on Tuesday. Instead of its normal date of June 1, the monsoon is expected to arrive by June 4. The IMD said its forecast had an error margin of four days, so a further delay was possible.
The IMD forecast is likely to fuel apprehensions of a bad monsoon, but the two events are not connected. The timing of the onset of monsoon over Kerala has no bearing on the quantity of rainfall during the season.
Onset
The onset of monsoon over the Kerala coast is just a marker of the official beginning of the four-month monsoon season that brings more than 75 per cent of India’s annual rainfall. An early or delayed onset over Kerala does not in any way dictate monsoon performance over the rest of the season.
In fact, the onset over the Kerala coast is not even the start of the monsoon rains over India. The Andaman and Nicobar Islands start getting the rains at least two weeks before Kerala does. Sometimes, even parts of Kerala get rains for a few days before the onset is officially announced.
The onset is just a declaration that a set of criteria defined by the IMD to identify the arrival of the monsoon over the Indian mainland has been met. For this purpose, the IMD tracks rains at 14 selected meteorological stations on the Kerala coast. If at least 60 per cent of these stations, that is at least nine of them, get at least 2.5 mm rainfall for two consecutive days anytime after May 10, the onset is said to have occurred. There are a few other accompanying criteria, relating to wind speed and pressure, which need to be satisfied.
The monsoon then moves northwards, the speed of its movement dictated by local atmospheric conditions and the development of low-pressure areas. The normal arrival dates at different locations in the country are known, but the progression of monsoon does not always follow these dates. Late arrival over Kerala sometimes delays the arrival at some other locations as well, but not always.
Onset and rainfall quantity
In the past five years, only once has the onset over Kerala happened on the normal date, in 2020. In the past 11 years, it has happened once more. On other occasions, the onset was either a few days earlier or later. But this has no correlation with the amount of rainfall during the monsoon season.
In 2016, for example, the onset happened on June 8, the most delayed in the past 12 years. The rainfall that year was normal, 97.5 per cent of the long period average (see table). In 2018, the monsoon arrived early, on May 29. Rainfall was deficient that year — only 90 per cent of the long period average.
Prospects of El Nino for India in 2023
While the four-day delay in the onset of the monsoon is not of concern, the real worry is the unfolding situation in the equatorial Pacific Ocean. Recent forecasts suggest that El Nino, the warming of the equatorial Pacific Ocean that impacts weather events worldwide, is likely to develop earlier than expected. El Nino has the effect of suppressing the Indian monsoon rainfall.
The IMD, in its first forecast for this year’s monsoon last month, predicted normal rainfall for the season. But since then, the situation in the Pacific has changed considerably. Latest forecasts predict the development of El Nino in the May-July period itself. In an alert on May 11, the Climate Prediction Centre (CPC) of the National Weather Service in the United States said El Nino was expected to develop “in the next couple of months”, and that there was “a greater than 90 per cent chance” that it would persist through the rest of the year.
The CPC analysis said there was a 10 per cent probability of this developing into a strong El Nino in the June to August period. This probability rises to about 50 per cent for the September to November period.
Some other scientists have been warning of the possibility of a very strong El Nino this year, which also means that global temperatures could be expected to rise.
The IMD is expected to update its forecast for the monsoon later this month, and that would include an assessment of the kind of impact the El Nino is likely to have on the season’s rainfall.
Increasing variability
The performance of the monsoon is usually measured by one indicator — the amount of rainfall the country as a whole received in the entire season as a percentage of long period average (the average of 1961 to 2010). A deviation of 4 per cent, in either direction, is categorised as normal. That means that since 2009, there have been only three years when monsoon rainfall has been below normal — in 2014, 2015 and 2018. In the other 10, the rainfall has been normal or excess.
But that single indicator hides an increasing variability in rainfall over the country, both in spatial and temporal terms. Within the season, the number of rainy days is getting concentrated — fewer days accounting for a larger proportion of seasonal rainfall — while the dry spells have been increasing in length. Simultaneously, different regions have been receiving very different rainfall. So, while India as a whole had below normal rainfall on only three occasions since 2009, northeastern India had it on nine occasions while northwestern India had it five times.
Floods and droughts have been happening at the same time. Extreme rainfall events have been on an increase, and under the impact of climate change, such patterns are expected to get exacerbated.
That is why it is largely immaterial whether the country as a whole receives normal rainfall or not. The regional variations in rainfall and extreme rainfall events are more effective indicators to gauge how normal the monsoon season is.
The normal date for the onset of monsoon over the Kerala coast is June 1. (Express photo by Nirmal Harindran/Archive) 
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