Though Kerala has been receiving copious rain in the past few years, especially after 2018, the year that witnessed mega floods in the State, the southwest monsoon rainfall over Kerala is showing a decreasing trend while there is a noticeable increase in the northeast monsoon precipitation.
While forecasting a normal southwest monsoon for this year, the India Meteorological Department (IMD) has changed its definition of the long period average (LPA), an indication of the average rainfall over a 50-year interval.
The LPA will have to be updated every 10 years and when the average rainfall was updated as per the new norm, there was a drop of 30 mm in the southwest monsoon, which constitutes around 80% of the total rainfall in the State. Similarly, there was an increase of 0.3 mm rainfall during the northeast monsoon.
In short, despite the copious rainfall in recent years, the new normal rainfall in the State, based on data from 1971 to 2020 for the southwest monsoon, is 2,018 mm. This will replace the previous normal of 2,049 mm that was based on data from 1961 to 2010.
In the case of the country as a whole, the LPA was 890 mm from 1951-2000 and 880.6 mm from 1961-2010. And now, it is 868.8 mm while counting the 1971-2020 interval.
According to sources in the IMD, the average southwest rainfall in Kerala has been on a downward trend for the past few decades. “Although climate change and subsequent warming of oceans have a direct bearing on the performance of monsoon, the State is passing through a dry epoch, a natural cycle of 30 years in which rain will be less, and this will be followed by 30 years of a wet epoch. Then this drop will be made up in normal circumstances,” says Madhavan Rajeevan, former secretary, Ministry of Earth Science.
As per a study by an IMD scientist (Inter-annual and long term variability of rainfall in Kerala), the southwest monsoon rainfall is decreasing at the rate of 10.9 mm per decade in Kerala, while there is an increase (7.5 mm in 10 years) in the northeast monsoon rainfall. Rainfall decline is noticed more significantly in June and July.
There is also an overall increase in pre-monsoon rainfall. The year 2021 was a classic example for the change in the rain pattern. Despite the State registering a record 3,610.2 mm in 2021, highest rainfall in the past 60 years, the southwest monsoon was 16% less than than the average of 2,049 mm.
