Is Kerala undergoing a change in Monsoon Rainfall Pattern?

COMK Analysis
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Humans have looked at the skies and starts from time immemorial to understand time. Sun has been his companion for many millennia to keep a sense of time. In a way the predictable and regular cycle of Sun allowed humans to develop calendars through which they started to keep a tab of seasons / rainfall etc. This also meant humans attached dates to these natural events thereby creating an easier way of keeping tab of rainfall cycles / monsoons etc. A R Rahman fans will never forget the mellifluous Anjali Anjali from the film Duet sung by the evergreen SPB.

கார்த்திகை மாதம் போனால்…கடும் மழை இல்லையே… கண்மணி நீயில்லையேல்…கவிதைகள் இல்லையே… Vairamuthu uses a weather event as a metaphor to convey his feelings. Similarly we have a lot of ancient sayings which relate weather phenomenon to times of the year. Even IMD uses 1st June as the traditional onset date of Monsoon over Kerala though over time we have now come to realize monsoon travels at its own pace based on large global scale factors which influence the development and movement of the monsoon train.

While Southwest Monsoon has always had intra seasonal oscillation bringing about peaks and troughs of rainfall episodes a closer look will also give at times monsoon dynamics go through shifts in terms of rainfall pattern both spatially and also temporally. Identifying these shifts becomes essential in order to ensure not only the agricultural community tweaks their schedule accordingly but also from the perspective of disaster management new potential risks are also identified. Before we head to discussing about the topic of today’s post it is essential to place the caveat as well. This exercise based on the average cumulative rainfall for Kerala over the monsoon months is not the best way to understand shift in monsoon dynamics. But it certainly shows enough merit to possibly look at a more detailed study to understand if there is change in monsoon dynamics and associated rainfall pattern for Kerala during the monsoon months. The poor monsoon season so far for Kerala triggered this thought to understand if there is change in monsoon dynamics that is resulting in unseasonal rains / lesser rains during peak monsoon season etc.

Football is a game of two halves goes the saying. It seems even Monsoon understands the football crazy Kerala very well. Southwest Monsoon also seem to have a season of two halves. The first half of the Monsoon season, June and July, provides for nearly 2/3rd of the average seasonal rainfall received during Southwest Monsoon season. July is incidentally the wettest month for Kerala during Southwest monsoon contributing more than 1/3rd of the seasonal tally. In what could be a not so good development it is this first half of the monsoon, and July in particular, that is showing clear signs of decline based on the rainfall records between 1901 and 2023. What is very worrying is out of the 10 years with the lowest rains during July since 1901, 5 of them have happened since 2000 indicating a clear weakening trend over the past couple of decades. June ’23 with 260.3 mm rainfall accumulation is the 3rd lowest in terms of overall rains received since 1901.

Just as the first half of the monsoon season is showing signs of weakening the second half of the monsoon season is showing signs of strengthening. While August is showing marginal signs of strengthening September shows a more pronounced strengthening. Unfortunately the strengthening during the second half is not sufficient enough to compensate for the weakening during the first half. While linear trend more often than not gives a fair indication of how things are evolving a slight different way of using decadal averages can be a good reinforcing mechanism if it dovetails with the linear trends. The decadal averages ensure the individual outliers are evened out thereby giving a smoother outlook and eliminating bias from individual outliers.

As mentioned above the first half of the monsoon season contributes to nearly 2/3rds of the overall seasonal tally. The decadal average rains cumulative for June and July dropped below 1200 mm for the first time in 1988 on the back of three poor years, 85 to 86, when the July rainfall numbers did not even touch 400 mm. 1988 to 1991 saw the decadal average continue to stay below 1200 mm before picking up. Between 2017 and 2023 only one year saw the decadal average cross 1200 mm during the first half of the monsoon season (June & July). While one may attribute these troughs to decadal oscillation, the worrying thing is 2000 was the last year when the June & July decadal average touched 1400 mm, every year since then the June & July cumulative decadal average is lower than 1400 mm. To give a perspective between 1901 to 1931 the cumulative June & July decadal average crossed 1500 mm on 8 occasions while it crossed 1400 mm every third year during the 20th century. The 21st century has not yet seen a year where the decadal average has crossed 1400 mm.

There is a fair bit of confidence based on the statistical analysis of monthly rainfall data during the Southwest Monsoon that indicates a shift in monsoon dynamics with the first half weakening and the second half strengthening marginally. The strengthening during the second half is not strong enough to compensate for the first half drop leading to an overall weakening trend of Southwest Monsoon over Kerala. A more detailed study with district wise rainfall data / gridded rainfall data could give a more clearer answer on possibly spatial shift as well in addition to temporal shift in rainfall pattern.