The northeast monsoon in Chennai over the last few years has been erratic to say the least. From the torrential December 2015 downpour that submerged the city, to a sub-par monsoon in 2016 that led to a drought in the earlier part of 2017, and now the very heavy rains in the first week of the season, we have experienced the season in all its diversity.

This year, the India Meteorological Department (IMD) announced the official commencement of the northeast monsoon on October 27. While this marked a slight delay in its arrival, Chennai had already received 648.4 mm of rain by November 7th.

Northeast Monsoon, also called the ‘post monsoon season’ or the ‘retreating southwest monsoon’ typically accounts for 48 per cent of the total annual rainfall in the state. According to data from the office of the Regional Meteorological Centre at Nungambakkam, the average rainfall during the season in Chennai is 867.5 mm, spread over the 90-day period from October 1st to December 31st. This means that with 648.4 mm of rain, Chennai has already received 74 per cent of the average northeast monsoon rainfall in a little over a week.

Another striking feature of the 2017 rains has been the extreme rainfall recorded within a span of 24-hours. The office of the Director General of Police (DGP) at Mylapore recorded a whopping 300mm rainfall on November 2nd, with one of the highest rain rates per hour recorded in the recent past.

Weather Blogger, Bhaskaran Shivaraman, who blogs at Keaweather says, “There were similar instances in the past due to non cyclonic activity. On 1 December 2015, Chennai received 294mm of rain.”

The intense rainfall in Chennai is not due to a cyclonic storm, but associated with a low pressure area located far away, says Pradeep John, who blogs at Tamil Nadu Weatherman, also attributing the showers to the convergence of winds at a lower level and the higher level divergence.

Effects of climate change?

Whatever the reason, the amount of rain received within a week of the onset of monsoon and the extreme 24-hour-rainfall in the coastal districts of Tamil Nadu including Nagapattinam and Chennai are both extraordinary weather events. This has led many in the city to question if Chennai is indeed witnessing the effects of what we call climate change.

While weather experts who study data on rainfall and temperature hesitate to label the phenomenon as ‘unusual’, climate researchers say that it could be due to climate resilience, a milder term used for climate change.

Y E A Raj, former Deputy Director General of the Regional Meteorological Centre says, “There is nothing unusual about the Chennai rains this time. Days of extreme rainfall were witnessed even in the 1970s, when the term ‘climate change’ was still largely unheard of.”

However, noted researchers from the city link the current rainfall to the effects of global warming. According to them, the rise in temperatures is the prime reason for many out-of-the-ordinary weather events, including the flash floods in Mumbai and Bengaluru and drought in North Indian states including Punjab and Haryana.

Dr Nambi Appadurai, Director of Climate Resilience Practice at the World Resources Institute said that the pattern of northeast monsoon should be thoroughly studied and researched. “The northeast monsoon has been abnormal over the past three years. This year, the expected rainfall over a month has happened in just a week. But it would be premature to attribute this to climate change without proper research. According to the defined standards of the World Meteorological Organisation standards, a window frame of 30 years must be studied to authentically conclude it to be a climate change event,”  says Appadurai. In his opinion, the fluctuations in monsoon could definitely be due to climate variability.

Climate variability

Climate variability refers to short term fluctuations in seasonal and multi seasonal timescales, normally ranging from months to a few years. Climate change, on the other hand, refers to changes in long term averages — changes occurring over decades (30 years in the least to millennia), explains Appadurai. Expecting such short term variations to occur again, he emphasises the need to focus on proactive measures to handle the situations arising from such weather events.

Quoting the fifth assessment report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC), Dr K Palanivelu, Director of Centre for Climate Change and Adaptation Research at Anna University, said that the 2017 Chennai rains could be due to global warming. “Although such extremes were witnessed in the past, the frequency at which it is happening has increased. Globally, the Sea Surface Temperature (SST) and atmospheric temperature has increased by about 1.1 degrees celsius since the late 19th century, according to the World Meteorological Organisation. This results in the rapid evaporation of seawater, resulting in more rains,” said K Palanivelu.

Palanivelu attributes this to the Indian Ocean Dipole, which had also resulted in excess rainfall in December 2015 in Chennai. The IOD is the difference in sea surface temperature between two poles  – a western pole in the Arabian Sea and an eastern pole in the eastern Indian Ocean, south of Indonesia. The IOD affects the climate of countries that surround the Indian Ocean Basin, and is a significant contributor to rainfall variability in this region.

Urban heating

Another factor called the Urban Heat Island Effect cannot be ignored too. Weather blogger Pradeep John says, “Paved surfaces and skyscrapers that have been increasing in a metropolitan city like Chennai could be the arterial reason. The heat produced by them gets into the atmosphere and collides with moist air from a nearby water source, resulting in high intense rainfall.” Pradeep, an ardent pluviophile, observed that the showers are getting intense and more torrential every year.

What could be done?

“Restricting the global atmospheric temperature rise to 1.5 degrees would keep a check on extreme weather events, which are becoming more common everywhere. Also, carbon dioxide emissions, which is now at 403 ppm should be capped at 450 ppm,” said K Palanivelu.

Suggesting immediate solutions, Dr Appadurai said, “Proper adaptation methods, including clearing the encroachments from wetlands and increasing greenery, are important.”