CHENNAI: The slow-moving well-marked low-pressure area in the Bay of Bengal has concentrated into a depression on Wednesday night and lay centered over the southwest Bay of Bengal, about 430 km from Chennai. It is very likely to move west–northwestwards and reach near north Tamil Nadu coast by Thursday morning and cross north Tamil Nadu and adjoining south Andhra Pradesh coasts between Karaikal and Sriharikota close to the north of Puducherry on the same evening.
Extremely heavy rainfall is likely in Chennai and neighboring districts from Wednesday night to Thursday afternoon, where rainfall in excess of 20 cm is forecast. The regional meteorological centre has issued a red alert for Chennai, Tiruvallur, Ranipet, Vellore, Tiruvanamalai, Kallakurichi, Salem and Tirupattur. Strong surface wind reaching 30 km/ph is likely over Chennai and suburbs during the daytime.
N Puviarasan, the director, Area Cyclone Warning Centre, said the landfall may happen slightly north of where it was forecasted initially. In the last 24 hours ending 8.30 am on Wednesday, Nagapattinam recorded a whopping 310 mm of rainfall followed by Karaikal 290 mm. Several observatories clocked in excess of 250 mm. However, during the day, the rainfall activity reduced with Ennore Port receiving 32 mm followed by Cheyyur 33.5 mm and Nungambakkam 23.7 mm.
Meteorologists say favourable oceanic conditions would continue to keep the northeast monsoon active and vigorous this November. Another low-pressure area is likely to form in the south Andaman Sea around November 13. Chennai has surpassed monthly average rainfall within the first week of November itself, Nungambakkam observatory has recorded a total of 464 mm of rains from November 1-9, while Menambakkam reported 369 mm of rains against the normal of 374 mm.
The 215 mm of rainfall on November 7 is so far the second-highest rainfall recorded in the span of 24 hours in the last decade, with the highest being 246.5 mm received on November 16, 2015. Meanwhile, the all-time record for 24 hours rainfall stands at 452.4 mm recorded on November 25, 1976.
Chennai has been recording good rains ever since the beginning of the northeast monsoon. The state capital is witnessing excess rains by 50%, with 612.2 mm of rains against the normal of 408.6 mm recorded from October 1 to November 10. Tamil Nadu too has rain excess of 50 per cent, with actual rains of 384.1 mm against the normal of 255.2 mm during the same time span.
So, what is causing such extreme rains and formations of back-to-back weather systems? As per meteorologists all the prime oceanic parameters-La Nina, Indian Ocean Dipole (IOD) and MJO (Madden–Julian oscillation) are in favourable positions at the same time. This would favour the formation as well as strengthening of the weather systems in the Bay of Bengal.
Presently, MJO is parked over the Indian Ocean, albeit with low amplitude. However, it remains favourable for enhancing the northeast monsoon rains over the south Peninsula., especially Tamil Nadu. La Nina is invariably linked with a good northeast monsoon.
This apart, contribution of climate change cannot be negated. The rise in global temperatures has increased the frequency of heavy rains. The Indian Ocean is warming at a faster rate, with sea-surface temperatures (SST) soaring above average. 26.5 degrees Celsius is the threshold value, past which the conditions are very favourable for cyclogenesis or rapid intensification of any weather system. The current SST value is close to 29 degrees.
GP Sharma, President-Meteorology and Climate Change, Skymet Weather said, “The impact of rising of 1-2 degrees in the ocean temperatures is much more than compared to the increase in land temperatures. The relation between temperature and oceanic heat potential can be exponential. The rise in SST means the weather systems would be gaining more strength and moisture. This has also increased the shelf life of the weather systems, which then tends to travel inland for a longer duration.”
Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, Pune, said the Indian Ocean is getting warmer at a faster rate than any other ocean in the world due to greenhouse gases. Although the rate of warming is more in the Arabian Sea than in the Bay of Bengal, the rise of a degree or two in oceanic temperatures can have a large impact.
“Climate change has led to an increase in sea surface temperatures that in turn has resulted in sea level rise by 10% to 15%. Further, it is an established fact that global warming has increased the frequency of heavy rains, even if it is associated with a low-pressure area,” said Mrutyunjay Mohapatra, Director General of Meteorology, India Meteorological Department (IMD).
“One aspect of climate change that is important for the east coast including Tamil Nadu is that extreme weather and climate events are overlapping. Now, we know that the sea level is rising in the background. Hence, the flood level due to storm surge and rains are rising year on year. Such kinds of compound events are increasing now due to climate change. Sometimes they coincide with a high tide that pushes water several kilometres inland,” added Roxy.