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Menopausal Mother Nature

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Multiple troughs, La Nina: Why Bengaluru is flooding repeatedly this monsoon

The city of Bengaluru has witnessed unprecedented rainfall in the past 24 hours and got flooded for the second time in a week.

The city received 131.6 millimetres of rainfall between the mornings of September 4 and September 5, 2022, according to the Bengaluru met office. This was the highest daily recorded rainfall for September since 2014. 

Historically, the city received the largest volume of rainfall (177.6 mm) in a single day on September 12, 1988, according to the India Meteorological Department (IMD). 

The Bengaluru Urban district received 79.2 mm rainfall during September 4 to September 5 against a normal of 4.5 mm — a staggering 1,660 per cent excess, data from IMD showed.

Exactly a week ago from August 29-30, Bengaluru Urban district received 71.2 mm rainfall against a normal of 2.4 mm, which was 2,867 per cent excess. This had also led to floods in the  city.

Since the beginning of the rainy season in June 2022, Bengaluru has witnessed a lot of heavy downpours that have caused intermittent flooding. 

Down To Earth analysed weekly data from IMD and found that out of the 13 weeks from June 1 through August 31, Bengaluru Urban district received large excess rainfall (more than 60 per cent above long period average) in five of the weeks. 

The district recorded excess rainfall in two weeks, normal in two others and had one week each of deficient and large deficient rainfall, according to the analysis. 

The largest excess rainfall (951 per cent) was recorded from June 16 and June 22 and the second-largest (625 per cent) from August 25 to August 31. 

The week from August 11 and August 17 witnessed 97 per cent less rainfall than normal — the lowest during the period. 

The overall monsoon rainfall recorded in the district till September 5 was 791.8 mm — 162 per cent more than normal. 

This gives an indication that in the weeks in which there has been large excess rainfall the city has been primed for floods. For instance, the week between August 25 and August 31. 

This combined with infrastructure that is not equipped to deal with frequent episodes of heavy rain and a topography that is prone to flooding have made the lives of the people of Bengaluru rather difficult this rainy season. 

The reasons for excessive rainfall at the end of August and early September have been weather systems associated with the ongoing southwest monsoon season. An active La Niña in its third year, however, could have also played its part. 

The IMD has been regularly forecasting heavy rainfall in southern India since August 28, 2022. On that day, in its press release, the weather agency had highlighted a trough — an extended low pressure region that causes rainfall — between central parts of north Andaman Sea and southern Tamil Nadu at a height of 3.1 km above sea level. 

Another trough that ran from Vidarbha in Maharashtra to south coastal Andhra Pradesh in the lower layers of the troposphere, the layer of the atmosphere where most weather occurs, may have also aided the rainfall, IMD mentioned. 

On August 30, the national weather forecasting body had talked about a cyclonic circulation, different from a cyclonic storm, over interior Tamil Nadu in the middle and lower layers of the troposphere. A trough also formed from this circulation to Vidarbha.

In its September 4 press release, in which IMD had again highlighted rainfall in southern India, it had mentioned a north-south trough running from Madhya Pradesh to the Comorin area in Tamil Nadu and a cyclonic circulation over the Comorin area. 

The La Niña, which is the cooler than the normal phase of the El Niño Southern Oscillation phenomenon in the central and eastern Pacific Ocean, generally brings more rainfall to India by enhancing the monsoon and bringing in moisture. 

The ongoing La Niña is in its record third year and has strengthened in July and August. This may cause excessive rainfall episodes over India, including may be in Bengaluru, in the coming months as well. 

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