The state witnessed a see-saw rainfall pattern since the beginning of October due to global warming
More rainfall may be on the way for flood-ravaged states Andhra Pradesh and Tamil Nadu. A cyclonic circulation has been predicted over southeast Bay of Bengal in the lower tropospheric levels by the India Meteorological Department (IMD) November 22, 2021.
This circulation may be moving towards the Tamil Nadu coast in the next 4-5 days.
Along with this atmospheric motion, there is a low-pressure trough (an extended area of low pressure which carries rainfall and winds) stretching from the cyclonic circulation to the Tamil Nadu coast.
This would start causing rainfall in Tamil Nadu, coastal Andhra Pradesh and Kerala from November 25 onwards. The exact regions where rainfall would occur and intensity of rainfall is anybody’s guess.
A cyclonic circulation forms at a different height in the atmosphere than a low-pressure area — the latter is closer to the surface while the former occurs in the lower and middle troposphere levels where air pressures are lower.
Usually, a prevailing cyclonic circulation induces the formation of a low-pressure area towards the surface.
A recent low-pressure area induced by the cyclonic circulation near the Andaman Sea intensified into a depression close to the Tamil Nadu coast and crossed over to land.
It brought devastating floods to five districts of Andhra Pradesh, killing 29 people. As many as 60 are still missing.
Before that, the combined effect of two low-pressure areas in the Bay of Bengal and the Arabian Sea as well as their associated cyclonic circulations and troughs had brought heavy rainfall to many parts of Tamil Nadu.
This resulted in floods in Chennai in the second week of November. After the floods, the remnants of the compound atmospheric system merged into the cyclonic circulation in the Arabian Sea, according to IMD.
In October, Kerala and Uttarakhand suffered deluges. The Uttarakhand floods also were caused by the interaction of two systems: Remnants of a low-pressure area from the Bay of Bengal that had moved north with the prevailing winds from the east and an active western disturbance coming in from the west.
Such a merger of different wind and rain systems is a sign of a changing climate and can be expected in the coming week as well, according to IMD.
From the beginning of October, the weather over Andhra Pradesh witnessed a see-saw of rain-rich and rain-deficient periods symptomatic of the impact of global warming on rain-carrying systems.
These alternating weather patterns are clearly visible in some districts, an analysis of the conditions in flood-affected districts Nellore, Anantapur, Chittoor, Kurnool and YSR Kadapa showed.
The October rainfall trends in Nellore is a glaring example. The district recorded 70 per cent excess rainfall in the first seven days of the month when the state had a deficit of 36 per cent.
By October 18, the situation reversed and the overall rainfall deficit in the district was 23 per cent since the beginning of the month. In the following week, too, there was not much rainfall and by October 24, the district had recorded a deficit of 45 per cent for the period since October 1.
But the teeter-totter of precipitation levels persisted: The rainfall deficit reduced to 9 per cent by November 8 and there was 15 per cent excess rainfall by November 15, which increased to 38 per cent by November 21, calculated since October 1.
In the neighbouring district of Chittoor, the rainfall volume for October was six per cent excess by October 7 and increased to 41 per cent by October 18. These 11 days since October 7 is when Nellore experienced no rainfall.
Since October 18, Chittoor experienced a slight slump in rainfall for a couple of weeks. After November 8, the rains picked up again and by November 15, the district again had an excess rainfall of 47 per cent for the period since October 1. This increased to 100 per cent by November 21 for the duration, which included the period of the floods.
In the other three districts, rainfall levels followed an almost linear increasing trend similar to the state-wide scenario. Anantapur ended up with a whopping 173 per cent excess rainfall (started with a deficit of 14 per cent on October 7) for the period October 1-November 21 and YSR Kadapa (started with a deficit of 18 percent on October 7) ended up with an excess of 169 per cent.
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