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Menopausal Mother Nature

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Delayed monsoon withdrawal: Why that should bother us

Data from recent years has shown that the seasonal gap between the southwest and northeast monsoons may be decreasing
 

The southwest monsoon started withdrawing October 6, 2021, a record 19 days later than the normal date of September 17. If the overall withdrawal does not happen by October 15, then this would be the eleventh year in a row that the primary rainfall season of the country will be withdrawing later than normal.

The India Meteorological Department (IMD) had come up with a new calendar for the monsoon season’s onset and withdrawal over different parts of the country in 2020.

It had done this after taking into account the late withdrawal and erratic progress of the monsoon winds over the country in recent years.

It had stated that the data with which it had earlier calculated the onset and withdrawal dates was old and sparse. It had further added that the monsoon regime in the country was changing due to a changing climate.

The last few years have shown that even after this correction, the monsoon is not keeping its dates with India, especially while withdrawing.

In every year since 2017, the season has started withdrawing later than September 17. It was late by 22 days in 2019:

  • September 28 in 2020
  • October 9 in 2019
  • September 29 in 2018
  • September 27 in 2017
  • September 15 in 2016

The season’s rainfall for IMD ends September 30, when it changes its categorisation of rainfall from monsoon (June to September) to post monsoon (October to December).

The actual rainfall has not yet stopped. Eighteen states have received excess (20 to 59 per cent excess) or large excess rains (greater than 60 per cent excess) in the first week of October.

Nearly half of all districts in the country have also received excess or large excess rains. Country-wide excess of rainfall for the first week of October stands at 23 per cent.

The rainfall has continued from the excesses of September. It was the month that received 35 per cent more rainfall than normal and accounted for 26 per cent of the total monsoon rainfall against the normal of around 19 per cent.

This happened mainly because of a series of five low pressure areas that formed in the Bay of Bengal and moved in a generally westward-north westward direction.

Two of these systems further intensified to a deep depression and Cyclone Gulab over the Bay of Bengal, before hitting land and causing torrents of rain over eastern, central and western India.

Factors like a developing La Nina phenomenon in the equatorial Pacific Ocean, favourable conditions of the Indian Ocean Dipole (IOD) and the Madden Julian Oscillation may still be the cause for the continuing October rainfall.

The IOD is a climatic phenomenon caused by the difference in sea surface temperatures in the east and west Indian Ocean.

This has two phases — negative and positive. It causes generally decreased rainfall over India in its positive phase and enhanced rainfall in its negative phase.

The negative phase of the IOD had weakened in August and the month witnessed less than normal rainfall but it came back to normal in September.

There were colder-than-normal sea surface temperatures over the equatorial Pacific Ocean at the same time. This indicates the formation of the La Nina phenomenon which also causes more-than-normal rainfall over India.

During most of the days, the Madden Julian Oscillation, which is an eastward moving pulse of cloud and rainfall, was in the phases 3, 4 and 5.

These are favourable for monsoon rainfall activity and formation of low-pressure systems. More West Pacific Typhoon activity and the remnants of these westward moving systems helped to form the low-pressure areas over the Bay of Bengal.

This rainfall is happening while the monsoon winds still prevail over India but they would not be accounted for under the official monsoon rainfall data by IMD.

Usually, the rainfall in October is accounted under the North East Monsoon (NEM) rainfall which officially begins around the second week of October.

The NEM season brings significant rains (up to 48 per cent of annual rainfall in Tamil Nadu and Puducherry) to south peninsular India from October to December.

Further rainfall may occur in the coming weeks as well as IMD has indicated the formation of another low-pressure area in the Bay of Bengal on October 10.

It also said that the system may intensify further and move in a generally westward direction.

The southwest and northeast monsoon distinct weather systems have come very close to each other or even overlapped in the past few years.

This is mainly because of the former’s late withdrawal. In 2019, the southwest monsoon season completely withdrew from the country October 16.

The NEM rainfall had already begun by this time. These massive climate systems, which have remained largely stable and timely for millions of years, may be changing their behaviour.

What is perhaps even more ominous is that when climate scientists take up this skewed data of the last few years for analysis and research, they may arrive at wrong inferences.

They may make problematic conclusions which would create a cascade of further decisions resulting in flawed policies for adaptation to climate change. 

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