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Why the temporal distribution of 2020 monsoon season is highly skewed

The southwest monsoon of 2020 has been one of the best in terms of spatial distribution, with just 26 per cent of the country’s 685 districts receiving deficient and below-normal rainfall, and a majority of the districts getting adequate showers till September 22.

But it is the temporal distribution of the showers that has caused a lot of problems, such as the spurt in food prices and disruption in supplies due to breakdown in the already Covid-19-battered transport in the hinterland.

Skewed temporal distribution

The southwest monsoon this year was 17 per cent above normal in June, and 10 per cent below normal in July.

However, the rains recovered sharply and the cumulative rainfall was 27 per cent above normal in August.

In the month of August, out of the 31 days, around 27 days had Low Pressure Areas (LPA). This could explain why the southwest monsoon in August across India was the best for that month since 1926.

As on date (till September 22), of the 36 meteorological subdivisions in the country, only five have received deficient rains

The met department expects the final season-end cumulative southwest monsoon for 2020 to be more than its forecast of 102 per cent of the Long Period Average (LPA), as rains are continuing over several parts of the country. However, it will be less than 110 per cent of LPA.

Till September 22, the monsoon has been cumulatively 108 per cent of the LPA, ot eight per cent above normal.

The LPA of the four-month southwest monsoon season is around 88 centimeters.

The monsoon entered India on June 1 this year and covered the entire country by June 26, almost 12 days ahead of its normal schedule.

The good progress and spread of the southwest monsoon this year has spurred sowing of kharif crops, which till last week has reached an all-time high of over 111 million hectares, with oilseeds and rice leading the way.

Buoyed by good rains, kharif food grain harvest in 2020-21 is expected to be over 144 million tonnes, an all-time high. Production of oilseeds and pulses is also expected to rise sharply.

IMD’s efforts to improve temporal monsoon forecast

The met department has been trying to improve its temporal forecast exponentially for the past several years and has achieved a fair degree of success as well.

The met department says that during monsoon, considerable variation in rainfall is seen with space and time.

This is due to several reasons, the prime among them being the onset, advancement and withdrawal of monsoon, which determine its duration at different places.

The position of the monsoon trough is another factor that determines variation of the monsoon. It can oscillate five degrees to north and five degrees to south within 24 hours.

If this trough is south of normal, strong monsoon conditions will be observed over India.

If this trough is north of the normal position or if it runs to the foothills of Himalayas or is not seen at all, then bleak monsoon conditions are observed.

Phenomena like cyclonic circulations, lows, depressions move along troughs and contribute to rainfall.

The met has been taking several measures to accurately predict temporal and spatial distribution of southwest monsoon.

It uses real time monitoring of monsoon using remote sensing techniques like satellites and radars, and also relies on various national and international weather forecasting models at different spatio-temporal scales.

IMD issues short- to medium-range forecasts for five days based on various national and international weather forecasting models and expertise from the scientists.

These forecasts are being used by various stakeholders for planning their routine activities.

Its different meteorological centres issue nowcast forecast for heavy rainfall during monsoon season with validity up to six hours. All these help in proper prediction of monsoon distribution both spatial and temporal. Nowcast refers to present weather conditions or a forecast of those immediately expected.

Has climate change made an impact on the monsoon?

The IMD affirms it has, and says several studies have attributed the rising trend in the frequency and magnitude of extreme rainfall events and decreasing trend in moderate rainfall events during monsoon season over the central Indian region to climate change and natural variability.

It says recent observations show that summer monsoon precipitation (June to September) over India has declined by around six per cent during the past 50 years, with notable decreases over the Indo-Gangetic Plains and the Western Ghats.

It is also observed that there has been a shift in the recent period toward more frequent dry spells and more intense wet spells during the summer monsoon season.

Over central India, the frequency of daily precipitation extremes with rainfall intensities exceeding 150 mm per day increased by about 75 per cent during recent decades.

It further elaborated that as the temperature of the Earth is increasing rapidly due to anthropogenic greenhouse gas emissions, thermodynamically, warm air holds more moisture as compared to the dry air.

According to Clausius-Clapeyron equation, the capacity of air to hold moisture increases by 7% for each degree of warming.

“Studies indicate that, in a changing climate, heavy rainfall events are expected to rise due to abundance of the moisture due to warming,” the met said.

Thus, when it comes to southwest monsoon, though 2020 might have been a better year, but the future remains highly uncertain due to growing climate uncertainties. 

Table: Progress of southwest monsoon from June 1 to September 22, 2020 (in ml)
Region  Actual rainfall  Normal rainfall  % variance 
North-west India  488.4 582.2 -16
Central India 1,090.1 943.9 +15
East & NE India 1,361.0 1,339.0 +2
South Peninsula  886.6 680.4 +30
All-India 908.8 844.0 +8
Source: India Meteorological Department

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