The monsoon is slated to make its earliest arrival in 13 years over Kerala, informs the India Meteorological Department (IMD).

What does the “Onset of Monsoon” mean?

• The onset of the monsoon over Kerala marks the beginning of the four-month, June to September southwest monsoon season over India.
• It brings more than 70 per cent of the country’s annual rainfall.
• It marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.
• The IMD announces it only after certain newly defined and measurable parameters, adopted in 2016, are met.
• The onset is a significant day in India’s economic calendar.

How does IMD predict the monsoon?

• Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed:

1. Rainfall: The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10.
2. Wind field: The depth of westerlies should be upto 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.
3. Heat: The INSAT-derived Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watt per sq m (wm2) in the box confined by 5-10ºN latitude and 70-75ºE latitude.

• The onset is not officially declared until the prescribed conditions (above) are met.

Factors considered by IMD

• The IMD uses a specialised model that forecasts the arrival dates within a four-day window.
• It uses six predictors:

1. Minimum temperatures over northwest India
2. Pre-monsoon rainfall peak over south Peninsula
3. Outgoing long-wave radiation (OLR) over the South China Sea
4. Lower tropospheric zonal wind over the southeast Indian Ocean
5. Upper tropospheric zonal wind over the east equatorial Indian Ocean, and
6. OLR over the southwest Pacific region

Where is the early arrival noticed?

• The monsoon’s arrival over India is marked by rain over south Andaman Sea, which then advances north-westwards across the Bay of Bengal.
• In general, the Andaman and Nicobar Islands start receiving monsoon rainfall between May 15 and May 20 every year.
• And it usually starts raining along the Kerala coast in the last week of May.

Does an early onset foretell a good monsoon?

• No, it does not — just as a delay does not foretell a poor monsoon.
• The onset is just an event that happens during the progress of the monsoon over the Indian subcontinent.
• A delay of a few days, or perhaps the monsoon arriving a few days early, has no bearing on the quality or amount of rainfall, or its regional distribution across the country.

Back2Basics: Long Period Average (LPA)

• The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
• The LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc.
• LPA refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month.
• The IMD’s prediction of a normal monsoon is based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
• It has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000.

Why LPA is needed?

• The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.
• Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month.
• An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made.
• A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall, as well as for the periodic drought years.
• It also takes into account the increasingly common extreme weather events caused by climate change.

Range of normal rainfall

The IMD maintains five rainfall distribution categories on an all-India scale. These are:

1. Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA;
2. Below normal, when departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA;
3. Above normal, when actual rainfall is 104-110% of LPA;
4. Deficient, when departure of actual rainfall is less than 90% of LPA; and
5. Excess, when the departure of actual rainfall is more than 110% of LPA.

Also read

Various terms related to Indian Monsoon

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To help out horticulturists who face crop damage due to hailstorms, the Himachal Pradesh government will be testing the use of indigenously developed ‘anti-hail guns’.

What are anti-hail guns?

• An anti-hail gun is a machine that generates shock waves to disrupt the growth of hailstones in clouds, according to its makers.
• It comprises a tall, fixed structure somewhat resembling an inverted tower, several metres high, with a long and narrow cone opening towards the sky.
• The gun is “fired” by feeding an explosive mixture of acetylene gas and air into its lower chamber, which releases a shock wave (waves that travel faster than the speed of sound, such as those produced by supersonic aircraft).
• These shock waves supposedly stop water droplets in clouds from turning into hailstones, so that they fall simply as raindrops.

Answer this PYQ in the comment box:

Q.During a thunderstorm, the thunder in the skies is produced by the

1. meeting of cumulonimbus clouds in the sky
2. lightning that separates the nimbus clouds
3. violent upward movement of air and water particles

Select the correct option using the codes given below:

(a) 1 only

(b) 2 and 3 only

(c) 1 and 3 only

(d) None of the above

How do they ‘prevent’ a hailstorm?

• It is this hail formation process that the shock waves from anti-hail guns try to disrupt in a radius of 500 meters, so that the water droplets fall down before they can be lifted by the updrafts.
• The machine is repeatedly fired every few seconds during an approaching thunderstorm.
• However, the effectiveness of anti-hail guns has remained a contentious issue.

How do Hailstorms occur?

• Hail is produced by cumulonimbus clouds, which are generally large and dark and may cause thunder and lightning.
• In such clouds, winds can blow up the water droplets to heights where they freeze into ice.
• The frozen droplets begin to fall but are soon pushed back up by the winds and more droplets freeze onto them, resulting in multiple layers of ice on the hailstones.
• This fall and rise is repeated several times, till the hailstones become too heavy and fall down.

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A new study has revealed how dust coming from the deserts in West, Central and East Asia plays an important role in the Indian Summer Monsoon.

Try this PYQ:

With reference to ‘Indian Ocean Dipole (IOD)’, sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?

1. IOD phenomenon is characterized by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.
2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.

Select the correct Option using the code given below:

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

Why study dust?

• Many studies have shown that the dust emission scheme is extremely sensitive to climate change.
• Understanding these mechanisms and effects of dust will help us understand our monsoon systems in the face of global climate change.

Impact of dust on Indian Monsoon

• Dust swarms from the desert when lifted by strong winds can absorb solar radiation and become hot.
• This can cause heating of the atmosphere, change the air pressure, wind circulation patterns, influence moisture transport and increase precipitation and rainfall.
• A strong monsoon can also transport air to West Asia and again pick up a lot of dust.
• The researchers say this is a positive feedback loop.

Role of the Iranian plateau

• Not just the dust from the Middle East, the Iranian Plateau also influences the Indian Summer Monsoon.
• The hot air over the Iranian Plateau can heat the atmosphere over the plateau, strengthen the circulation over the deserts of the Arabian Peninsula and increase dust emission from the Middle East.
• The researchers also explain how the Indian Summer Monsoon has a reverse effect and can increase the winds in West Asia to produce yet more dust.

Transported aerosols

• Deserts across the globe play important roles in monsoons.
• The dust aerosols from deserts in West China such as the Taklamakan desert and the Gobi Desert can be transported eastward to eastern China and can influence the East Asia summer monsoon.
• And in the southwest United States, we have some small deserts that influence the North African monsoon.

Anthropogenic contributions

• Some studies have found that the anthropogenic aerosols emitted from the Indian subcontinent can decrease summer monsoon precipitation.
• However, some others found that absorbing aerosols such as dust can strengthen the monsoon circulation.

Minor components

• Earlier it was believed that dust from deserts across the globe will have the same components.
• But it was found that different deserts have different chemical compositions and this can influence the dust’s properties.
• For example, dust from the Middle East has the more absorbing ability of solar radiation than dust from North Africa and this difference in absorbing ability might influence monsoon systems.

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Rainfall over the Southern peninsular region has been deficient so far due to prevailing La Nina conditions according to the India Meteorological Department (IMD).

Try this PYQ:

Q.La Nina is suspected to have caused recent floods in Australia. How is La Nina different from El Nino?

1. La Nina is characterized by unusually cold ocean temperature in equatorial Indian Ocean whereas El Nino is characterized by unusually warm ocean temperature in the equatorial Pacific Ocean.
2. El Nino has an adverse effect on south-west monsoon of India, but La Nina has no effect on monsoon climate.

Which of the statements given above is/are correct?

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

What is the Northeast monsoon?

• India receives rainfall during two seasons.
• About 75 per cent of the country’s annual rainfall is received from the Southwest monsoon between June and September.
• The Northeast monsoon, on the other hand, occurs from October to December and is a comparatively small-scale monsoon, which is confined to the Southern peninsula.
• After the complete withdrawal of the Southwest monsoon from the country takes place by mid-October, the wind pattern rapidly changes from the south-westerly to the north-easterly direction.
• Also called the winter monsoon, the rainfall associated with the Northeast monsoon is important for almost entire South India.

Why it is important?

• Tamil Nadu records about 48 per cent (447.4 mm) of its annual rainfall (943.7 mm) during these months, making it the key factor for undertaking agricultural activities and reservoir management in the state.
• Some South Asian countries such as Maldives, Sri Lanka and Myanmar also record rainfall from October to December.

Why there are distortions?

• The majority of districts in Tamil Nadu remain highly rain-deficient up this time.
• The period after the Southwest monsoon season, from October to December, is the peak time for cyclonic activity in the North Indian Ocean region — covering the Arabian Sea and the Bay of Bengal.
• The winds associated with the formation of low-pressure systems, depressions, or cyclones influence this monsoon, and therefore, the rainfall.
• Officials at IMD have linked it to the prevailing La Niña conditions in the Pacific Ocean.

La Nina  link with the NE monsoon

• While La Niña conditions enhance the rainfall associated with the Southwest monsoon, it has a negative impact on rainfall associated with the Northeast monsoon.
• During La Niña years, the synoptic systems — low pressure or cyclones — formed in the Bay of Bengal remain significantly to the north of their normal position.
• Besides, instead of moving westwards, these systems recurve. As they lie to the north of their normal position, not much rainfall occurs over southern regions like Tamil Nadu.

Back2Basics: El Nino and La Nina

• While El Niño (Spanish for ‘little boy’), the more common expression, is the abnormal surface warming observed along the eastern and central regions of the Pacific Ocean (the region between Peru and Papua New Guinea).
• The La Niña (Spanish for ‘little girl’) is an abnormal cooling of these surface waters.
• Together, the El Niño and La Niña phenomena are termed as El Niño Southern Oscillation (ENSO).
• These are large-scale ocean phenomena which influence the global weather — winds, temperature and rainfall. They have the ability to trigger extreme weather events like droughts, floods, hot and cold conditions, globally.
• Each cycle can last anywhere between 9 to 12 months, at times extendable to 18 months — and re-occur after every three to five years.
• Meteorologists record the sea surface temperatures for four different regions, known as Niño regions, along this equatorial belt.
• Depending on the temperatures, they forecast either as an El Niño, an ENSO neutral phase, or a La Niña.

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