Explained: Monsoon in India, interesting facts and its impact

The India Meteorological Department (IMD) announced on Thursday (June 8) that the monsoon has officially begun in Kerala.

Kerala experienced widespread rainfall across various regions on Wednesday (June 7). Experts emphasised that a slightly delayed monsoon onset in Kerala does not necessarily indicate a late arrival in other parts of the country.

India’s agricultural landscape heavily depends on regular rainfall, as 52 per cent of the net cultivated area relies on it. The importance of rainfall extends beyond agriculture, as it plays a critical role in replenishing reservoirs that provide drinking water and support power generation across the country.

Rainfed agriculture is a significant component, accounting for approximately 40 per cent of India’s total food production. This reliance on rainfall makes it a crucial contributor to the nation’s food security and economic stability. Below are a few important questions about the monsoon with answers from an Indian Meteorological Department (IMD) report.

Also read: IMD declares that monsoon has onset over Kerala

Here is all you need to know about monsoon in India:

1. What is the monthly and seasonal rainfall for the entire country?

The monthly rainfall for India refers to the accumulated rainfall received over the country during a specific month. For example, the All-India monthly rainfall for June 2018 was recorded as 155.7mm. Similarly, the seasonal rainfall for India represents the accumulated rainfall received over the country during a particular season, such as the South-West monsoon (June to September). For instance, the All-India seasonal rainfall for the South-West monsoon of 2018 was 804.1mm. It’s important to note that these quantities are not fixed and vary from year to year.

2. What does long period average (LPA) of rainfall mean?

The long period average (LPA) of rainfall is the average rainfall recorded over a specific region for a given period, typically spanning several years, such as 30 or 50 years. It serves as a benchmark for forecasting the expected quantitative rainfall for that region during a specific month or season. For example, the LPA of South-West monsoon rainfall over Kerala for June, July, August, and September are 556mm, 659mm, 427mm, and 252mm, respectively. Currently, the LPA of all-India South-West monsoon rainfall, based on the average rainfall between 1961 and 2010, is 880.6mm.

3. What are the categories of rainfall termed as large excess, excess, normal, deficient, and large deficient?

These categories describe the realized rainfall averages over various temporal scales, such as daily, weekly, or monthly, and spatial scales, such as districts or states. The classifications are operational in nature. Based on the deviation from the LPA, the categories are as follows:

Large excess: Realized rainfall is equal to or greater than 60% of the LPA.

Excess: Realized rainfall is between 20% and 59% of the LPA.

Normal: Realized rainfall is between -19% and +19% of the LPA.

Deficient: Realized rainfall is between -59% and -20% of the LPA.

Large deficient: Realized rainfall is between -99% and -60% of the LPA.

Also read: Citizens in Mumbai to receive weather updates on phones during monsoon: BMC

4. How is below normal, normal, and above normal rainfall defined for the entire country of India?

To determine whether rainfall is below normal, normal, or above normal for the country, statistical measures are employed. For the monsoon season (June to September) rainfall over India as a whole, the LPA is 88 cm, with a standard deviation of 9 cm (approximately 10% of the mean value). Rainfall below 90% or above 110% of the LPA is categorized as “below normal” or “above normal”, respectively.

5. What role does the monsoon trough play?

The monsoon trough is an elongated low-pressure area that stretches from the heat low over Pakistan to the Bay of Bengal. It is a semi-permanent feature of the monsoon circulation. When the monsoon trough migrates southward, it triggers an active and vigorous monsoon over a significant portion of India. This results in widespread rainfall and favorable monsoon conditions. On the other hand, when the trough moves northward, it leads to a break in the monsoon over much of India. Instead, heavy rains occur along the foothills of the Himalayas, and there is an increased risk of floods in the Brahmaputra River region.

6. What is a heat low, and what is its impact on monsoon rainfall?

During the northward movement of the sun in the northern hemisphere, the region surrounding the Arabian Sea receives substantial heat from solar radiation and heat flux from the Earth’s surface. This leads to the formation of a trough of low pressure known as the heat low — a semi-permanent feature of the Indian monsoon. An intense heat low, characterized by below-normal pressure, acts as a suction device for moist air along the monsoon trough, contributing to a good monsoon over India.

Also read: Normal southwest monsoon expected in India, says IMD

7. What is a monsoon low, and how does it influence the monsoon?

A monsoon low refers to a region with the lowest pressure at its centre, characterized by a closed shape with winds blowing counter clockwise in the Northern Hemisphere. It involves a swirling motion of air, convergence, and upward movement of air. Typically, clouds and rainfall are present within the low-pressure area during the monsoon season, hence the term “monsoon low”.

Monsoon lows can intensify into monsoon depressions, which are the primary rain-bearing systems during the southwest monsoon period in India. Monsoon depressions, forming in the Bay of Bengal, contribute significant amounts of rainfall as they move westward. On average, two depressions form each month during the monsoon season (June-September).

8. What is the Tibetan High, and how is it related to monsoon rainfall?

The Tibetan High refers to a warm anticyclone located over the Tibetan Plateau in the middle to upper troposphere during the monsoon period. At times, the Tibetan High may deviate considerably to the west of its usual position. In such cases, the monsoon may extend further westward into Pakistan and, in extreme situations, even into northern Iran.

9. What is the Mascarene High, and how does it influence monsoon rainfall?

The Mascarene High is a high-pressure area that forms around the Mascarene Islands in the southern Indian Ocean during the monsoon season. It plays a crucial role in facilitating cross-equatorial airflow through the south Arabian Sea, acting as a linkage between the hemispheres. The intensity variation of the Mascarene High causes monsoon surges, leading to heavy rains along the west coast of India.

10. What is the Somali Jet?

The Somali Jet refers to a low-level cross-equatorial flow of air, reaching jet speeds at the western end of the monsoon regime along the east coast of Africa. It eventually reaches the west coast of India in June, attaining maximum strength in July. Its strengthening gives rise to strong monsoon over peninsular India.

11. How does IMD monitor monsoon?

To monitor the monsoon, the Indian Meteorological Department (IMD) employs several techniques:

A. IMD collects data on temperature, humidity, wind speed, rainfall, and other meteorological parameters from a network of weather stations across the country.

B. Satellites and radars are utilized to monitor the monsoon in real-time. Satellite images provide information on cloud cover, rainfall patterns, and atmospheric conditions, while radars help detect and track rain-bearing clouds.

C. IMD analyses various meteorological charts, such as synoptic charts, weather maps, and pressure charts. These charts help identify weather systems, track their movement, and assess their impact on the monsoon.

D. IMD uses national and international weather forecasting models that provide predictions at different spatiotemporal scales.

Also read: Cyclone Biparjoy: Maharashtra, Karnataka, Goa on high alert

12. How does the IMD declare onset and advance of monsoon?

The onset of monsoon occurs in many stages and represents a significant transition in the atmospheric and ocean circulations in the Indo-Pacific region. At present, IMD uses a new criterion adopted in 2016 for declaring the onset of monsoon over Kerala. It was based on the daily rainfall recorded in 14 stations over Kerala and neighbouring areas along with wind field and Outgoing Longwave Radiation over southeast Arabian Sea. In future, it plans to use Artificial Intelligence and Machine Learning techniques (AIML) too.

13. Further advance of monsoon over the country

Further advance is declared based on the occurrence of rainfall over parts/sectors of the sub-divisions and maintaining the spatial continuity of the northern limit of monsoon.

14. How does the IMD define withdrawal of monsoon?

The current operational criteria used by IMD to declare monsoon withdrawal from extreme north-western parts of the country was adopted in 2006 and consist of the following major synoptic features, which are considered only after September 1:

i) Cessation of rainfall activity over the area for continuous 5 days

ii) Establishment of anticyclone in the lower troposphere

iii) Considerable reduction in moisture content

Monsoon withdraws from the southern peninsula and hence from the entire country around October 15.

15. Is there any impact of climate change on monsoon?

Yes. Several studies have attributed the rising trend in the frequency and magnitude of the extreme rainfall events and decreasing trend in moderate rainfall events during monsoon season over central Indian region to climate change and natural variability.