• India is expected to get above-normal rainfall in the upcoming Southwest Monsoon season, the India Meteorological Department (IMD) said on April 15.

• The seasonal rainfall is likely to be 105 per cent of the Long Period Average (LPA) with a model error of 5 per cent. 

• 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. It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season.

• The LPA of the seasonal rainfall over the country as a whole based on data of 1971-2020 is 87 cm.

There are 5 categories of monsoon rainfall: 

i) Deficient (less than 90 per cent of LPA)

ii) Below normal (90-95 per cent of LPA)

iii) Normal (96-104 per cent of LPA)

iv) Above normal (105-110 per cent of LPA)

v) Excess (more than 110 per cent of LPA).

• The Southwest Monsoon lasts from June 1 to September 30.

• According to the long-range monsoon forecast, large parts of Tamil Nadu and the northeastern region were likely to get below-normal rainfall, while the rain-deficient parts of Marathwada and adjoining Telangana are expected to witness above-normal showers.

• Parts of Jammu & Kashmir, Ladakh, Tamil Nadu, Bihar and the northeastern states are likely to experience below-normal rainfall during the monsoon season.

• Normal to above-normal monsoon rainfall is expected in large parts of Madhya Pradesh, Rajasthan, Maharashtra, Odisha, Chhattisgarh, Uttar Pradesh and West Bengal, which form the core monsoon zone (agriculture primarily rain-fed) of the country.

How does the IMD forecast monsoon?

• The India Meteorological Department (IMD) issues operational forecast for the Southwest Monsoon season (June to September) rainfall for the country as a whole in two stages. 

• The first stage forecast is issued in April and the second stage forecast is issued in May/June.

• Since 2021 IMD has implemented a new strategy for issuing monthly and seasonal operational forecasts for the Southwest Monsoon rainfall over the country by modifying the existing two stage forecasting strategy. 

• The new strategy uses both dynamical and statistical forecasting system. Multi-Model Ensemble (MME) forecasting system based on coupled global climate models (CGCMs) from different global climate prediction centres, including IMD’s Monsoon Mission Climate Forecast System (MMCFS) is used in dynamical forecast system. 

• As per the new LRF strategy, the first stage forecast issued in middle of April consist of quantitative and probabilistic forecasts for the country as a whole and the spatial distribution of probabilistic forecasts for the tercile categories (above normal, normal, and below normal) of the seasonal rainfall (June-September) over the country.

• The second stage forecast issued around end of May consists of an update for the seasonal rainfall forecast issued in April along with the probabilistic forecasts for the seasonal rainfall over the four homogenous regions of India (northwest India, central India, south Peninsula and northeast India) and monsoon core zone (MCZ). 

• In addition, quantitative and probabilistic forecasts for the country as a whole and the spatial distribution of probabilistic forecasts of rainfall (above normal, normal, and below normal) over the country during June are also issued during the second stage forecast.

Importance of normal monsoon in India

• Monsoon brings relief to dry and parched land in the form of rain, and affects Indian agriculture in a very substantial measure. The impact of the  monsoon on the Indian economy is more pronounced. 

• Normal rainfall is critical for India’s agricultural landscape, with 52 percent of the net cultivated area relying on it. It is also crucial for the replenishing of reservoirs critical for drinking water apart from power generation across the country.

• June and July are considered the most important monsoon months for agriculture because most of the sowing for the Kharif crop takes place during this period.

• Rainfed agriculture accounts for about 40 percent of the country’s total food production, making it a crucial contributor to India’s food security and economic stability. 

• Excessive rain leads to floods in certain areas, while little or no rain in other parts bringing drought and famine resulting in acute distress to millions.

Southwest Monsoon & Northeast Monsoon

• The word monsoon comes from the Arabic word “mausim”, which means weather. 

South Asia experiences two monsoons:

i) Southwest Monsoon or summer monsoon during June to September.

ii) Northeast Monsoon or winter monsoon during October to December.

• The annual oscillation in the apparent position of the Sun between the Tropics of Cancer and Capricorn causes the annual oscillation in the position of the thermal equator (region of maximum heating) on the Earth’s surface. This is associated with the annual oscillation of temperature, pressure, wind, cloudiness, rain etc.

• On the Earth’s surface, there are asymmetries of land and Ocean. The differential heating of land and Ocean causes variations in the intensity of the annual oscillation of the thermal equator and hence regional variations in the intensity of monsoon.

• The south-westerly wind flow occurring over most parts of India and Indian seas gives rise to Southwest Monsoon over India from June to September.

• While the Southwest Monsoon is responsible for a major portion of annual rainfall over India, rainfall received during the Northeast Monsoon season (October-December) is also important, especially for the south peninsula, Sri Lanka and Maldives. 

• In India, the Northeast Monsoon season contributes about 11 per cent of its annual rainfall over the country as a whole. 

• The south peninsula receives much more rainfall compared to northern parts of the country. Many districts over the south peninsula receive 30–60 per cent of the annual rainfall during this season.

• During the Southwest Monsoon season, surface pressure gradient is directed from the Indian Ocean to land with southwesterly monsoon winds in the lower troposphere. 

• During the Northeast Monsoon season, the pressure gradient reverses (from land to the Indian Ocean) resulting in northeasterly trade winds, which is the basic state of the system. 

• This change in surface pressure gradient and lower tropospheric winds is associated with the southward movement of the continental tropical convergence zone (CTCZ) and the subtropical anticyclone in the upper troposphere. 

• Therefore, the Northeast Monsoon is often described as the retreating phase of the Southwest Monsoon.

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