‘Onset’ of the Monsoon

Context: According to the India Meteorological Department (IMD), the southwest monsoon has set in over the Kerala coast.
Probable Question: Q. What are potential impacts of Climate change on Indian Monsoon and how is the government of India addressing these challenges?
 About Monsoon:
  • ‘Monsoon’ refers to the seasonal reversal in the wind direction during a year.
  • Types:
    • South West monsoon: It starts with the onset of monsoon rains over the southern state of Kerala.
    • Northeast Monsoon: It is also known as the winter monsoon or the retreating monsoon, affects the southern and eastern coastal regions of India.
Mechanism of South-West Monsoon:
  • By early June, the low-pressure condition over the northern plains intensifies. It attracts the trade winds of the southern hemisphere.
  • These south-east trade winds originate over the warm subtropical areas of the southern oceans.
  • They cross the equator and blow in a southwesterly direction entering the Indian peninsula as the south-west monsoon.
  • As these winds blow over warm oceans, they bring abundant moisture to the subcontinent.
  • These winds are strong and blow at an average velocity of 30 km per hour. With the exception of the extreme north-west, the monsoon winds cover the country in about a month.
Key Features of Indian Monsoon:
  • Monsoon Breaks: Indian monsoon is characterised by ‘breaks’ in rainfall. Thus, it has wet and dry spells.
  • Southwest Monsoon: The southwest monsoon is the major monsoon season in India, lasting from June to September.
  • Monsoon Trough: The monsoon trough is a low-pressure area that forms along the foothills of the Himalayas during the monsoon season. It acts as a major rainfall zone, bringing widespread precipitation to northern and central India.
What is “Onset of the Monsoon”?
  • The onset of the monsoon refers to the arrival of the southwest monsoon winds over the Indian subcontinent.
  • It usually occurs around the end of May or early June in the southern state of Kerala and progresses northwards over the subsequent weeks
  • In the South-West Monsoon India gets more than 70% of its annual rainfall.
  • Contrary to popular assumption, the onset does not mean the first rain of the season as that can start happening in certain places even before the onset is declared.
  • According to the IMD, the onset of the monsoon happens when there is a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.
  • IMD announces the onset of the monsoon only after certain precisely defined and measurable parameters are met.
Conditions for the Onset of Monsoon:
  • Rainfall: 
    • The onset is declared 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. 
    • The onset over Kerala is declared on the second day, as long as specific wind and temperature criteria are also fulfilled.
  • Wind Field: The IMD says that the depth of westerlies should be up to 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area that is bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE.
    • The 10th parallel North passes through Kochi; and the area bounded by the 55th and the 80th meridians East stretches from the middle of Iran to about Chennai.
    • The zonal wind speed over the area bound by 5-10ºN latitude (Maldives to Kochi) and 70-80ºE longitude (Arabian Sea to Chennai) should be of the order of 15-20 knots (28-37 kph) at 925 hPa.
  • Heat: The INSAT-derived Outgoing Longwave Radiation (OLR) value should be below 200 watt per sq m (wm2) in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes
Is a delay in the onset of the monsoon unusual?
  • The normal date of the onset of the monsoon over the Kerala coast is June 1, and this year’s delay is significant. However, neither an early nor a late onset of the monsoon is unusual.
    • For Example: So, in 2021, 2019, 2016, 2015, 2014, 2012, and 2011, the monsoon onset was released after June.
  • A delay or an early arrival has no bearing on the quality or amount of rainfall, or its regional distribution across the country.
  • In a recent year, the onset occurred two days in advance of the normal date and it rained heavily for about 10 days after that — however, the season as a whole ended with 14% less rain than normal.
Does a delayed onset mean cascading delays across the country and for the rest of the season?
  • A delay in the onset over Kerala can potentially delay the arrival of the monsoon in other parts of the country, especially in the southern states, which normally start getting rain within days of the monsoon reaching the Kerala coast.
  • The northward progression of the monsoon after it has hit the Kerala coast is not uniform — it depends on local factors, including the creation of low-pressure areas.
    • The monsoon may stall over certain places in certain years; or it may progress faster than usual.
  • However, it has been observed at times that despite a late onset over Kerala, other parts of the country will start getting monsoon rain on time.
Impact of Monsoons life in India:
  • Agriculture: The monsoon is crucial for agriculture in India, as a large portion of the population depends on rain-fed farming.
  • Water Resources: The monsoon plays a vital role in replenishing water resources such as rivers, reservoirs, lakes, and groundwater.
  • Flooding: The intense rainfall associated with the monsoon can lead to flooding in many parts of India.
  • Public Health: During the monsoon season Stagnant water and increased humidity create favourable conditions for the breeding of mosquitoes, leading to the spread of diseases like malaria, dengue fever, and chikungunya.
Potential Impacts of Climate change on the Indian monsoon:
  • Changes in Rainfall Patterns: Climate change may alter the spatial and temporal distribution of rainfall during the monsoon season.
  • Monsoon Variability: Climate change may lead to increased variability in the monsoon, with fluctuations in the onset, duration, and withdrawal of the monsoon season.
Way Forward: Ensuring the sustainability and effective management of the Indian monsoon requires a multi-faceted approach which could include:
  • Improving Forecasting and Early Warning Systems: Investing in advanced meteorological technologies, satellite monitoring systems, and data analysis can help improve the accuracy of predictions.
  • Diversification of Agriculture: Reducing dependence on rain-fed agriculture by promoting crop diversification and alternate farming practices can enhance resilience to monsoon variability.
  • Climate-Resilient Infrastructure: Building infrastructure that is resilient to monsoon impacts is crucial. This includes robust drainage systems in urban areas to manage excessive rainfall and prevent flooding.
  • Strengthening Disaster Preparedness and Response: Enhancing disaster preparedness and response mechanisms is crucial to minimise the impact of monsoon-related disasters.
Additional Information: About Outgoing Longwave Radiation (OLR):
  • The INSAT-derived Outgoing Longwave Radiation (OLR) value — which is a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere.
Monsoon Mission:
  • Under the Monsoon Mission, the Ministry has developed state-of-the-art weather and climate prediction models.
  • These models include models for short range to medium range (1-10 days), extended range (10 days to 30 days) and seasonal (up to one season).
 Source: The Indian Express 
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