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Onset of Monsoon

Context:

The southwest monsoon has arrived in Kerala three days before its normal onset date of June 1. As per the India Meteorological Department (IMD) records, this is only the fourth time since 2010 when the monsoon has arrived well ahead of its normal date.

Relevance:

GS I- Indian Geography

Dimensions of the Article:

  1. What does the “onset of monsoon” mean?
  2. Is it unusual for the monsoon to hit the Kerala coast early?
  3. What is Monsoon?
  4. Causes of Monsoon
  5. Importance of Monsoon for India
  6. Issues with Prediction of monsoon in India
  7. Two models for monsoon prediction

What does the “onset of monsoon” mean?

  • The onset of the monsoon over Kerala marks the beginning of the four-month — June-September — southwest monsoon season over India, which brings more than 70 per cent of the country’s annual rainfall. According to the IMD, the onset of the monsoon marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, and the IMD announces it only after certain newly defined and measurable parameters, adopted in 2016, are met.
  • Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed.
  • Neither early nor late onset of the monsoon is unusual.
Rainfall
  • The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep.
  • The 14 enlisted stations are: Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru.
  • It records at least 2.5 mm of rain for two consecutive days at any time after May 10.
  • In such a situation, the onset over Kerala is declared on the second day, provided specific wind and temperature criteria are also fulfilled.
Wind field
  • The depth of westerlies should be up to 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.
Heat
  • According to IMD, 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).
  • This is measured in the box confined by 5-10ºN latitude and 70-75ºE latitude.

What is Monsoon?

  • Monsoon connotes the climate associated with a seasonal reversal in the direction of winds. India has a hot monsoonal climate which is the prevalent climate in the south and southeast Asia.
  • The Indian summer monsoon typically lasts from June-September with large areas of western and central India receiving more than 90% of their total annual precipitation during the period, and southern and northwestern India receiving 50%-75% of their total annual rainfall.
  • Overall, monthly totals average 200-300 mm over the country as a whole, with the largest values observed during the heart of the monsoon season in July and August.

Causes of Monsoon

  • During the summer months, sunlight heats the surfaces of both lands and oceans, but land temperatures rise more quickly due to a lower heat capacity.
  • As the land’s surface becomes warmer, the air above it expands and an area of low pressure develops.
  • Meanwhile, the ocean remains at a lower temperature than the land and so the air above it retains a higher pressure.
  • Since winds flow from areas of the high-pressure area to low, this deficit in pressure over the continent causes winds to blow in an ocean-to-land circulation (a sea breeze).
  • As winds blow from the ocean to the land, moist air is brought inland. This is why summer monsoons cause so much rain.

Importance of Monsoon for India

The Monsoon is one of the most important single variables in the Indian economy as a good monsoon can reduce the burden on the government, while a bad one can make it spend more.

Positive effects of Monsoon

  • The agricultural prosperity of India depends very much on time and adequately distributed rainfall. If it fails, agriculture is adversely affected particularly in those regions where means of irrigation are not developed.
  • Regional variations in monsoon climate help in growing various types of crops.
  • Regional monsoon variation in India is reflected in the vast variety of food, clothes and house types.
  • Monsoon rain helps recharge dams and reservoirs, which is further used for the generation of hydroelectric power.
  • Winter rainfall by temperate cyclones in north India is highly beneficial for Rabi crops.

Negative effects of Monsoon

  • Variability of rainfall brings droughts or floods every year in some parts of the country.
  • Sudden monsoon burst creates a problem of soil erosion over large areas in India.
  • In hilly areas, sudden rainfall brings landslide which damages natural and physical infrastructure subsequently disrupting human life economically as well as socially.

Issues with Prediction of monsoon in India

Prediction of the exact behaviour of monsoon is very difficult and this makes it a problem for the Indian Metrological Department (IMD) as every year millions of Indians are dependent on its forecast.

  • The topography of the Indian subcontinent makes the monsoon system very complex. Tropical weather is difficult to predict because weather systems in the tropics aren’t understood very well.
  • The weather systems destabilise faster in the tropics than they do in the extra-tropics, where they persist for longer durations.
  • Since it is difficult to predict the exact amount of rainfall, IMD relies just on a probabilistic forecast.
  • A major problem has been to identify a small set of stable and independent parameters that influences the monsoon rainfall and the bulk of its variance. Many of the once strongly influencing parameters have declined in their correlations over the years – Hence, the search for a minimal set of stable and strongly enforcing parameters thus remains a constant one.
  • Presently, the lack of enough and quality data (the IMD collects weather data like temperature, humidity, wind and precipitation through automatic weather stations, surface observatories, radiosonde or weather balloons, radars and three satellites) is one of the biggest challenges. There are also major data gaps, like those involving dust, aerosols, soil moisture and maritime conditions.
  • Further, the automatic weather stations are of sub-standard quality. The upkeep of instruments is a major problem.
  • Another issue is that dynamical models require a huge number of computations, for which supercomputers are required. As such, the need for an increased number of supercomputers remains a challenge for India.
  • The correlation between El-Nino and Indian monsoon is still under research. And it is difficult to forecast exactly how much the El-Nino will affect.
  • In addition to this, Global warming has also emerged as a factor that affects the monsoon forecast.

Two models for monsoon prediction

  1. Statistical model: This is specific to the monsoon and is based on 16 parameters determined by IMD, for which data is collected and fed into models. These models calculate the numbers based on mathematical equations. ‘Statistical models’ try to match prevailing conditions with historical records to see how the monsoon had behaved in years when similar conditions had prevailed.
  2. General circulation method/Dynamic model: This model makes continuous observation of some selected physical phenomena, and notes how the conditions for monsoon behave over a period of time. It then follows those changes to extrapolate for the future, and comes up with a forecast. IMD has recently started using this model for weather forecasts. However, this model has its own limitations.

-Source: Indian Express


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