From UPSC perspective, the following things are important :
Prelims level : Onset of Monsoon, ITCZ, etc.
Mains level : Factors that influence the onset of south-west monsoons, Indian Monsoon
- The monsoon’s arrival over Kerala has been delayed to June 3, according to an update by the India Meteorological Department (IMD).
- Private weather forecast agency, Skymet, however, said that the monsoon had arrived.
- This was because two of the three criteria — as defined by the IMD — had been met.
- Currently, IMD’s own data indicated that except for the OLR, the other criteria were met. Thus, there is an element of subjectivity in arrival.
What are those criterias defined by IMD?
- Rain-bearing westerlies being at a minimum depth and speed;
- At least 60% of the available 14 stations in Kerala and coastal Karnataka, reporting rainfall of 2.5 mm or more for two consecutive days after May 10;
- A certain degree of clouding, indicated by a parameter called ‘outgoing long wave radiation’ (OLR), being below 200 W/square meter.
What is meant by ‘Outgoing Long Wave Radiation’ (OLR)?
- Outgoing Long-wave Radiation (OLR) is electromagnetic radiation of wavelengths from 3–100 μm emitted from Earth and its atmosphere out to space in the form of thermal radiation.
- It is also referred to as up-welling long-wave radiation and terrestrial long-wave flux, among others.
- The flux of energy transported by outgoing long-wave radiation is measured in W/m.
- In the Earth’s climate system, long-wave radiation involves processes of absorption, scattering, and emissions from atmospheric gases, aerosols, clouds and the surface.
- Over 99% of outgoing long-wave radiation has wavelengths between 4 μm and 100 μm, in the thermal infrared part of the electromagnetic spectrum.
Factors responsible for south-west monsoon formation:
- Intense heating of Tibetan plateau during summer months.
- Permanent high pressure cell in the South Indian Ocean (east to north-east of Madagascar in summer).
Factors that influence the onset of south-west monsoons:
- Above points +
- Subtropical Jet Stream (STJ).
- Tropical Easterly Jet (African Easterly Jet).
- Inter Tropical Convergence Zone.
Factors that influence the intensity of south-west monsoons:
- Strengths of Low pressure over Tibet and high pressure over southern Indian Ocean.
- Somali Jet (Findlater Jet).
- Somali Current (Findlater Current).
- Indian Ocean branch of Walker Cell.
- Indian Ocean Dipole.
Factors responsible for north-east monsoon formation:
- Formation and strengthening of high pressure cells over Tibetan plateau and Siberian Plateau in winter.
- Westward migration and subsequent weakening of high pressure cell in the Southern Indian Ocean.
- Migration of ITCZ to the south of India.