South-West and North-East Monsoon in India
The word monsoon is derived from the Arabic word ‘Mausim’ which means season. Monsoon refers to the seasonal reversal in the wind direction during a year. During summer, the interior parts of North Indian Plains covering Rajasthan, Punjab, Haryana, and Western Uttar Pradesh are intensely hot. The daily maximum temperature in some of these parts is as high as 45° to 47° C.
The average maximum temperature is above 33°C in the month of May at Delhi, Jodhpur and Jaisalmer. Such high temperature heats up the air of that region. Hot air rises and due to this a low pressure area is created under it. This low pressure is also known as monsoonal trough. It lies between western Rajasthan to Odisha.
On the other hand temperature over Indian Ocean is relatively low. So a relatively high pressure region is created over the sea.
The pressure difference between Indian Ocean and North Central Indian Plains causes the air from high pressure region of the sea move towards the low pressure region of North India. This implies that the general movement of air is in June is from equatorial region of Indian Ocean to the Indian subcontinent in the South-West to North-East direction. This direction is exactly opposite to that of the trade winds (North – East to South-West) prevailing during winter in India. This complete reversal of wind direction from North-East to South West and vice-versa is known as monsoons. The winds contain a lot of moisture. When these moisture laden winds move over the Indian sub-continent they cause wide spread rain throughout India and from June to September. Thus, most of the total rainfall in India is confined to these four months only.
During the winter season, North-East trade winds prevail over India. They blow from land to sea and that is why that for most part of the country, it is a dry season. A part of North-East trade winds blow over Bay of Bengal. They gather moisture which causes rainfall in the Coromandal coast while the rest of the country remains dry. Strictly speaking these winds are planetary winds known as Northeast Trades. In India they are essentially land bearing winds.
Irregularity of Monsoon
Monsoons winds are irregular in nature affected by different atmospheric conditions. They are also not equally distributed. Coastal areas like Kerala West Bengal and Odisha receive heavy rain fall, whereas interior regions like Haryana, Madhya Pradesh, receive less rainfall. When monsoon arrives, it gives heavy rainfall which continues for several days. This is known as ‘burst of monsoon’, which generally occurs at the Kerala coast.
The monsoon tends to have ‘breaks’ in its rainfall which causes wet and dry spells. This means that monsoon rains occur only a few days at a time. Rainless dry spells occur in between.
The above simple story is based upon a mechanism proposed by Halley and is also known as Thermal Concept. However, it fails to answer the following questions:
- Why the low pressure areas on land are not stationary and why they suddenly change their location?
- Why there is no antimonsoon circulation in the upper troposphere, which must be there if the monsoon winds are thermally induced?
- Low Pressure are in northern India is in April and May, but rains start in the end of June or beginning of July.
- Monsoon rains are an amalgamation of convectional, orographic and cyclonic rainfall, the thermal concept is unsatisfactory to explain in details.
Another gentleman Fohn tried to link the Monsoon with the ITCZ or Intertropical Convergence Zone, which is called Dynamic Concept.
This concept says that monsoon is the result of seasonal migration of planetary winds and pressure belts around Equator. The Inter-Tropical Convergence Zone (ITCZ) is formed due to the convergence of north-east and south-east trade winds near the equator. In summer when the rays of Sun are directly above the Tropic of Cancer, the Northern Intertropical Zone gets extended up to 30° N latitude, thus covers the South Asia as well as South East Asia, where a low pressure area develops.
When this happens, the trade winds of the Southern Hemisphere need to cross the equator in order to reach the ITCZ. Thus, the trade winds of the Southern Hemisphere cross the equator but are deflected towards right under the Coriolis Effect. In this manner a new belt of “equatorial westerlies” is developed and Indian landmass receives the south west monsoon due to these winds.
This theory further explains that in winter, the ITCZ shifts towards south of Equator and the North East Trade winds have to cross the equator to reach the ITCZ. These winds blowing from the northern hemisphere to southern hemisphere deflected left due to Coriolis Effect and blow as North westerly Monsoon there. Since the winds blowing over the Indian subcontinent at this time are usual trade winds of these latitudes, they blow from North East to South West and so become the North East Monsoon.