The Indian Monsoon
Definition : The word monsoon comes from the Arabic word ‘mausam’ which means season. Monsoon is thus rain bearing winds which reverse their direction with the change of ‘mausam’.
Facts about mechanism of Monsoons :
(A) The differential heating and cooling of land and water creates low pressure on the landmass of India while the sea around experiences comparatively high pressure.
(B) The shift of the position of Inter Tropical Convergence Zone (ITCZ) . In summer over the Ganga plain (This is the equatorial trough normally positioned about 5º N of the equator - also known as the monsoon trough during the monsoon season).
(C) The presence of high pressure area, east of Madagascar approximately at 20ºS over the Indian ocean. The intensity and position of this high pressure area affects the Indian monsoon.
(D) The Tibetan plateau gets intensely heated during summer which results in strong vertical air currents and the formation of high pressure over the plateau at about 9 km above sea level.
(E) The movements of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
Southern Oscillation or SO : When the tropical eastern south Pacific Ocean experiences high pressure the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure condition is known as Southern Oscillation or SO.
El Nino Southern Oscillations : A warm ocean current EIL-Nino, flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years. The changes in pressure conditions are connected to the El Nino. Hence the phenomenon is referred to as ENSO. The presence of the EIL-Nino leads to an increase in sea surface temperatures and weakening of the trade winds in the region.