The Indian Monsoon
Following facts are important in formation of monsoon:
The Sun causes differential heating and cooling of land and water. This creates low pressure on the landmass of India and high pressure over the ocean surface.
Shifting of ITCZ
The Inter Tropical Convergence Zone (ITCZ) is normally positioned about 5°N of the equator. It shifts over the Ganga plain during summer. It is also known as the monsoon trough during the monsoon season.
1: (a) The Ganga Plain, 2: (b) North of the Himalayas, 3: (c) Ganga Plains, 4: (d) Indian Peninsula, 5: (a) Warm ocean current, 6: (b) El Nino Southern Oscillation, 7: (a) Eastern Pacific and eastern Indian oceans, 8: (c) Indian Ocean < Pacific Ocean, 9: (d) Periodic change in pressure differential, 10: (b) Peruvian coast
High pressure near Madagascar
The high pressure area, east of Madagascar is approximately 20°S over the Indian Ocean. This area affects the Indian Monsoon.
High pressure over Tibetan Plateau
The Tibetan plateau gets intensely heated during summer. This results in strong vertical air currents and formation of high pressure over the plateau. This high pressure zone is about 9 km above the sea level.
Movement of Jet Streams
The westerly jet stream move to the north of the Himalayas, and the tropical easterly jet stream moves over the Indian Peninsula during summer.
In normal circumstances, when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. Such changes in the pressure conditions over the southern oceans also affect the monsoon.
But in certain years, there is a reversal in the pressure conditions. In this case, the eastern Pacific Ocean has lower pressure compared to the eastern Indian Ocean.
This periodic change in pressure conditions is known as the Southern Oscillation or SO.
The difference in pressure over Tahiti and Darwin is computed to predict the intensity of the monsoons. Tahiti (18°S/149°W) lies in the Pacific Ocean and Darwin (12°30’S/131°E) lies in northern Australia. If the pressure differences are negative, it means a below average and late monsoon.
This is a feature which is connected with the SO. El Nino is a warm ocean current that flows past the Peruvian coast; in place of the cold Peruvian current, every 2 to 5 years. El Nino significantly affects the changes in pressure conditions. Hence, the phenomenon is called ENSO (El Nino Southern Oscillations).