The climate of India is strongly influenced by the monsoon winds. It refers to a season in which the wind system reverses completely. The monsoons are experienced in the tropical area roughly between 20° N and 20° S.
Various atmospheric conditions influence the monsoon winds. The first condition is the differential heating and cooling of land and water. This creates low pressure on the landmass, while high pressure is created over the seas around during day time, but is reversed during the night time.
The second condition is the shift in the position of Inter-Tropical Convergence Zone (ITCZ). In summer, the equatorial trough normally positioned about 5°N of the equator moves over the Ganga plain creating a monsoon trough during the monsoon season.
The third condition is the presence of the high-pressure area that develops east of Madagascar. It is approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.
The fourth condition develops during the summer. The Tibetan Plateau gets intensely heated resulting in strong vertical air currents and high pressure over the plateau about 9 km above sea level. The fifth condition develops during the summer due to the movement of the westerly jet streams to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian Peninsula.
Changes in pressure over the southern oceans also affect the monsoons. In certain years, there is a reversal in the pressure conditions. This periodic change in pressure conditions is known as the Southern Oscillation, or SO.
The Southern Oscillation is connected to El Nino, which is a warm ocean current that flows past the Peruvian Coast. It flows every two to five years in place of the cold Peruvian current. The phenomenon is, referred to as ENSO (El Nino Southern Oscillations). In India, the monsoon lasts for 100 to 120 days from early June and to mid-September. The monsoon winds encounter various atmospheric conditions on their way and hence are pulsating in nature, and not steady.
The monsoon arrives with a sudden downpour of rainfall that continues for several days. This is known as the ‘burst’ of the monsoon.
The monsoon arrives at the southern tip of the Indian Peninsula generally by the first week of June. By early September, the monsoon starts to withdraw or retreat and is a more gradual process. By mid-October, it withdraws completely from the northern half of the peninsula. The withdrawal takes place progressively from north to south from the first week of December to the first week of January. This is the start of the winter season.
The retreating monsoon winds move over the Arabian Sea and the Bay of Bengal, and collect moisture on the way. These monsoon winds reach the southern states of India by October, and are responsible for a second round of rainfall. These are called the winter monsoons. The winter monsoon is experienced in the states of Tamil Nadu, Kerala and Andhra Pradesh in the first week of January.