Here meteorologists have tried to find out relationship between Indian Monsoon and El Nino, Southern Oscillation, Walker Cell and the Somali Ocean current.
El Nino meaning ‘Child Christ’ is a warm ocean current appearing along the Peru coast in December. It replaces the Peru or Humboldt Cold Ocean current flowing over this region in normal years. Under normal conditions the mixed layer over the eastern Pacific (Peru and Ecuador) is cool and shallow, while over the western Pacific (Indonesia and Western Australia) it is warm and deep. Such conditions are helpful for strong south-west monsoons.
The appearance of El Nino called ‘El Nino anomaly ‘reverses the condition (warm condition over eastern Pacific and cold in western Pacific). Since El Nino represents large atmospheric perturbations to which the ocean responds with warmer or colder surface temperatures, it leads to extreme events, such as droughts, floods and poor monsoons. The El Nino in 1986-87 did cause a poor monsoon in 1987. But there has been exception to these findings.
“The Southern Oscillation is the name ascribed to a seesaw pattern of meteorological changes that are often observed between the Pacific and the Indian Ocean. It has been noticed that whenever the surface pressure is high over the Pacific, the pressures over the Indian Ocean tend to be low, and vice- versa.
This oscillation was discovered by Sir Gilbert Walker, the first director general of Indian meteorological service, in 1924″ (Das, P.K., 1988, p. 257). “As pressures are inversely related to rainfall this suggests that when low pressures prevail over the Indian Ocean in the winter months (positive SOI) the chances are that the coming monsoon rains will be good. The oscillation has a period varying from 2-7 years. The intensity of the Southern Oscillation (SOI) is measured by the difference in sea level pressures of Tahiti (18° S, 149° W), a station in the mid- Pacific, and Port Darwin (12° S, 130° E), a representative station of the Indian Ocean.
A negative value of the Southern Oscillation Index (SOI) implies higher pressure over north Indian Ocean and a poor or indifferent monsoon (Das, P.K., 1990, p. 46). There is close relationship between the appearance of the El Nino and the negative SOI. This low or a negative phase of the SOI in combination with an El Nino is called an ENSO event.
The Southern Oscillation is closely linked with the Walker Circulation (named after Sir Gilbert
Walker). With a high positive SOI there would be a zone of low atmospheric pressure over Australia and the Indonesian archipelago. It will be accompanied by large convective clouds, heavy rainfall and rising air motion. This air eventually turns eastwards and after traversing the Pacific as a high level westerly wind at 200 mb it descends over South America. The Walker circulation, thus, has a rising limb over Australia-Indonesia with a descending branch over South America.
In terms of global winds the Walker circulation suggests a strong bell of convergence between the trade winds of both hemispheres at a location slightly to the north of the equator.
These trade winds pile up huge quantity of warm water in the western Pacific, produce equatorial counter current from the Indonesian coast and facilitate upwelling of cold water from below near Peru coast giving rise to cold Humboldt Current. Hence, high positive value of SOI indicates (a) a cold Humboldt current and upwelling off the Peru coast, (b) strong trade winds, (c) accumulation of water in the western Pacific which is balanced by the equatorial counter current and under current, (d) a rise in the depth of the thermo cline as we proceed from the cast to the western half of the Pacific, and (e) an ascending branch of the Walker circulation over Australia- Indonesia with its descending branch over South America. Such normal condition leads to normal south-west monsoon.
The appearance of El Nino leads to ‘warm’ phase of the Pacific or negative SOI. Now the ascending branch of the Walker cell shifts to the central regions of the Pacific and the descending branch to the south-eastern parts of the ocean. As upwelling off the South American coast decreases, the sea surface temperature rises. This leads to weaker trade winds, less accumulation of water on the western half of the Pacific, weakening of the equatorial undercurrent, heavy rain and floods along the South American coast and poor monsoon over India.
Meteorological observations indicate that the circulation of air over the tropics is dominated by two gigantic cells. The first is known as a Hadley Cell which is oriented in north-south direction. During the summer monsoon its ascending limb is over the plateau of Tibet. This ascending air from Tibet moves southwards as tropical easterly jet and eventually descends over the Indian Ocean.
The sect cell is known as the Walker Cell which is oriented in east-west direction. Its ascending and descending limbs are closely linked to sea surface temperature (SST). Temperatures at the surface of the sea are usually cool in regions of descending motion (Precast), while they are warm in zones of ascend (Indonesia-Australia coast). During El Nino effect or negative SOI the descending limbs of Walker Cell are moved eastward. Thus, a larger area over Indi now becomes a zone of descent rather than ascend leading to poor monsoon and drought conditions.
(d) The Somali Current-The Somali cure” is one of the few warm ocean currents which reverses its direction in sympathy with the overlying wind. During the northern summer it flows north ward from a location to the south of the equator about 10° N before turning towards the Indian coast during winter, however, it flows southward following the north-east winter monsoons.
The Somali current is the western limb oft gigantic whirl over the Arabian Sea called ‘gyre’. Recent observations suggest that the current is made up of two gyres: (a) northern gyre between 5° and 9″ N latitudes, and (b) southern gyre between the equator and 4° N latitudes. As the summer monsoon sets in over India, the southern gyre begins to move northwards and finally the two gyres coalesce. Bui in some years instead of coalescence the north gyre just moves away. Also the southern gyre is more prominent in years of good monsoon, while years of weak monsoon it is either weak or abase the area lying between these two gyres is in the of a wedge and region of intense upwelling.
The leads to thermal anomaly between the coast of S Malia (June, 15°C) and Mumbai (30°C). This gradient of temperature influences the radiation banal of the monsoon air (Das, P.K., 1990, p. 15).