Structure:

Introduction: (upto 30 words) Write about the normal conditions and the mechanism therefore

Body: (upto 100 words) Explain El Nino and La Nina

Conclusion: (upto 30 words) Conclude by mentioning how they affect monsoon in India. El Nino – reduced monsoons and La Nina – increased monsoons in India.

Supporting Points:

El Nino:

El Niño is the name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru.  When this warming occurs the usual upwelling of cold, nutrient rich deep ocean water is significantly reduced.  El Niño normally occurs around Christmas and usually lasts for a few weeks to a few months.

Normal Conditions:

1. In a normal year, a surface low pressure develops in the region of northern Australia and Indonesia and a high pressure system over the coast of Peru. As a result, the trade winds over the Pacific Ocean move strongly from east to west. The easterly flow of the trade winds carries warm surface waters westward, bringing convective storms (thunderstorms) to Indonesia and coastal Australia. Along the coast of Peru, cold bottom cold nutrient rich water wells up to the surface to replace the warm water that is pulled to the west.
2. The Walker circulation (walker cell) is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific ocean, and a low pressure system over Indonesia.
3. The Walker cell is indirectly related to upwelling off the coasts of Peru and Ecuador. This brings nutrient-rich cold water to the surface, increasing fishing stocks.

During El Nino year:

1. In an El Niño year, air pressure drops over large areas of the central Pacific and along the coast of South America.
2. The normal low pressure system is replaced by a weak high in the western Pacific (the southern oscillation). This change in pressure pattern causes the trade winds to be reduced - Weak Walker Cell. Sometimes Walker Cell might even get reversed.
3. This reduction allows the equatorial counter current (current along doldrums) to accumulate warm ocean water along the coastlines of Peru and Ecuador.
4. This accumulation of warm water causes the thermocline to drop in the eastern part of Pacific Ocean which cuts off the upwelling of cold deep ocean water along the coast of Peru.
5. Climatically, the development of an El Niño brings drought to the western Pacific, rains to the equatorial coast of South America, and convective storms and hurricanes to the central Pacific.

Effects of El Nino:

1. The warmer waters had a devastating effect on marine life existing off the coast of Peru and Ecuador.
2. Fish catches off the coast of South America were lower than in the normal year (Because there is no upwelling).
3. Severe droughts occur in Australia, Indonesia, India and southern Africa.
4. Heavy rains in California, Ecuador, and the Gulf of Mexico.

How El Nino impacts monsoon rainfall in India?

El Nino and Indian monsoon are inversely related. However, not all El Nino years led to a drought in India. For instance, 1997/98 was a strong El Nino year but there was no drought.

La Nina:

After an El Niño event weather conditions usually return back to normal.  However, in some years the trade winds can become extremely strong and an abnormal accumulation of cold water can occur in the central and eastern Pacific. This event is called a La Niña.

1. A strong La Niña occurred in 1988 and scientists believe that it may have been responsible for the summer drought over central North America. During this period, the Atlantic Ocean has seen very active hurricane seasons in 1998 and 1999.
2. One of the hurricanes that developed, named Mitch, was the strongest October hurricane ever to develop in about 100 years of record keeping.

Effects of La Nina:

1. Some of the other weather effects of La Niña include:
2. Abnormally heavy monsoons in India and Southeast Asia.
3. Cool and wet winter weather in southeastern Africa, wet weather in eastern Australia.
4. Cold winter in western Canada and northwestern United States.
5. Winter drought in the southern United States.