La Niña or “Little girl” is the opposite of El Niño. La Niña sees cooler than average sea surface temperatures in the equatorial Pacific region. It is the “Cool phase”. Trade winds are stronger than usual, pushing warmer water towards Asia.
- 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.
The Conditions of La Nina.
- It is caused by a build-up of cooler-than-normal waters in the tropical Pacific, the area of the Pacific Ocean between the Tropic of Cancer and the Tropic of Capricorn.
- It is characterized by lower-than-normal air pressure over the western Pacific. These low-pressure zones contribute to increased rainfall.
- La Nina events are also associates with rainier-than-normal conditions over southeastern Africa and northern Brazil.
- However, strong La Nina events are associates with catastrophic floods in northern Australia.
- It is also characterizes by higher-than-normal pressure over the central and eastern Pacific.
- This results in decreased cloud production and rainfall in that region.
- Drier-than-normal conditions are observe along the west coast of tropical South America, the Gulf Coast of the United States, and the pampas region of southern South America.
- It tends to lead to milder winters in Northern Europe and colder winters in southern/western Europe leading to snow in the Mediterranean region.
- It is continental North America where most of these conditions are felt.
- It causes drought in the South American countries of Peru and Ecuador.
- It usually has a positive impact on the fishing industry of western South America.
- In the western Pacific, La Nina increases the potential for landfall in those areas most vulnerable to their effects, and especially into continental Asia and China.
- It also leads to heavy floods in Australia and high temperatures in Western Pacific, Indian Ocean, off the Somalian coast and a comparatively better monsoon rains in India.
Triple dip La Nina – The extended spell of La Nina lasting across three winter seasons in the northern hemisphere is called ‘Triple dip La Nina’.
Walker circulation – The air circulation as a result of difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean is known as Walker circulation.
Read more : Pressure belts