Q+A: Why do El Nino and La Nina trigger weather chaos?

Wed Jan 12, 2011 6:49am EST

(Reuters) - The record floods sweeping Australia's Queensland state are the result of a natural swing in Pacific Ocean temperatures that can trigger climate chaos around the globe.

Scientists say the floods are caused by one of the strongest La Nina episodes on record with experts forecasting it will last for several more months. The disaster, which has killed 16 people, could cut Australia's economic growth by 1 percent, a central bank board member said.

Following are some questions and answers on the La Nina weather pattern and its sibling El Nino and their billion-dollar impacts on economies.

WHAT IS EL NINO?

El Nino, also called "Little Boy" or "Christ Child," is a warming of ocean surface temperatures in the eastern and central Pacific.

Peruvian fishermen noticed the arrival of the warm waters occurred usually around Christmas time. The phenomena, which occurs every three to seven years, led to more rains in that part of South America and a drop in the fish catch.

Strong El Ninos can lead to a dramatic weakening of the trade winds that blow west across the Pacific, triggering drought in Southeast Asia and Australia. Some El Ninos can also affect the Indian monsoon by reducing rainfall, threatening crops and livelihoods.

WHAT IS THE IMPACT?

Globally, El Nino can trigger above average rains in northern Peru and Bolivia, drought in Southeast Asia, Australia, India and northeast Brazil, cyclones in the central Pacific and stormy weather in southern and western United States.

El Ninos also tend to cut the number of Atlantic hurricanes but boost the number of storms in the eastern Pacific.

Globally, the result can be disastrous in terms of loss of human life, infrastructure, crops and business. In Australia, strong El Ninos can slash wheat crop output, threaten water supplies by cutting river flows, shrink city reservoirs and dry out forests, triggering deadly bush fires.

Major El Ninos occurred in 1982-82 and 1997-98 but the weak El Nino of 2002-2003 also led to severe drought in Australia.

WHAT IS LA NINA?

La Nina, or "Little Girl," is the opposite to El Nino, leading to abnormally cool ocean temperatures in the eastern and central Pacific.

This triggers stronger trade winds across the Pacific that pile up very warm warmers in the western Pacific and around northern Australia.

This fuels tropical weather by leading to more cloud cover and rain during the monsoon as well as a jump in the number of cyclones during the December-April Australian cyclone season.

IMPACTS

Just like El Nino, the weather impact can be global. In Indonesia and parts of Australia, La Nina can bring flooding rains, affecting wheat, sugar, palm oil and rubber crops as well as coal and iron ore mining.

In Argentina and the U.S. Plains, La Nina can trigger drought, hurting the Argentine corn and soy crops and the main U.S. hard red winter wheat crop.

La Nina tends to lead to wind patterns that favor the formation of more hurricanes in the Atlantic and fewer in the eastern Pacific, potentially meaning a greater threat to U.S. Gulf oil and gas assets.

Major recent La Nina events occurred in 1988-89 and 1973-76.

WHAT ARE THE GLOBAL LINKS?

Heat from the tropics helps drive the global climate by fuelling ocean and atmospheric patterns that shift the warmth around the globe.

Warm tropical waters fuel evaporation and add moisture to the atmosphere needed for clouds to form. The rising air also drives atmospheric circulation patterns that help shift the moisture and warmth around to other parts of the globe. So disruption of these patterns can alter the climate elsewhere.

WILL CLIMATE CHANGE AFFECT EL NINO AND LA NINA?

The jury is still out on this. Some computer climate models tend to show a future trend toward more El Nino episodes as the world warms.

Climate scientists say it will be some years before a clearer pattern is likely to emerge. By then computers will be much more powerful to run detailed, high-resolution simulations to test the impacts of warmer oceans and atmosphere over time-scales of several decades to a century.

Scientists say a warmer world will mean more extreme droughts and floods and possible sudden shifts in ocean or atmospheric patterns, with devastating impacts.

(Sources: NOAA, Australian Bureau of Meteorology, National Center for Atmospheric Research, Reuters)

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