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Predicting El Ni

DOE/Los Alamos National Laboratory : 24 May, 2007  (Technical Article)
The Spanish term 'El Ni
In modern usage, the term refers to the oceanic and atmospheric condition in which the sea-surface temperature in the tropical eastern Pacific area becomes significantly warmer than normal, and the prevailing westward trade winds either disappear or reverse direction. The El Niño cycle consists of a cold phase and a warm phase. The cold phase is called La Niña.

The largest sources of interannual climate variability on a global scale, El Niño effects have far-reaching societal consequences. While scientists know the sequence of phenomena once an El Niño event begins, they are still unable to predict when they will occur.

Two Los Alamos researchers have mapped the life cycles of El Niño events and identified categories of patterns that may lead to a model to predict occurrences up to a year in advance. Chung-Chieng 'Aaron' Lai and Zhen Huang of the Laboratory's Atmospheric and Climate Sciences Group will present a paper detailing the variations documented in several ocean basins.

'People have tried to forecast the regional weather based on the El Niño/La Niña cycle, but have discovered that every one of these cycles is unique,' Lai said. 'We can't assume that the general weather patterns in two winters will be similar merely because they are both in El Niño years.'

The key element of the El Niño phenomenon is the interaction between the winds in the atmosphere and the sea surface water. In normal years, when there is no El Niño, the trade winds tend to blow from east to west across the coastal waters of the eastern Pacific. They tend to drag the surface warm waters westward across the ocean and form a 'warm pool' in the tropical western Pacific. This in turn causes deeper, colder waters to rise to the surface in the eastern Pacific.

At the beginning of an El Niño event, the westward trade winds weaken, causing the upwelling of deep, cold water to cease. One of the fundamental questions researchers must answer to understand the phenomenon is: what causes the original weakening of the westward trade winds?

The Los Alamos researchers have studied the El Niño/La Niña cycle with data analysis and modeling using patterns of anomalies in sea surface temperatures. By examining these over a long time period, 1871 to 1994, they found the cycle in all tropical and extratropical ocean basis.

'We have found El Niños, and subsequently La Niñas, appearing in the North Pacific, South Pacific, Indian Ocean, Equatorial Pacific, South Atlantic and North Atlantic,' Lai said. 'Sometimes they appear simultaneously over all world ocean basins except the Arctic and Southern Oceans. It's not uncommon for an El Niño to appear in one ocean basin while a La Niña appears in another ocean basin.'

Lai believes the Antarctic Circumpolar Wave, a series of weather anomalies originating in the Southern Oceans, may hold a key to understanding the El Niño phenomenon. The Southern Oceans are the only oceanic domain encircling the globe, and they contain the strong eastward flow of the Antarctic Circumpolar Current, the unifying link for exchanges of water masses at all depths between the world's major ocean basins.

'Because these exchanges are an important control of mean global climate, the Southern Oceans are expected to play an important role in transmitting climate anomalies around the globe,' Lai said.

The Lai and Huang paper is one of several being presented by Los Alamos scientists at the various symposia that are part of the 81st annual meeting of the AMS at the Albuquerque Convention Center. In addition, the Annual Meeting Exhibition includes a Los Alamos booth, designed to inform meeting attendees about the Laboratory's contributions to meteorology, hydrology and oceanography. The exhibit's creators hope it will help recruit graduate students and post-doctoral researchers.
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