El Niño-La Niña cycle needs watching

Katrina moves in. An image taken by the US National Oceanic and Atmospheric Administration shows Hurricane Katrina in the Gulf of Mexico as it headed inland. The El Niño is widely credited for restricting the impact of hurricanes in the Atlantic Ocean last year. (Picture © NOAA/AP) View larger version of image In normal, non-El Niño conditions (top panel of schematic diagram), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 metre higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface. During El Niño (bottom panel of the schematic diagram), the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific. (Diagrams © NOAA) View larger version of image

La Niña, the abnormal cooling of Pacific Ocean surface temperatures, is as powerful as her brother El Niño and the effects of global warming on their cycle need to be monitored, UK scientists said.

The La Niña weather pattern could prevent sweltering summers across the world this year, although it has also been associated with flooding in Asia and could bring more hurricanes to the Atlantic Ocean, according to forecasters.

"La Niña is just the flipside of El Niño, it's a cycle that goes around," said weather observation scientist Matt Huddleston.

"It's the biggest phenomena after the seasons. It's a natural cycle of warm water moving backwards and forwards across the Pacific, causing a lot of storms, a lot of atmospheric activity that has impact all around the globe."

The better-known El Niño refers to an abnormal warming of surface waters in the eastern Pacific. It tends to occur every two to seven years and lasts for several months.

Forecasters warn that if the El Niño-La Niña cycle were disrupted, the effects could be devastating.

"Some people have even linked the rise and fall of civilisations to persistent El Niño phenomena in the past. If that balance did change, the impact on some communities could be catastrophic," Huddleston said.

The El Niño was widely credited for restricting the impact of hurricanes in the Atlantic Ocean last year, and despite triggering floods and droughts like its counterpart, both parts of the cycle are important to global communities, Huddleston said.

La Niña tends to follow the El Niño pattern. In 1997-1998, the biggest El Niño on record killed thousands of people.

"People have talked about mega El Niños where if it gets stuck in a very strong phase it could have an incredible impact," Huddleston said.

The Met Office pays close attention to the cycle when making global forecasts. "Much research goes into connections between El Niño and La Niña and their global impacts," climate scientist Richard Graham said.

One worry among some forecasters is that global warming could disrupt the balance between the two parts of the cycle. "That's the killer question. There is not such a global consensus amongst scientists on the actual impact of global warming on El Niño and La Niña," Huddleston said.

"But global temperatures are warming, sea temperatures are warming, so we may experience more El Niño -like conditions even though that natural cycle continues."

"We need to keep an eye on it," he added.

Source: Reuters News Service