La Nina - how it works La Nina is mainly a Pacific event. Atlantic weather in the Caribbean is affected at the same time, but for different reasons. The jury is still out on whether the Indian Ocean is also affected in similar ways. In the Pacific there is always a big difference in sea temperatures along the equator. It is up to 10 degrees colder in the eastern Pacific off the coast of Peru than in the waters to the north of Australia (some of the warmest deep ocean waters in the world). This warm water makes the air above it unstable and helps drive convection. In a simple sense, warm air rises in the west drawing cooler air in from the east.
La Nina begins with even cooler water appearing in the east Pacific along the equator towards South America, while to the north of Australia sea temperatures rise. A La Nina event, therefore, makes the difference in ocean temperatures across the Pacific even larger. This translates into stronger trade winds: South easterlies in the southern hemisphere and north easterlies in the northern hemisphere. These stronger trades also draw colder water up from the depths of the eastern Pacific and push warmer surface water towards the west, helping to fuel the cycle. Scott Power, climate scientist with the National Climate Centre in Melbourne, Australia, calls it positive feedback and relates it to the awful squeal that music fans are subjected to at poorly engineered concerts. "You start off with a little thing and it ends up big," he says. Funny enough, meteorologists don't really know whether the cold water triggers the stronger winds, or vice versa. "It's like the chicken-and-the-egg kind of thing," says Brisbane's severe storm centre meteorologist Jeff Callaghan. What meteorologists now have, however, is greater access to satellite data of all kinds, from cloud pictures to sea surface temperatures. And this new data is helping to direct new science. Satellite information helped forecasters witness the dramatic demise of the 1996-'97 La Nina, when Cyclone Justin sat in the Coral Sea for about three weeks, cooling sea surface temperatures with its constant rain. At the same time it peppered the equator with constant westerly gales to the north of its clockwise-spinning eye. According to Callaghan these westerly winds pushed a slow-moving wave of warm water towards Peru. "We saw what was happening and we said to ourselves, 'If this works, we're in for a big El Nino in about three months'." Sure enough, the El Nino that followed was one of the century's strongest. The normally dry west coast of South America went wet. Warm water evaporating off the eastern Pacific travelled with prevailing high altitude westerly winds and collided with the great Andes, hammering Peru with flash floods and mudslides. The wet weather created commotion all over the US and Central America. However, the beginning of the 1982-'83 El Nino, during which Australia suffered its worst drought this century, began with little more than a whimper. The easterly trade winds simply moderated along the equator. "There was no one big event," recalls Callaghan. The moral of this story? We still have a lot to learn about what drives our planet's weather. © weather-wise.com 2000 |