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El Nino: Building toward wet winter and spring

It is official. It is an El Nino year because the government has said so. Problem is for those living in the Desert Southwest, it just doesn't seem like it with the very dry winter experienced so far.

Paul Brown, University of Arizona vegetable specialists at the UA Yuma Agricultural Center, agrees with the government forecast because since September Pacific trade winds along the equator have weakened and ocean temperatures off South America have cooled and water off Indonesia and Australia is warming, the telltale signs of El Nino.

Brown told the 17th annual Desert Vegetable Crops Workshop in Yuma, Ariz., recently that this portends a wet winter weather cycle in the Southwest, and wet spring weather for the Southern tier of the U.S.

The weather so far is more like La Nina, the drought-producing opposite of El Nino. El Nino and La Nina cycle ever three to seven years.

This weather pattern along the equator, said Brown, is one of the best indicators for long range forecasting meteorologists have and therefore is closely watched.

El Nino and La Nina have nothing to do with global warming, said Brown. Indian tribes along the South American coast have been noticing changes in ocean water temperatures for 600 years with changing fishing patterns.

While trade winds and ocean temperatures are good indicators of changing dry-wet cycles, how wet or dry it becomes depends on how strong the warm /cool ocean water patterns become. The strength of these cycles is determined by the barometric pressure difference between Tahiti and Darwin, Australia in December. A strong pressure difference between the two locations is an indication of a strong El Nino.

‘Right on border’

Right now that difference is “right on the border” of becoming a full-fledged El Nino. If that happens, history tells desert farmers they can expect twice the normal (about 2.5 inches) rainfall from October through March. And the U.S. Sunbelt can expect more spring rain than normal.

Since 1950, there have been 8 El Ninos in the Desert Southwest with the majority of the rain in those years coming from December through March in the form of a Pineapple Express pushing moisture from Hawaii through the Desert Southwest in a jet stream.

This year El Nino was late to the party. Weather patterns along the equator did not start changing until September. A strong El Nino is usually in place by summer.

Brown calls this government-forecasted El Nino moderate in strength now, but building.

Brown says the federal government's “fearless forecast” calls for the wet El Nino weather pattern to continue early into 2007 and likely grow in intensity; therefore farmers can expect above normal precipitation this winter and into the spring in the U.S. Sunbelt.

If it is a strong El Nino, there is a 75-85 percent chance of above normal precipitation. If it is moderate, the chance of heavier than normal rain is just 50-65 percent.

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