   La Nina Weather Conditions - January 26, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Our winter weather has been very dry for the second year in a row. Much of the state has received little or no precipitation since the end of the monsoon season. Forecasts predict that the dry weather may continue through spring. This dry spell has been blamed on a phenomenon known as "La Nina." La Nina is one phase of an important fluctuation in sea surface temperature and wind flow in the equatorial Pacific Ocean. The other corresponding phase of this fluctuation is called "El Nino." I'm fairly certain that many readers are familiar with these names, but do you know what causes these conditions? Knowing more about it won't increase the likelihood of rain, but at least we'll all know what we are swearing about. The tropical regions just to the north and south of the equator have consistent east to west wind flow patterns known as the trade winds. In the Pacific, these winds drive the warm surface water to the west creating an area of warm water near Indonesia. As the wind pushes the water to the west, the displaced water in the eastern equatorial Pacific is replaced by much cooler subsurface water during a process called upwelling. Scientists have discovered that the intensity of the trade winds fluctuates on a two to seven year cycle. When the trade winds are strongest, warm water is driven westward at a higher rate resulting in above normal sea surface temperatures in the western Pacific. This also results in greater upwelling and below normal surface temperatures in the eastern Pacific. These periods of strong trade winds are known as La Nina conditions. Conversely, below average trade wind speeds result in below average sea surface temperatures in the western Pacific and above average surface temperatures in the eastern Pacific. This condition known as El Nino. So, what does water temperature in the Pacific have to do with Arizona weather? These changes in sea surface temperature impact the general circulation of the atmosphere, especially during the winter months. This is reflected in the weather patterns of the subtropical and temperate regions of the United States. La Nina conditions push the main winter storm track northward into the Pacific Northwest reducing the frequency and intensity of the winter storms that bring precipitation to the Southwest. In contrast, the El Nino phase of the cycle tends to move the winter storm track further south bringing more precipitation to the Southwest. El Nino winters usually bring above normal precipitation to Arizona. Historically, nearly all of the region's serious floods have been during El Nino phases. Last winter, La Nina conditions were very evident. This year, they are much less well defined. Ocean surface temperatures and other measurements currently show that we are leaving the La Nina phase and approaching an intermediate phase where neither La Nina nor El Nino conditions prevail. However, the National Oceanic and Atmospheric Administration's Climate Prediction Center observed some strengthening of La Nina during November 1999 and forecast models indicate that La Nina will continue through April 2000. This translates into a forecast of lower than average precipitation for Arizona through May. The National Climate Data Center has said that most of the state of Arizona is in the mild drought category. Northwest Arizona had a drier monsoon season and consequently is in a severe drought category. As backyard gardeners, this is certainly not what we want to hear. This also puts undue stress on agricultural producers and communities already coping with limited water supplies. Can you say xeriscape? The above information was excerpted from a report prepared by Dr. Paul Brown, a University of Arizona Cooperative Extension Specialist in Biometeorology. Dr. Brown was also awarded 1999 Arizona Extensionist of the Year. The University of Arizona Cooperative Extension has publications and information on climate and irrigation. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |