Cold La Niña Surprises Meteorologists and Growers

by | Jul 1, 2011 | Weather

Clyde Fraisse
Assistant Professor
University of Florida Agricultural and Biological Engineering
cfraisse@ufl.edu

According to the NOAA National Climatic Data Center (NCDC), December 2010 was the 3rd coldest winter on record in the Southeast U.S. and the coldest on record in Florida and Georgia. The statewide temperature for December in Florida was more than 9oF (5oC) F below the 20th century average. In addition; several cities including Miami, West Palm Beach, Ft Lauderdale, Daytona, Orlando, Tampa, and Tallahassee had their coldest December on record. We had expected above average temps due to La Niña.

December temperature departure from normal – NOAA National Climatic Data Center. December 2010 was the third coldest December on record in the Southeast U.S. and the coldest on record in Florida and Georgia

 

A federal disaster declaration has been issued for two-thirds of Florida’s counties due to losses caused by frosts and freezes that occurred between Nov. 5 and Dec.17, most counties listed are located in central Florida. Farmers have eight months to apply for loans. Revenue assistance applications will be accepted later this year when the 2010 farm data are available.

The pattern that brought us the cold weather last winter in early January and February is the same one affecting us this year: the North Atlantic Oscillation (NAO). The NAO is a measure of surface pressure differences between the North Atlantic Ocean over Iceland and the tropical Atlantic near the Azores.  The NAO has been consistently in what scientists call its “negative” or “cold” phase, causing Arctic air to surge farther south into the Central and Eastern USA.  Circulation patterns over the higher latitudes of the Northern Hemisphere are also tracked with another index known as the Arctic Oscillation (AO), which has also been strongly and consistently negative.  When the NAO index is strongly negative like we have seen in the last two months, it can overwhelm the effects of the more well known El Niño/La Niña climate patterns.

You may be asking why no one predicted this. Unfortunately, NAO is more an indicator of the jet stream pattern over the Eastern U.S. and the North Atlantic, not a direct physical driver like El Niño/La Niña and temperatures of the Pacific Ocean.  Unlike El Niño and La Niña events, which can be predicted 6 months in advance, the NAO changes are not yet predictable on seasonal time scales. This causes problems with seasonal forecasts in general and in particular for the winter-season temperature forecast.  Medium-range weather prediction models are forecasting a break from this negative NOA pattern which would allow the strong La Niña to re-establish its influence over the weather patterns in the Southeast. Researchers are actively working on models that may eventually better predict the NAO shifts.

John Wells
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