Arctic Oscillation

[Kathryn41 3,205]

http://earthobservatory.nasa.gov/IOTD/view.php?id=42260

(follow the link for the image)

Winter Temperatures and the Arctic Oscillation

"If you live nearly anywhere in North America, Europe, or Asia, it’s

no news that December 2009 and early January 2010 were cold. This image

illustrates how cold December was compared to the average of

temperatures recorded in December between 2000 and 2008. Blue points to

colder than average land surface temperatures, while red indicates

warmer temperatures. Much of the Northern Hemisphere experienced cold

land surface temperatures, but the Arctic was exceptionally warm. This

weather pattern is a tale-tell sign of the Arctic Oscillation.

The Arctic Oscillation is a climate pattern that influences winter

weather in the Northern Hemisphere. It is defined by the pressure

difference between air at mid-latitudes (around 45 degrees North, about

the latitude of Montreal, Canada or Bordeaux, France) and air over the

Arctic. A low-pressure air mass usually dominates the Arctic, and while

higher pressure air sits over the mid-latitudes. This pressure

difference generates winds that confine extremely cold air to the

Arctic. Sometimes, the pressure systems weaken, decreasing the pressure

difference between the Arctic and midlatitudes and allowing chilly

Arctic air to slide south while warmer air creeps north. A

weaker-than-normal Arctic Oscillation is said to be negative. When the

pressure systems are strong, the Arctic Oscillation is positive.

Throughout December 2009, the North Atlantic Oscillation was

strongly negative, said the National Weather Service.

This image shows the impact of the negative Arctic Oscillation on land surface

temperatures throughout the Northern Hemisphere as observed by the

Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra

satellite. Cold Arctic air chilled the land surface at midlatitudes,

while Arctic land, such as Greenland and Alaska, was much warmer than

usual."

Edited January 13, 2010 by Kathryn41