Nao Going Negative

[Chris Jennings]

what does this mean does this mean colder winters ahead.

[vizzy2004]

No mate, it only effects weather patterns in the short term as it fluctuates up and down. Its basically represents the pressure patterns in the Atlantic, so if we have High pressure over Northern Atlantic towards Greenland ( not a Greenland surface high ) and lower pressure down towards the Azores then that's a negative NAO and favors colder weather for us. Positive NAO is pretty much the opposite and is the normal typical zonal weather we see here with lower pressure in the Northern Atlantic and higher pressure towards the Azores.

So if we see the NAO drop negative then it increases the chance of snow in the short term and will not affect years ahead. Hope this helps. I'm only a novice and im sure someone could explain it better.

[Jason T]

what does this mean does this mean colder winters ahead.

The Arctic Oscillation (AO) is calculated based on seal level pressure differences (now measured by satellite for the 1000 mb height). (See: http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/hgt.shtml )

“The AO index describes the relative intensity of a semipermanent low-pressure center over the North Pole. A band of upper-level winds circulates around this center, forming a vortex. When the AO index is positive and the vortex intense, the winds tighten like a noose around the North Pole, locking cold air in place. A negative AO and weak vortex … allow intrusions of cold air to plunge southward into North America, Europe, and Asia. … the index has been mostly positive in wintertime since the late 1980s. The Arctic Oscillation has strengthened in recent decades, contributing to the unusual warmth over the Northern Hemisphere land masses.†[http://www.ucar.edu/communications/newsreleases/2003/deser.html]

The following figures show the effect of positive AO (left) and negative AO (right) [http://biogeo.botanik.uni-greifswald.de/fileadmin/user_upload/_temp_/Martin/lesson/CC_Wilmking_05.ppt]. The colder stratosphere associated with the positive AO results in reduced ozone.