WHAT IS THE NAO?

1.

The North Atlantic Oscillation (NAO) is part of an overall High-Latitude Atmospheric Blocking scheme, which oscillates between a North-South Center dipole of two anomalies. When the NAO is in a Negative Phase or North Centered, an intensifying High pressure anomaly center becomes established over the *southern-tip of Greenland... commonly referred to as the Greenland Block. Which in-return blocks, buckles the jet-stream (upper-air flow) from moving straight west to east... establishing a Meridional north to south flow. Sending air of polar origin south into middle latitudes of eastern North America & western Europe. *The Negative NAO influence is often noted as West-based or East-based, and although the overall -NAO loading-pattern effects New England with below to well below normal temperatures...where the blocking takes place in the North Atlantic is important. The West-based -NAO directly impacts New England & especially the Mid-Atlantic, as a result of the blocking high pressure anomaly center near or just off the southern-tip of Greenland. This pattern, teleconnecting with the Arctic Oscillation (AO) often produces text-book...                         

2.

Nor'easters. Due to a deep trough associated with an intense cold building High to the north, over Ontario, Canada ... that often becomes negatively-tilt as the downstream trough lifts-up along the U. S. Eastern Seaboard. 
For more information on Nor'easters, please visit... 

The East-based -NAO effects are less pronounced over New England & especially the Mid-Atlantic... due to the blocking High pressure anomaly to the east of Greenland. This pattern is still associated with below normal temperatures, however it usually does not teleconnect with the Arctic Oscillation... allowing it's influence to be short-lived. Also, with the High further east... the downstream trough is also further east, that is positively-tilt. In return, this significantly limits development along the U. S. Eastern Seaboard & promotes Miller Type B cyclogenesis that does develop... to move East-northeast from the East Coast.

3.

When an intensifying High pressure anomaly center builds off the southeastern 
United States coast or near Bermuda... commonly referred to as the 
Bermuda High,
the NAO is in a Positive Phase or South Centered. Unlike the Negative Phase, the Positive Phase enhances the westerlies... establishing a Zonal flow, or west to east orientation of the jet-stream, preventing buckling. Allowing for a pronounced south to southwest flow... at all levels of the atmosphere in middle latitudes.


Other major steering mechanism's of the
Northern Annular Mode (NAM), such as the
Arctic Oscillation (AO) strongly influence the NAO. When an intensifying High pressure anomaly center becomes established over the Arctic or 
North Pole, blocking the Polar jet-stream from moving straight west to east, expanding a developing Polar/Arctic Vortex southeast (over Hudson Bay, Ontario, Canada)... the
AO is in a Negative Phase or North Centered. Due to building heights in high northern latitudes of central & eastern North America... a Greenland Block often becomes established, producing a classic 
-AO / -NAO Couplet, fostering below to well below normal temperatures and strong potential of
Miller Type A cyclogenesis over central & eastern North America.
      
When an intensifying High pressure anomaly center builds over the U. S. Gulf Coast states or
Gulf of Mexico, encouraging a stronger than normal middle latitude & sub-tropical jet-stream, with above to well above normal temperatures over central & eastern North America... the AO is in a Positive Phase or
South Centered. As a result, High pressure also intensifies over the central Atlantic, encouraging establishment of a Bermuda High. Due to the highly influential AO, over NAO anomalies... it significantly influences the 
500-millibar Height Pattern over central and eastern North America.
 
Both the AO & NAO are part of the
Northern Hemisphere Teleconnection Longwave Pattern, and are observed through current trends & Indice Forecasts (of Operational Models & perspective Ensemble Members) from NOAA's -
NCEP Global Forecast System (GFS) and the
European Centre for Medium-range Weather Forecasts (ECMWF) in the
United Kingdom. Each Indice Forecast from it's perspective model measures the AO & NAO in-respect to normal-mean by an algebraic measure of variability, known as the
 Standard Deviation (SD)... defined by Wikipedia, the free online encyclopedia.

1. The first image above from NOAA's Climate Prediction Center (CPC) - demonstrates the NAO in a
classic Positive Phase.

2. The second image above from NOAA is of a March 2014 Nor'easter. 

3. The third image above of the
Trans-Atlantic continents - an area highly influenced by the
AO & NAO Teleconnection. 

4. The fourth image below from NOAA's Earth System Research Laboratory (Physical Sciences Division) & NCEP - demonstrates the NAO in a
classic Negative Phase.

4.

       

                 
In meteorological history the NAO was originally identified in 1924 by 
British Meteorologist Sir Gilbert Thomas Walker, with particular attention placed on sea-level pressure changes in the North Atlantic, as a result of Low pressure over Iceland and intensifying High pressure through the Azores.