http://www.wetterzentrale.de/archive/ra ... 050118.gif
A negative NAO has higher pressure over Greenland and lower pressure over the Azores, this stalls the Atlantic and promotes northerlies or easterlies.
http://www.wetterzentrale.de/archive/ra ... 101129.gif
...and some links for a professional explanation

http://www.cpc.ncep.noaa.gov/products/p ... ensm.shtml
http://www.cpc.ncep.noaa.gov/products/p ... ensm.shtml
http://www.nc-climate.ncsu.edu/climate/ ... s/NAO.html