North Atlantic Oscilation

The NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa (source:http://www.cpc.ncep.noaa.gov/products/outreach/glossary.shtml#NAO)

Here is another link for the AO and NAO: http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html