what is a negative NAO?

A negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States. this may also have an effect on this winter. here is an article about the negative nao -- http://climate.ncsu.edu/climate/patterns/NAO.html