This is the first essay of the series of commonly analyzed teleconnections. Teleconnections are weather patterns that help produce long term forecasts (beyond 5 days). Pressure patterns in the atmosphere tend to promote certain weather patterns. The first teleconnection we will look at is the North Atlantic Oscillation (NAO).

The NAO is an index based on the pressure difference between the subtropical Azores high and the subpolar low. The polar pressure is taken near Iceland and the subtropical pressure is taken near the Azores.

A value of 0 using the index is the typical expected value. A positive NAO indicates the pressure difference is more than typical while a negative NAO indicates the pressure pressures is less than typical.

Positive NAO: A positive value leads to more intense subpolar lows and a greater frequency of subpolar lows that track from the U.S. toward Europe. The tracks of the storm systems tend to be more northerly. The Eastern U.S. has relatively mild and moist conditions in winter. Europe tends to have more storm systems and warmer weather in winter. The subtropical high tends to be stronger and the hurricane season tends to be more active.

Negative NAO: A negative value leads to less intense subpolar lows and a slackened frequency of subpolar lows that track from the U.S. toward Europe. The Eastern U.S. tends to have colder and snowier winters. Europe tends to be colder in the northern portions and wetter in the southern portions. There tends to be more blocking due to a stronger Atlantic ridge.

A graphical update of NAO is available at: