Regional and hemispheric circulation patterns in the northern hemisphere winter, or the NAO and the AO

[ 1] Two different scales of teleconnection patterns are found over the Euro- Atlantic region during the winter. One is a regional seesaw between Iceland and the Azores, and the other is a hemispheric seesaw between the Mediterranean and the polar- Siberia region. The North Atlantic Oscillation ( NAO) and the Arctic Oscillation ( AO) indices are sensitive to both modes, although the NAO index reflects the more regional one. The principal process producing a hemispheric seesaw pattern in the surface pressure is changing in vertical winds in the polar troposphere driven by wave forcings. Upward motion in the polar region induces meridional flows near the surface, especially around the Norwegian Sea and Bering Strait, which induce wavy patterns in the surface pressure. The importance of distinguishing the two modes in understanding recent trends is also addressed.