The ?northern annular mode? is a natural shift of air masses back and forth between the North Pole and mid-latitudes. At some times (left), a surplus of air mass and pressure (blue) exists over the pole and a deficit (red) exists at around 45?N; at other times (right), the air mass is redistributed to create a deficit at the pole and a surplus in mid-latitudes. This seesaw exchange of air masses shifts wind patterns (blue arrows), as well as temperature and storm conditions, and affects the severity of winters over the Northern Hemisphere. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)[back]
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