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Climate Seesaw

The severity of wintertime climate over North America and Europe is strongly linked to the most prominent atmospheric pattern in the Northern Hemisphere, a seesaw exchange of air massed called the “northern annular mode.” This 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. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)