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Images: Why the West Wind Wobbles

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)

This graph depicts a simulated "random walk," in which one repeatedly flips a coin: If it comes up heads, take a step forward; if it's tails, step backward. This portion of the "walk" seems to show a periodic oscillation, with peaks or valleys every 500 coin flips, but no real cyclic process is occuring, just random chance. This illusion is common in these "stochastic" processes. (Jason Goodman, Woods Hole Oceaonographic Institution)

The blue lines show daily variations in the the northern (upper panel) and southern (lower panel) annular mode indexes for the winter months of 1995-2004 (Dec.-Feb. and June-Aug., respectively). The red line shows the output of our stochastic "coin- flipping" model. The similarity in patterns suggests that the behavior of annular modes could be a consequence of random chance, such as the north-south motion of air mass caused by daily storm activity. (Jason Goodman, Woods Hole Oceanographic Institution)