Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

FIGURE 5.22 Minimum air temperatures in the Arctic and Antarctic lower stratosphere. (Courtesy of Paul Newman, NASA Goddard Space Flight Center.)

5,7.3 Arctic Ozone Hole

The ozone hole phenomenon was first discovered in the Antarctic stratosphere. What about the Arctic stratosphere? Recall that two ingredients are necessary to produce the ozone hole; very cold temperatures and sunlight. The Arctic winter stratosphere is generally warmer than the Antarctic by about 10 K. Figure 5.22 shows the distribution of minimum temperatures in the Antarctic and Arctic stratospheres. We see the distribution of generally lower minimum temperatures in the Antarctic versus the Arctic and the overall lower frequency of PSC formation in the Arctic. Thus polar stratospheric clouds are less abundant in the Arctic and, where they do form, they tend to disappear several weeks before solar radiation penetrates the Arctic stratosphere. Also, the Arctic polar vortex is less stable than that over the Antarctic, because Antarctica is a land mass, colder than the water mass over the Arctic, As a result, wintertime transport of ozone toward the north pole from the tropics is stronger than in the Southern Hemisphere. All these factors combine to maintain relatively higher levels of ozone in the Arctic region. Ample evidence for perturbed ozone chemistry does, however, exist over the Arctic including observed CIO levels up to 1.4ppb (Figure 5.23) (Salawitch et al. 1993; Webster et al. 1993). CIO concentrations are routinely as high in February in the Arctic as in September in the Antarctic but the PSCs disappear sooner in the Arctic, As a result, denitriiication is the most important difference between the Antarctic and Arctic; the Arctic experiences only a modest denitrification, whereas denitrification in the Antarctic is massive. The reason for the difference is the sufficient persistence of NAT PSCs in the Antarctic to allow time to settle out of the stratosphere.

ANTARCTIC (S.H.) Powr Vortex Minimum Temperature, 20 km

1978 to 2000

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