Planetary Waves and Ozone Holes

Huge planet-girdling atmospheric waves suppress ozone holes over Earth's northern hemisphere.

Most of the world's ozone-destroying pollutants come from the northern half of our planet. Yet Earth's yawning ozone hole straddles the south pole - not the north.

Why? New research confirms what scientists have long thought: Giant atmospheric waves spawned by land features such as the Himalayas damp the formation of a northern ozone hole and, as a result, Arctic cities remain safe from unwelcome doses of solar ultraviolet radiation - at least for now. Researchers caution that climate change could undo the work of those waves and make Arctic ozone holes more common in the future.

Small ozone holes have in fact formed over the Arctic before; the spring of 1997 is a recent example. But such events are the exception rather than the rule. The chemistry of ozone destruction requires very cold air temperatures in the stratosphere, and the Arctic stratosphere just isn't as cold as its Antarctic counterpart.

The north-south difference is an indirect result of the way land is distributed around Earth - that is, unevenly. Most of our planet's land and its highest mountains are in the northern hemisphere.

High mountains and land-sea boundaries combine to generate vast undulations in the atmosphere called "planetary-scale waves," or "long waves," which act to heat polar air. Planetary-scale waves are so large that some of them wrap around the whole Earth! Unlike water waves, which displace the water up and down, planetary waves displace air north and south as they travel around our planet. They form in the troposphere (the lowest part of the atmosphere) and propagate upward, transferring their energy to the stratosphere.

Stronger planetary waves in the northern hemisphere warm the Arctic stratosphere and suppress ozone destruction. Land forms in the southern hemisphere also produce planetary waves, but they tend to be weaker because there are fewer tall mountain ranges and more open ocean around Antarctica

In years when planetary waves (or "long waves") in the Northern Hemisphere are unusually weak, an ozone hole can form over the Arctic. Blue and purple indicate regions of low stratospheric ozone.

"The Himalayan plateau is a terrific forcing function for these waves in the north," says Paul Newman, an atmospheric physicist at NASA's Goddard Space Flight Centre."If you didn't have the Himalayas, the stratosphere over the Arctic would be much colder than it is."