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Planetary Waves and Ozone Holes

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

By Patrick L.Barry and Dr Tony Phillips

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."

Newman was the lead author of a paper published in September 2001 which presents satellite and meteorological data linking planetary waves to bursts of warming registered in the Arctic - a connection that scientists long-ago recognised but have only now quantified. (His paper appeared in the September 16th issue of Journal of Geophysical Research - Atmospheres.)

"Typically a wave will warm the polar region by 5° to 10° C," Newman continued. "A 'warm' polar stratosphere is typically in the temperature range -73° to -63° C. Of course, as soon as the wave has dissipated, the polar region begins to cool down again."

Credit: Lamont Poole, NASA

Dangerous beauty. Polar stratospheric clouds (PSCs) are common in Antarctica, but a rare sight in the Arctic. They form when temperatures in the stratosphere become extremely cold - below -78° C. PSCs spell trouble for ozone; tiny ice crystals and droplets within the clouds provide surfaces where CFCs are converted into ozone-destroying molecules.

Indeed, planetary waves in the northern hemisphere don't always heat the stratosphere enough to prevent substantial ozone destruction. In 1997, for example, the waves were weak because of capricious weather. That triggered a rare springtime ozone hole over the Arctic.

Scientists are concerned that climate change could make such times more common. "If our models of Arctic stratospheric cooling are correct, we would expect lower ozone values across the Arctic during this century," says Newman.

It so happens that stratospheric cooling can be a curious result of global warming. Greenhouse gases, which trap the heat radiating from Earth's surface in the lowest layer of the atmosphere, reduce the heat that reaches the stratosphere. In effect, greenhouse gases cool the stratosphere by insulating it from the warmer Earth below.

Spring is the season when sunlight can trigger the chemistry of stratospheric ozone destruction. But Earth's two poles react differently to the coming of spring. Springtime in Antarctica heralds a large ozone hole, while springtime in the Arctic (six months later) often brings above-average ozone concentrations. Global warming could alter this familiar pattern, though, by chilling the northern stratosphere and producing an ozone hole there as well. [more]

Climate changes associated with global warming might also weaken planetary waves - so say some computer models. The cooling of the stratosphere due to this indirect effect could be more significant than the cooling caused directly by greenhouse gases. However, Newman cautions that this result is still very uncertain because of questions about the fidelity of the computer models.

Another wild card is the changing concentration of ozone-destroying pollutants. The number of CFC molecules in the lower atmosphere, for example, peaked in 1994 and have since declined. Computer simulations show that CFCs in the high stratosphere could return to pre-1980 levels in 30 to 50 years. Because climate change occurs on similar time scales, it's difficult to say which trend would dominate: the cooling of the stratosphere, which would encourage an Arctic ozone hole, or the decline of CFCs, which would suppress it.

Perhaps only time will tell if far-northern cities will continue to enjoy the good fortune of year-round polar ozone. But many researchers aren't content to wait decades for an answer. With the aid of Earth-watching satellites and ever-improving computer climate models, scientists hope to unravel the puzzle of Arctic ozone before it becomes a problem.

After all, one planetary ozone hole is more than enough!


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First Science 2014