Arctic Ozone

Other Factors Affecting Arctic Ozone Depletion

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Although ozone-depleting chemicals and the unique depletion processes of the polar stratosphere have the most dramatic impact on Arctic ozone, several other factors can also affect ozone amounts. For example, day-to-day changes are often associated with the movement of weather systems and pressure patterns in the troposphere and with changes in the height of the tropopause. Longer-term changes have been linked to a variety of other natural processes, of which the most significant are the periodic reversal of stratospheric winds over the equator (the quasi-biennial oscillation), El Niños, the solar or sunspot cycle, and volcanic eruptions (Figure 5).

THE QUASI-BIENNIAL OSCILLATION

Over the equator, stratospheric winds circle the globe in either an easterly or a westerly direction. Every 20-30 months the direction is reversed. This phenomenon is known as the quasi-biennial oscillation (QBO). Its ultimate cause is not fully understood, but it is known to influence a variety of atmospheric phenomena, including ozone amounts over the middle and high latitudes. When the QBO is in its westerly phase, polar stratospheric temperatures are generally lower, the poleward transport of ozone is reduced, and ozone depletion tends to be greater. In 1993, 1995, and 1997, for example, the QBO was in its westerly phase and large ozone losses were recorded in the Arctic. In 1994, 1996, and 1998, it was in its easterly phase. Depletion was minimal, as expected, in 1994 and 1998, but losses as high as 30% occurred in 1996. These unexpected results suggest that factors other than the QBO had a more substantial effect on ozone levels that year.

EL NIÑO SOUTHERN OSCILLATION

El Niños are periodic abnormal warmings of the eastern equatorial Pacific. Occurring roughly every three to seven years, they are accompanied by a reversal of normal pressure patterns over the Southern Hemisphere (a phenomenon known as the Southern Oscillation) and result in disturbances of prevailing weather patterns in much of the world. Because El Niños also change normal pressure patterns in the upper troposphere, they can alter the height of the tropopause, affect the poleward transport of ozone, and cause changes in ozone amounts over many parts of the world.

One of the consequences of the strong El Niño of 1997-1998 was a deepening of the Aleutian Low, a large quasi-permanent low pressure area over the northeastern Pacific Ocean. It is an important feature in the large-scale circulation of the atmosphere, and its strengthening may well have made the Arctic vortex less stable and more subject to the sudden stratospheric warmings that moderated temperatures within the vortex during the winter of 1998. By making conditions less favourable for PSC formation, the El Niño may have made a significant contribution to the comparatively low rate of Arctic ozone depletion that was observed in the spring.

THE SOLAR CYCLE

Over a period of approximately 11 years, the energy output of the sun varies by only about 0.1%, changing with the growth and decay of sunspots on the solar surface. When the number of sunspots is greatest, the sun produces more energy than when the number of sunspots is least. Although the change in energy is quite small, much of it is concentrated in the ultraviolet range. Since ozone is created as a result of the breaking of oxygen molecules by the sun's ultraviolet radiation, more ozone will be produced in years when the solar cycle is at its maximum. The observed variation in ozone amounts over the cycle is approximately 1-2%.

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Recent studies have linked the solar cycle maximum to a slight warming of the winter stratosphere (thus making conditions less favourable for PSC formation) and a strengthening of the poleward transport of ozone (Figure 6). Both of these conditions tend to diminish the extent of ozone depletion in the Arctic. Other factors being equal, ozone depletion over the Arctic is therefore likely to be lower in years when the solar cycle is at a maximum (as it was in 1968, 1979, and 1990 and will be again in about 2001).

VOLCANOES

Major volcanic eruptions can have a significant impact on ozone depletion over a one- or two-year period. That is because chemical reactions similar to those that take place on PSCs can also take place on the surfaces of sulphate aerosols (fine droplets or particles) that have formed in the stratosphere as a result of the eruptions. These aerosols may also stimulate the formation of PSCs by various indirect processes.

The eruption of Mt. Pinatubo in the Philippines in 1991, for example, put about 120 million tonnes of sulphur dioxide into the stratosphere, and within a week or two the gas was converted to sulphate aerosols by atmospheric reactions. During the next two years, severe ozone depletion was observed both in the midlatitudes and at the poles. Over Canada, ozone amounts in the spring of 1993 were 10-17% below normal, the largest decrease observed until then. At the same time, in the high Arctic, ozone amounts in the lower stratosphere (between 10 and 20 km) fell to about 110 DU, a low value not seen again until 1997.

Because the timing and magnitude of volcanic eruptions are unpredictable, volcanoes function as a kind of wild card in estimates of future ozone depletion.

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