The seasonal phenomenon of the Antarctic ozone hole immediately raises the question of whether the same losses might take place in the northern hemisphere (the Arctic) during the northern spring period. This question has lead to ongoing, active research since the late 1980s. In this section, we will take a look at observations of Arctic ozone since the late 1980s to see if there are any indications of significant, seasonal ozone loss occurring there. We will then compare and contrast the Arctic and Antarctic to determine why a comparable large hole has not has not been found to date in the northern polar stratosphere.
As with the southern hemisphere, research into possible Arctic ozone loss has included ground-based observations, aircraft missions, and satellite measurements. The first attempt find out if ozone losses were occurring in the Arctic began with the Airborne Arctic Stratospheric Expedition I (AASE-I) in the northern winter of 1988-89. The mission was to determine whether stratospheric heterogeneous processes and photochemical reactions known to be occurring over Antarctica might be causing Arctic ozone losses. Subsequent research in the winter of 1991-92 included the AASE-II mission and the European Arctic Stratospheric Ozone Experiment (EASOE). In addition, the UARS satellite with its array of instruments began measuring various trace gas constituents over the Arctic.
6.1.1 The AASE-I Mission: Various Findings -- The AASE-I mission of 1988-89 found high concentrations of chlorine monoxide, ClO, inside the Arctic vortex suggesting that significant ozone depletion could occur during the Arctic spring given the proper atmospheric conditions. McKenna et al. (1990) calculated localized ozone loss rates of more than 20 ppbv per day for this winter using the AASE-I data. Chipperfield et al. (1993) carried out a sophisticated model calculation of ozone evolution during the Arctic winter of 1989-1990. They calculated an ozone loss rate of 15 ppbv per day during February and pointed out that this rate would increase to 25 ppbv per day if polar stratospheric clouds (PSCs) persisted into March.
All of these modeling and observational studies indicate that large ozone losses are possible, given the appropriate conditions of high chlorine levels (something likely to be true for at least the next couple of decades), and sufficiently cold temperatures persisting into spring. However, it was also clear from these early studies that a large Antarctic-like ozone hole was not apparent over the Arctic.
Numerous other papers have been published based on AASE-I and other data sources that have deduced various amounts of ozone loss in the Arctic winter and spring. Evans (1990) described a "depleted layer" of stratospheric ozone between 18 and 24 km altitude in balloon- launched ozonesonde profiles taken above Alert, Canada in 1989. Salawitch et al. (1990) deduced a loss rate of 0.5% per day from AASE-I data for January and February of 1989. Hofmann and Deshler (1991) launched a series of balloons with ozonesondes during January and February of 1990 from Kiruna, Sweden. They deduced an ozone loss rate over the station that reached as high as 4 ppmv per month at the 22 km altitude that then steadied out to 0.6 ppmv per month. These losses were "episodic," meaning that they occurred only at certain times (episodes). Koike et al. (1991) also deduced a loss rate of about 1.5% per day on the 525K potential temperature (approximately 21 km) surface during February of 1990 from balloon-borne ozonesondes launched from Kiruna, Sweden.
6.1.2 UARS Measurements and the AASE-II and EASOE Missions: Various Findings -- The Arctic winter of 1991-92 has been extensively studied because of the launch of the UARS (Upper Atmospheric Research Satellite), the AASE-II campaign, which involved the ER-2 aircraft, and the European Arctic Stratospheric Ozone Experiment (EASOE).
Using ozone and chlorine monoxide data gathered by the Microwave Limb Sounder (MLS) instrument aboard the UARS, Waters et al. (1993) compared the evolution of ozone over the Arctic with that over the Antarctic. Data from the AASE-II measurements were used by Salawitch et al. (1993). They deduced a 15-20% loss of ozone in the lower stratosphere over the course of the northern winter. Proffitt et al. (1993) and Browell et al. (1993) also used AASE-II data to quantify ozone loss during the Arctic winter of 1991-92. Using a variety of data sources obtained during EASOE, an number of researchers -- including Hughes et al. (1994), Wege and Claude (1994), Dahlback et al. (1994), Braathen et al. (1994), Lutman et al.(1994), and Müller et al. (1994) -- deduced various ozone loss rates inside the Arctic vortex. As is pointed out in most of these papers, interpreting loss rates during the winter of 1991-92 was complicated by the large amount of aerosol in the vortex from the 1991 eruption of Mt. Pinatubo.
Since that time, the UARS MLS data has been used to deduce Arctic ozone loss for the winter of 1992-93 (Manney et al., 1994), 1993-1994 (Manney et al., 95), and 1994-95 (Manney et al., 1996). These studies separated chemical and dynamical ozone changes by using long-lived tracer measurements and by conservative trajectory calculations. Müller et al. (1996) used data gathered by the Halogen Occultation Experiment (HALOE) instrument, also onboard the UARS, to deduce ozone loss during the northern winters of 1991-92, 1992-93, 1993-94, and 1994-95. They used the simultaneously measured tracer methane to isolate photochemical (i.e. production and loss) changes from strictly dynamical (i.e. transport) changes in total ozone. For the northern winter of 1994-95, they found a loss in the total column of ozone of about 100 DU.
Prior to 1996, most of the studies on Arctic ozone loss showed a rather small impact in comparison to the very large ozone losses recorded over Antarctica. Thus, there was no ozone "hole" comparable to the one present over Antarctica from 1988 to 1995. The winters of 1995-96 and 1996-97, however, were the first to show very large ozone losses and a polar ozone low that was very similar to the one observed over Antarctica each year.
The Arctic winter of 1996-1997 had the long-lasting polar vortex in the data record (Coy et al., 1997). The Arctic vortex, centered over the North Pole, lasted in a relatively undisturbed state into late-April. A similar vortex occurred previously in 1990, but it lasted only until about mid-March. During most other recent winters, the vortex has been significantly distorted and most low ozone amounts were located well off of the pole.
The TOMS total ozone measurements provide an ideal set of data to map out the evolution of ozone over the entire winter of 1996-97. TOMS and BUV (Backscatter Ultraviolet) data also provide more than 15 years of data from previous years that permits us to compare and contrast this winter with other winters. This allows us to place the winter of 1996-97 in context.
Total ozone amounts have been decreasing over the Arctic during the last two decades (Newman et al., 1997). Figure 11.57 displays the March average of total ozone from 63°N to 90°N.
The March observations show a clear downward trend between the 1970s and the 1990s. The March 1997 amounts are the lowest on record for the 20 years of observations. The polar averaged March 1997 amount of 354 DU is 21% lower than the pre-1990s March average of 450 DU. The 63°N to 90°N region represents a conservative estimate of the polar vortex average, since the polar vortex is typically not centered on the pole. This large March trend is much larger than the uncertainties of a few percent determined for all TOMS and BUV ozone observations. The March 1998 amounts recovered to near normal (i.e. pre-1990s) levels, indicating the sensitivity of the March ozone levels to dynamical activity and temperatures during the winter period. During 1998, a stratospheric major warming eroded the polar vortex, warmed temperatures above the threshold necessary for the formation of PSCs, and transported more ozone poleward and downward from the photochemical production region. Hence, ozone was near "normal" in 1998.
Figure 11.58 displays the March average ozone for both early years (1971, 1972, 1979, and 1980: top), and later years (1993, 1996, 1997, and 1998: bottom).
The distribution of ozone during the early years shows high amounts of ozone in the polar region and low amounts in the tropics. An ozone low is observed in March 1980 centered near the Norwegian Sea between Norway and Greenland. In contrast to the high polar amounts observed in the early years, the ozone amounts observed during the 1990-97 period were quite low, and showed a stong localized minimum in the Arctic region. The localized ozone minimum is most distinct and polar centered during March 1997. During 1998, as noted above, a stratospheric major warming eroded the polar vortex and inhibited the formation of PSCs. The Brewer-Dobson circulation (see Chapter 6) was able to transport more ozone into the polar stratosphere, hence keeping ozone amounts at pre-1990s levels in the spring.
The difference between levels of Arctic and Antarctic ozone is directly linked to the difference between the stratospheric climate of each region. The difference in climate, in turn, is due to differences in circulation patterns at each pole.
In the northern hemisphere, there are large-scale topographic features (i.e., mountain ranges), as well as greater land-sea temperature contrasts. These induce more frequent and intense planetary wave activity (see Chapters 2 and 6). The wind field becomes more meridional (north-south) in the northern hemisphere, which contributes to mixing of tropical and polar air. Storm systems bring together and mix these different air masses. The Arctic stratosphere does not reach the extremely low temperatures found in the Antarctic stratosphere. Planetary wave activity also results in a stronger Brewer-Dobson circulation, which carries ozone-rich air from the tropical upper stratosphere downward and poleward in the winter hemisphere (see Chapter 6). Ozone accumulates in the northern polar regions. In addition to keeping overall temperatures warmer, the greater amounts of mixing between polar and midlatitude air in the Arctic vortex result in less chemical isolation of air inside the polar vortex. Reactive nitrogen compounds, such as NO and NO2, which act as a brake on the ClO-ClO and ClO-BrO catalytic loss cycles for ozone, are found in greater concentrations in midlatitude air. The greater isolation of the vortex, the less reactive nitrogen can enter into the vortex.
In the southern hemisphere, by contrast, the lack of significant topography and weaker land-sea temperature contrasts result in much less planetary wave activity. The wind flow is primarily zonal (west-east), and there is less large-scale north-south mixing of air. The Antarctic stratosphere by midwinter is very isolated from outside influences, including warmer midlatitude tempertures and reactive nitrogen species. This is why the Antarctic vortex cools to such extremely cold temperatures by midwinter. These colder temperatures permit the formation of PSCs, both Type I and Type II, which then allow the heterogeneous chemistry (discussed above) to occur, setting the stage for depletion of ozone to occur when sunlight returns in the spring. Figure 11.59 displays 80°N (top) and 80°S (bottom) longitudinally averaged temperatures over the course of the year at the 50 hPa level. The temperatures at which Type I and II PSCs develop are noted by the lines.
Northern hemisphere temperatures in Figure 11.59 have been shifted in months for easy seasonal comparison to their southern hemisphere temperatures. We see first that the temperatures in the northern hemisphere are substantially warmer during midwinter than those in the southern hemisphere. Second, the cold temperatures in the southern polar region persist well into spring, while temperatures in the northern polar region warm by early spring. Because of these warmer northern temperatures, the frequency of PSC occurrence is much less there. As a result, heterogeneous chemistry and the activation of reactive chlorine is much less frequent and extensive in the northern hemisphere during the critical spring period as the sun rises over the polar region. Exceptions to the rule occur in winters like 1996-97, when very cold temperatures persisted into March, activating chlorine, and causing the observed greater ozone loss. In general, the northern winter circulation tends to break down earlier in the season, such that chlorine is not activated when the sun rises over the northern polar region. Hence, the warmer temperatures of the northern hemisphere prevent the massive losses of ozone observed in the southern polar region that form the Antarctic ozone hole.
Future ozone losses over the Arctic are difficult to predict because of the uncertainty about future stratospheric temperature trends. Increases in greenhouse gases such as CO2 are expected to lead to warming at the surface. However, the opposite occurs in the lower stratosphere, where increased concentrations of greenhouse gases cause greater radiative cooling and hence lower temperatures (Austin and Butchardt, 1992). Recent work by Rind et al. (1998) and Shindell et al. (1998) suggests that greenhouse gases may also modify the large-scale planetary wave processes. Any weakening of planetary waves in the northern hemisphere will act to weaken the poleward and downward motions associated with the Brewer-Dobson circulation. It is this circulation that increases Arctic ozone levels and warms the lower polar stratosphere. The effect will exacerbate ozone losses because of the resulting greater cooling of the polar lower stratosphere, which in turn, will lead to greater chlorine activation. This potential Arctic cooling is a serious concern, since stratospheric chlorine levels are forecasted to begin to decrease any time now. As this decrease will occur very slowly, taking decades, the polar cooling may lead to severe ozone losses even as chlorine levels are decreasing.
In the next chapter, we will examine the forecasts of different computer modeling studies for stratospheric ozone. As we shall see, these models allow us to explore different scenarios (such as curbing CO2 output) and make assessments about likely future trends in ozone and other trace gases. However, as we shall also see, the modeling studies are limited in their predictive capabilities because of model simplifications. These issues are addressed in Chapter 12.
The ozone hole is a clear example of how observations, laboratory work, and theory can interact to determine the causes of a phenomena. Initially, observations defined the issue of the ozone hole. A large amount of time and effort was expended determining the precision and accuracy of observations conducted from the 1940s to the present. Hypotheses of the cause were quickly put together, which were tested via continued observational programs, and via ground-based, aircraft, and satellite measurements. The laboratory work determined key reactions, and established probabilities that certain gas reactions would occur and under what circumstances. These lab observations were then combined with meteorological observations to produce simulations of the ozone hole.
The conditions that lead to the seasonal formation of the ozone hole over Antarctica are now fairly well understood. Both historical and current observations give atmospheric scientists exceptional confidence in descriptions and evaluation of Antarctic stratospheric conditions. Details of transport, kinetics, chemistry, and particle physics continue to refine our understanding of the ozone hole. The future holds the promise that the ozone hole will disappear sometime in the middle of the next century as chlorine concentrations decrease.
|
|
|
|