The polar stratospheric regions of both hemispheres are surrounded by a narrow band or stream of fast-moving winds very high up blowing from west to east. Similar to the upper tropospheric jet stream, this jet stream develops along a zone of sharp temperature contrasts (or a tight temperature gradient; see Chapter 2, Section 4.1). In this case, it is the temperature gradient that develops along the line between sunlight and the 6-month long, wintertime polar night. This high altitude jet stream is commonly referred to as the polar night jet. The Antarctic polar vortex is the region inside (i.e., poleward) of this jet stream. During winter, the jet stream can reach speeds of over 100 mph at altitudes of 70,000 ft. The Antarctic polar vortex completely circumnavigates the continent of Antarctica. Figure 11.12 displays a plot of temperatures and winds for August 1, 1994, at four different levels in the stratosphere.
The figure shows extremely cold temperatures are found over Antarctica during winter, averaging about 183K (-90°C or -130°F) at 50 hPa (20km) in early August. These temperatures are contained inside the polar vortex region. The winds are predominantly westerly, with a slight north-south component, which means that little mixing of warmer air from lower latitudes occurs. The very cold temperatures that develop lead to the formation of polar stratospheric clouds (PSCs), which require for their formation temperatures below -80°C or -112°F. PSCs provide the necssary solid surfaces for the heterogeneous chemical reactions that free chlorine from inactive or "reservoir" forms (like ClONO2 and HCl) and convert it into highly reactive forms that catalytically destroy ozone.
The westerly circulation of the Antarctic polar vortex is strongest in the upper stratosphere and strengthens over the course of the winter. The polar night jet is important because it acts as a barrier to transport between the southern polar region and the southern midlatitudes. It is a barrier because it effectively blocks any mixing between air inside and outside the vortex during the winter. Thus, ozone-rich air in the midlatitudes cannot be transported into the polar region. The isolation of polar air allows the ozone loss processes to proceed unimpeded with no replenishment by intrusions of ozone-rich air from midlatitudes. This isolation of the polar vortex is a key ingredient to polar ozone loss, since the vortex region can evolve without being disturbed by the more conventional chemistry of the midlatitudes. The polar night jet over the Arctic is not as effective in keeping out intrusions of warmer, ozone-rich midlatitude air. This is because there is more wave activity (see Chapter 2, Section 4.0 and Chapter 6, Section 3.0) and hence more north-south mixing of air in the northern hemisphere than in the southern hemisphere.
Figure 11.13 displays the balanced zonal winds for August in the southern hemisphere averaged. ("Zonal" means, of course, that the wind is averaged for all longitudes at each latitude considered. As for "balanced," recall from Chapter 2, this means that extra momentum and curvature terms are considered in the zonal wind average. For our purposes, we can say that the balanced zonal wind is about the same as the zonal wind.) These zonal winds are averaged over a 17 year period (1979-1995). The peak August wind value is 84 m/s (190 mph) at 5 hPa (about 37 km). This peak tends to occur at latitudes around 60°S. In the figure, we can also see a weaker jet maximum in the upper troposphere around 12 km, centered at about 30°S. This jet is referred to as the subtropical jet.
3.1.1 Evolution of the Polar Night Jet and the Polar Vortex -- The evolution of the Antarctic polar vortex and the southern polar night jet over the course of the winter is illustrated in Figure 11.14 in an altitude-versus-time contour plot at 60°S. As the amount of sunlight decreases and temperatures drop in southern polar region, the night jet winds increase. So too does the degree of isolation. At higher altitudes, the polar vortex begins to develop in the March-April (early fall) period and is fully developed by May, corresponding to the onset of the period of complete polar night darkness. At lower altitudes, the vortex develops more slowly, not becoming fully developed until the June-July (early to midwinter) period. This vortex development is also illustrated in Figure 11.15 in a latitude-versus-time plot showing the evolution of the winds on the 50 hPa (approximately 20 km) surface. Both figures are based on data from 1979-1995.
The polar night jet reaches its maximum wind speed in the August-September (mid- to late winter) period. It breaks up in the November-December (mid- to late spring) period. Figure 11.13 shows us that the polar night jet is almost always centered at 60°S. In contrast to this southern polar night jet, the northern hemisphere polar jet is weaker in mid-winter, and has decreased in strength by late winter (February-March). The Antarctic polar night jet breaks up in mid-to-late spring (October-December), nearly 2 months later in the southern seasonal cycle than the breakup of the Arctic polar night jet in the northern seasonal cycle. This is due to the faster winds of the southern polar jet and the colder temperatures and greater degree of isolation of air inside the Antarctic polar vortex than their northern counterparts.
Antarctic polar vortex temperatures are extremely cold during the winter period. Figure 11.16 displays a multiyear zonal mean plot of temperature for August.
The thick black line in Figure 11.16 is the tropopause. It is basically defined by the temperature minimum between the stratosphere and troposphere. The tropopause is typically around 16 km (53,000 ft.) in the tropics, around 10 km (33,000 ft.) in the midlatitudes, and around 12 km (40,000 ft.) in the polar regions. (See Chapter 2 for more details.)
Temperatures are below 192 K (-114°F) over a deep layer (15-27 km), and extending from the pole to 70S. These cold temperatures develop during the polar night because of the lack of sunlight, which causes the air to cool radiatively towards its equilibrium temperature, which without sunlight is quite cold (see Chapter 4). The lack of north-south mixing due to the isolation of the polar vortex region allows this cooling to occur all winter long without any intrusions of warmer air.
3.2.1 Evolution of South Polar Cold Temperatures During the Winter -- The development of these cold temperatures over the south polar region is illustrated in Figure 11.17 using a zonal mean plot of the temperatures at 80°S as a function of altitude. The polar region cools over the course of the fall period at all altitudes. This cooling is extremely strong at the highest altitudes in early fall (40-48 km), with warming beginning in the June-July period. The coldest temperatures (i.e., temperatures less than 192 K or -81°C) first appear in July at approximately 24 km (30 hPa). These cold temperatures begin to appear at lower altitudes later in the season. Hence, at higher stratospheric altitudes, the coldest period is in early winter, while at lower stratospheric altitudes, the coldest temperatures occur in late winter. The temperatures rapidly warm during the breakup of the polar vortex. This breakup occurs earliest at the highest altitudes, and occurs the latest at the lowest altitudes. Because the ozone hole is observed at the lower altitudes below 30 km (100,000 ft.), the temperature region of greatest concern is between 10 and 30 km.
Temperatures inside the southern polar vortex are colder than inside the northern polar vortex. The cold temperatures inside of the polar vortex are crucial to the large polar ozone losses. This is because the formation of polar stratospheric clouds (PSCs) require such cold temperatures, and the PSCs are the key to the loss process. PSCs have been observed in both the Arctic and Antarctic stratosphere for quite some time, though the colder Antarctic leads to more frequent PSC formation there. Nacreous clouds, a form of PSC, have been observed for centuries over Scandinavia. PSCs have been the subject of research interest since the mid 1970s (Stanford, 1973; Stanford and Davis, 1974; Stanford 1977). McCormick et al. (1982) showed that PSC sightings from satellite data were strongly correlated with the cold temperatures below approximately 195 K (-78°C or -108°F). Both McElroy et al. (1986) and Solomon et al. (1986) recognized that PSCs which formed in the cold temperatures of polar night, could provide efficient sites for heterogeneous chemical reactions. Hence, it is the cold temperatures of the lower stratosphere which provide the first element in understanding the Antarctic ozone hole.
To identify stratospheric air that is inside or outside of the polar vortex, we use a quantity called potential vorticity. From Chapter 2, we learned that potential vorticity is a quantity derived from wind and temperature fields, and is conserved (i.e., remains the same) in the atmosphere from weeks to days.
The wind field enters the calculation through a concept called relative vorticity, which refers to the "spin" of horizontally moving air about a point, while the temperature field enters the calculation through a quantity called potential temperature. It refers to the temperature an air parcel would have if it were adiabatically compressed to 1000 mb. Both relative voriticity and potential temperature are discussed in Chapter 2.
Whereas the temperature and wind speeds of a particular air parcel might vary in time, the potential vorticity remains almost the same from day-to-day. Hence, potential vorticity is a key tool for following the motion of air in the stratosphere. Surfaces of constant potential vorticity are called isentropic surfaces. Motions of air in the stratosphere are mostly in the horizontal- east and west or north and south. Vertical motions are weak. Horizontal motions often occur on these isentropic surfaces, which are given in units of absolute temperature (kelvin).
Figure 11.18 displays a set of images of potential vorticity during the southern winter of 1994 near about 18km (an isentropic value of 440K). This period encompasses the development of the Antarctic ozone hole. Note the large, negative values of potential vorticity over the South Pole (indicated by the purple colors), and the somewhat higher values in the midlatitudes (indicated by the orange colors). (Potential vorticity is negative in the southern hemisphere.) The edge of the polar vortex (indicated by the dark lines) indicates the location of the polar night jet stream.
3.3.1 The Polar Vortex: Isolated and Contained -- We've already noted that there is a high degree of isolation of the Antarctic polar vortex from midlatitude air in the southern hemisphere. This is because of the nearly zonal flow of air that results in little north-south mixing of air. This, in turn, is due to the lack of topography in the southern hemisphere to induce planetary scale wave activity, such as exists in the northern hemisphere. Temperatures get very cold inside the vortex, where there is six months of polar night. This isolation produces air with a very different potential vorticity inside the vortex. This difference in potential vorticity acts to further isolate the vortex regardless of how zonal is the flow.
McIntyre (1989) argued that the Antarctic vortex ought to be highly contained because of the barrier set up by the sharp transition in potential vorticity values between the polar and midlatitude regions. Because of the high amounts of potential vorticity inside the vortex (very positive in the northern hemisphere and very negative in the southern hemisphere), any displacement of air outside of the vortex into the midlatitudes, where potential vorticity values are lesser creates a flow field that displaces the polar vortex air back towards the greater potential vorticity values inside the vortex. Thus, air of greater potential vorticity cannot get outside the vortex. The containment of the Antarctic vortex has been demonstrated with both trace gas observations (e.g. Schoeberl et al., 1989) and with modeling studies (e.g. Bowman, 1993). This containment has important ramifications for any mixing of air parcels with different characteristics, such as ozone amount.
3.3.2 Demonstrating Characteristics of Potential Vorticity with Trajectory Modeling Studies -- We can explore some of these characteristics of potential vorticity by using trajectory modeling studies. These characteristics include conservation of potential vorticity on isentropic surfaces, the fine structure of stratospheric air motions, and the high degree of containment inside the Antarctic polar vortex. Figure 11.19 displays a plot of potential vorticity on September 20, 1992, as constructed from a parcel trajectory model run. The plot is on the 440K isentropic surface, and the trajectories are initialized with potential vorticity on August 31, 1992. The 440K (18 km) level is centered on time and vertical region of maximum ozone loss.
By comparing the initialization plot of August 31 and the September 20 plot, we can demonstrate some of these characteristics of potential vorticity. First, the potential vorticity values on August 31 are of a similar magnitude to the analyzed potential vorticity, indicating that potential vorticity is a conserved quantity during the ozone hole period. Second, the trajectory model is produced at a higher resolution than the analysis, revealing details of fine structure that are not resolved by the analysis. This helps us better understand the motion of air parcels in the stratosphere. Third, the 15-day trajectory run shows virtually no escape of air into the midlatitudes, illustrating the high level of containment of the Antarctic polar vortex.
This transport barrier is a crucial element of the ozone hole theory. The barrier to transport caused by the potential vorticity prevents intrusions of ozone-rich midlatitude air into the ozone depleted region over Antarctica. In addition, nitrogen compounds that might inhibit ozone loss by reacting with chlorine are also not transported over Antarctica. Hence, polar ozone loss processes can occur undisturbed until the polar vortex breakup in late November.
The stratosphere is heated primarily by absorption of solar ultraviolet (UV) radiation by ozone (known as shortwave heating), while the stratosphere is primarily cooled by emission of IR radiation to space by carbon dioxide, ozone, and water vapor (known as longwave cooling). As the polar winter begins, solar UV heating by ozone ends and the Antarctic stratosphere cools to very low temperatures. Figure 11.20 displays a 17-year (1979-1995) average of net heating (sum of the shortwave heating and longwave cooling) in the stratosphere at 20 km as computed from the Goddard radiation model.
From Figure 11.20, we see that during the March-April period in the south polar latitudes, the stratosphere cools quite dramatically. This coincides with the disappearance of the sun over the polar region in late March. However, this process does not continue indefinitely. Stratospheric temperatures eventually become cold enough that longwave radiation emission by carbon dioxide, ozone, andwater vapor slows. By August, the temperatures have become so cold that the net cooling is near zero. Sunlight returns in early October as spring arrives, warming the stratosphere. Net cooling remains small until after the polar vortex breakup in late November, by which time the polar region has warmed to relatively high temperatures, and IR cooling to space once again becomes important.
The next image, Figure 11.21, displays the 17-year June zonal mean temperatures (top) and zonal mean heating rates (bottom).
We see that between 22 and 25 km, mean temperatures have fallen under 190 K. These temperatures are cold enough that little additional cooling takes place, and the cooling rate becomes quite small. These heating rates are extremely important, since they induce vertical motions by virtue of the fact that warming air rises and cooling air descends. The result of such heating and cooling is that in the tropics, where there is heating, there are rising motions, and in the polar regions, where there is cooling, there are sinking motions.
Because of the prevailing westerly winds in the Antarctic, air tends to move in a clockwise sense when viewed from space. The air at the 50 hPa (approximately 20 km) level circles the South Pole about once every 4 to 6 days in midwinter. This basic background circulation is illustrated in Figure 11.22 with a set of trajectories initialized on September 20, 1992 at midnight (00) Greenwich Mean Time (GMT) and run forward for 3 days to 00 GMT September 24, 1992. These trajectories are superimposed on an image of total ozone for September 21, 1992. The black dots indicate the location of each parcel at 00 GMT. We can observe how far each parcel moved over the four day period.
These trajectories represent the motion of a piece of air. You can think of them as showing the location of a balloon released in the flow. The air parcels move in a clockwise sense (white lines) around the polar vortex, with the fastest movement near the core of the jet stream. This westerly motion has relatively small displacements in the north-south directions (for the reasons discussed already). Note that the fastest motion of the air parcels occurs near the edge of the ozone hole in the core of the jet stream.
3.5.1 Vertical Motions and Ozone Transport -- Because air parcels are relatively isolated inside the polar vortex, the vertical motion of air is extremely important for evaluating the evolution of the ozone hole. One of the original theories of what causes the ozone hole was based on the transport of low ozone air from the troposphere into the lower stratosphere. Rosenfield et al. (1994) has shown that the air below 20 km inside the ozone hole during September has descended from altitudes near 25 km over the course of the southern hemisphere winter. This rules out transport of ozone-poor tropospheric air upwards into the stratosphere. This descent has also been determined via observations of the descent rates of long lived tracers in the UARS satellite data (Schoeberl et al., 95).
Figure 11.23 shows the motion of 10 air parcels contained within the polar vortex from early April to late November, 1992. The top panel of the figure shows the descent of these parcels from their initial altitudes between 23 and 26 km, to a final altitude of about 17.5 km (an average descent rate of about 0.9 km/month). The lower panel displays the evolution in terms of the potential temperature of the parcels, indicating a mean descent rate of between 12.5 and 25 Kelvin per month.
3.5.2 Nitrous Oxide as Tracer of Stratospheric Motions -- Nitrous oxide (N2O) is a crucial diagnostic tool for determining stratospheric motions. Figure 11.24 displays a plot of N2O on the 650K isentropic surface (approximately 24 km) for 8 separate days in 1992 and early 1993, as measured by the CLAES instrument aboard the UARS satellite. N2O has a tropospheric source from both biogenic and manmade (industrial) processes, and an upper stratospheric sink by reactions with O atoms, as well as through direct photodissociation by solar UV radiation. Because these loss processes are quite slow, N2O has a very long lifetime in the lower stratosphere, and can be used to trace atmospheric motion there. Refer back to Figure 11.10 for a vertical profile of N2O over Antarctica. The key point in the figure is the falloff in N2O amounts with altitude, indicating lower tropospheric source and upper stratospheric sink.
Because N2O amounts fall off with altitude, the presence of air with high amounts of N2O in the lower stratosphere is an indication that this air has tropospheric characteristics and origins. Air with low amounts of N2O in the lower stratosphere is an indication that this air has upper stratospheric characteristics and origins.
Superimposed on the images in Figure 11.24 are contours showing the boundary of the Antarctic polar vortex. Initially in January, the N2O values are low over much of the southern hemisphere. As the vortex begins to form (28 April 1992 panel), N2O decreases as low N2O air from the upper stratosphere is transported downward into the polar vortex. (Recall the motion of air from the Brewer-Dobson circulation, see Chapter 6.) This process continues through September. The vortex breaks up in late November, mixing the polar vortex's low N2O air into the midlatitudes, and the high N2O midlatitude air into the polar region. In addition to the low N2O values in the polar region, the CLAES observations also reveal high N2O in the tropics, providing evidence of the tropical uplifting from the troposphere, which is also in sync with the Brewer-Dobson circulation cell. The CLAES N2O observations provide clear evidence for: (1) strong downward motion into the polar vortex; (2) isolation of the polar vortex from midlatitude influence; (3) vertical uplift in the tropics, and (4) confirmation that potential vorticity is an extremely important diagnostic of the ozone hole.
3.5.3 Wintertime Stratospheric Circulation: The Brewer-Dobson Cell -- The overall winter stratospheric circulation is summarized in Figure 11.25, where it is shown for the southern hemisphere, though it applies to both hemispheres in winter. This circulation involves rising motion in the tropics and sinking motion in the polar region. It has been recognized since the early work of Brewer (1949) on water vapor observations, and is now known as the Brewer-Dobson circulation cell.
As discussed in Chapter 6, the Brewer-Dobson cell is a result of weather systems ("waves") that propagate into the stratosphere from the troposphere. These waves deposit energy that act to decelerate the polar night jet, the existence of which is due to the tight temperature gradient at the edge of the polar night. (The polar night jet is discussed in Chapter 2.) A very weak poleward circulation develops to balance this wave driven deceleration of the polar night jet, thus maintaining the jet and producing the gentle uplift in the tropics and sinking motion in the polar region. An alternative way of thinking about it is to recognize that the deceleration of the polar night jet results in a thermodynamical and radiative imbalance in the polar region. Warmer air that intrudes inside the vortex quickly cools in the polar night. Cooling air sinks. Due to mass continuity requirements, air rises in the tropics. The result is the Brewer-Dobson circulation cell. See Chapter 6, Section 3 for a more detailed explanation.
The characteristic "collar" region of high ozone surrounding the ozone-poor Antarctic polar vortex shown in Figure 11.02 arises because the downward transport by the Brewer-Dobson cell occurs in the polar region at the edge of the vortex, as opposed to inside the vortex. Ozone thus accumulates in the collar region.