Climate Prediction Center - Stratosphere: Winter Bulletins

Concerns about global ozone depletion (e.g. WMO, 1999) have led to major international programs to monitor and explain the observed ozone variations in the stratosphere. In response to these, as well as other long-term climate concerns, NOAA has established routine monitoring programs utilizing both ground-based and satellite measurement techniques (OFCM, 1988).

Selected indicators of stratospheric climate are presented in each Summary from information contributed by NOAA personnel. A Summary for the Northern Hemisphere is issued after each April, and for the Southern Hemisphere, after each December. These Summaries are available on the World-Wide-Web, at the site

Further information may be obtained from Craig S. Long
W/NP52, RM 806, WWB
NOAA Climate Prediction Center
5200 Auth Road
Camp Springs, MD 20746-4304
Telephone: (301) 763-8071, ext. 7557
Fax: (301) 763-8125
E-mail: Craig.Long at noaa.gov

Southern Hemisphere winter and spring conditions in 2008 were such that extensive ozone depletion occurred creating the fifth largest ozone hole at 25 million sq kilometers. The polar circulation was consistently zonal and rarely impacted by a poleward flux of heat. This resulted in a very prolonged polar vortex and consequently the ozone hole persisted well in to December. Although the ozone hole was quite large in 2008, this year had one of the latest starting dates for observations of ozone amounts depleted to the ozone hole threshold of 220 DU for the last ten years. Once established, the large area of depleted ozone persisted through October and November. Both the polar vortex and ozone hole persisted to the latest date since 1979. Observations from the South Pole did not show complete destruction throughout the 14-21 km zone. However, within this zone several days exhibited complete destruction at some altitudes. This year’s lowest observation of total ozone of 107 DU was observed relatively early on September 28, 2008. Extremely low ozone values did not persist as long as observed in some previous years, but increased such that the mean total ozone amount for the second half of October was about 150 DU. Estimated UV Indices over Antarctica under the ozone hole exceeded 10 units and daily dosages of erythemal UV exceeded 5000 Joules per sq meter.

I. DATA RESOURCES

The data used for this report are listed below. This combination of complementary data, from different platforms and sensors, provides a strong capability to monitor global ozone and temperature.

Parameter	Ground-Based	Satellite/Instrument
Total Ozone	Dobson	NOAA/SBUV/2
		Nimbus-7/SBUV
Ozone Profiles	Balloon-Ozonesonde	NOAA/ SBUV/2
		Nimbus-7/SBUV
Temperature Profiles	Balloon - Radiosonde	NOAA/TOVS

We have used total column ozone data from the NASA Nimbus-7 Solar Backscatter UltraViolet (SBUV) instrument from 1979 through December 1988; NOAA-11 SBUV/2 from January 1989 to December 1993; NOAA-9 SBUV/2 from January 1994 to December 1995; NOAA-14 SBUV/2 from January 1996 to December 1998; NOAA-11 SBUV/2 from January 1999 to December 2000; NOAA-16 SBUV/2 from January 2001 to December 2005; and NOAA-17 from January 2006 to December 2008. Solar-backscatter ultraviolet measurements are not available at polar latitudes during winter darkness.

Figure 1 presents time series of the size of the Ozone Hole, the size of the polar vortex, and the size of the polar area where lower stratosphere temperatures were below -78ºC (polar stratospheric cloud, PSC area). The daily 2008 values are shown, along with the extreme and average daily values for the most recent 10 years. The Ozone Hole is defined as the area in which total ozone amounts fall below 220 Dobson Units (DU). The polar vortex is defined as the area within the -32 potential vorticity units on the 450K isentropic surface (near 70 mb). The total ozone in this report is determined from SBUV/2 analyses. The coverage of the SBUV/2 on NOAA polar orbiting satellites increases poleward on a daily basis during August and September as sunlight returns to the southern high latitudes. This is also the same time that ozone depletion occurs. As Figure 1a shows the date at which the ozone column was depleted to 220 DU occurred very late in 2008. Initial explanations were the progression of NOAA-17 (and 18) to later earlier (later) equator crossing times thus delaying the observations of the higher latitudes. Another reason would be very zonal flow around the South Pole thus containing the colder temperatures (and ozone depleting PSCs) to higher latitudes. However, examination of Aura/MLS temperature and chemical observations showed that the flow around the South Pole was not entirely zonal and that ozone depletion was occurring on the outer regions of the SH polar vortex during mid to late August. Further examination of the MLS data for the entire time period of the SH winter disclosed that total ozone amounts over the SH high latitudes were greater this year than in previous years. Ozone depletion over Antarctica did occur without delay, but required several days longer to deplete to the 220 DU “ozone hole” threshold. Once this threshold had been reached the ozone hole size grew very rapidly. At the same time the areas at which temperatures were below 78C (the temperature to form Polar Stratospheric Clouds) also grew to above normal levels as shown in Figure 1c. The result was that the 2008 ozone hole grew to have the fifth largest single day size. Figure 1b shows that at this same time the SH polar vortex became larger than normal. The large size of the SH polar vortex persisted through October, November and December. In fact this year the polar vortex persisted beyond any previous year back to 1979. Consequently, the area of depleted ozone below 220 DU also persisted to the latest date.

As one would expect there is a strong relationship between the PSC area size (temperatures below -78C) and the area size of the ozone hole. Figure 2 shows the mean PSC area for the month of September for all years back to 1979. The five years with the largest single day ozone hole area happen to also be the five years with the largest September mean PSC area. Interestingly, all five of these years occurred in the past 10 years.

Of interest is to see what the longevity was of these five largest ozone holes. Figure 3 shows the time series of the five largest ozone hole years. After the maximum size for each year had been achieved, three years (including 2008) persisted with large areas, while the other two did not persist as long. By examining the time series of the poleward heat flux in Figure 4 we can understand why three years persisted to later dates while the other two did not. All three years in which the ozone hole persisted to later a date show minimal heat flux activity throughout the winter and spring months. While the two years that did not persist to later dates do show minimal wave activity in the winter which accounts for the growth of the area of cold temperatures, but then show wave activity in October which prompted the decrease in size of the polar vortex and the hastening of the transition from winter to summer circulation patterns.

Maps of monthly average Southern Hemisphere SBUV/2 total ozone for August, September, October, and November 2008 are shown in Figures 5a, b, c , and d (5a,b,c,d combined), respectively. “Ozone Hole” values (defined as total ozone values less than 220 DU) appear over most of Antarctica with the area centered over the South Pole, and the highest total ozone amounts are found poleward of Australia. Figures 6a, b, c , and d (6a,b,c,d combined) show the difference in percent between the monthly mean total ozone for each month, August-November 2008, minus the respective average (1979-86) monthly means (Nagatani et al., 1988). Extreme negative anomalies in total ozone of greater than 45 percent occurred in each of those months over almost all of Antarctica and adjacent ocean areas. Also, of note is that the tropics have positive anomalies, which are correlated with the west phase of the QBO, while the mid-latitudes have negative anomalies. These are in contrast with 2007.

Figure 7 compares the maximum Ozone Hole size of 2008 with those of the previous 29 years. With a maximum Ozone Hole size of 25.0 million sq km, 2008 ranked 5th largest of all previous Ozone Hole years back to 1979.

Figures 8a and 8b, 8c , and 8d (8a,b,c,d combined), show the individual monthly average Ozone Hole size for, respectively, August, September, October, and November, 1980 to 2008. As Figure 1 indicates, 2008 had a very late date of ozone hole detection. Consequently, Figure 5a shows that August 2008 had one of the lowest average ozone hole sizes. However, as Figure 1 also shows, the ozone hole size grew very rapidly in September and on average was the seventh largest on record. Figure 1 also indicates that the vortex and the ozone hole had extreme longevity in 2008. Figure 8a and Figure 8d show that 2008 had the seventh largest average October and third largest November monthly average ozone hole sizes.

Figure 9 displays monthly average anomaly values (percent) of zonal mean total ozone, as a function of latitude (80ºN to 80ºS) and time (January 1979 to December 2008). The anomalies are derived relative to each month's 1979 to 2008 average. Long-term ozone changes may be readily seen in the polar regions, where ozone values were substantially lower in the 1990s than in the 1980s. The largest anomalies are found for the polar regions in each hemisphere in winter-spring months, with positive anomalies of more than 10 percent in the earlier years changing to negative anomalies of greater than 25 percent for the 1990s. Note that in the Austral 2008 summer the ozone amounts at extremely high latitudes are 4-6% above normal which is quite rare for this latitude at that time of year. Note also that the positive anomalies in the tropics, which are QBO driven, are negatively correlated with those in the SH mid latitudes.

This year’s polar circulation patterns, and consequently the Ozone Hole, were relatively concentric about the South Pole. Figure 10 shows a time series of the past three years (2008, 2007, 2006) of total ozone, measured over the South Pole using balloon-borne ozonesonde instruments. Also shown in Figure 10 are the average range of total ozone values between 1986 and 2007 and the daily maximum and minimum envelope. Several things are to note from this figure. First is that from February through August total ozone values over the South Pole are relatively constant. Maximum mean total ozone values are observed in December. While the lowest ozone values are historically observed in late September and early October. The 2008 minimum total column ozone at South Pole was 107 Dobson Units measured on 28 September 2008 (see Figure 11 ) which is several days earlier than the historic mean of early October. This is similar to what happened in 2007 but not 2006. 2008 and 2007 behaved similarly immediately after the minimum ozone date. They both show relatively rapid increases throughout the remainder of October but then the total ozone values rate of increase slows in November such that the observations are on the low side of the 1986-2007 envelope. Figure 10 also shows via the daily minimum and maximum extremes that for much of the year the daily variability is relatively low except for the 2002 SH sudden warming that occurred in late September and throughout November when the SH polar vortex is breaking down and mid-latitude ozone values make their way to the polar region.

The sequence of profiles in Figure 11 shows the gradual destruction of ozone in the 14 to 21 km region, with the greatest depletion occurring on September 28 and October 5. Unlike 2006 when the Ozone Hole was centered upon the South Pole for many days, this year’s Ozone Hole was not, thus limiting the number of observations of vertical profiles with near complete destruction. The vertical time series in Figure 12 from ozone profiles at the South Pole during 2008 also shows the limited time of large depletion in ozone between 14 and 21 km in late September and very early October.

The dynamics of the middle and upper stratosphere strongly dictate the variability of the size of the Ozone Hole and its longevity. Atmospheric waves can transport heat from lower latitudes into the polar latitudes thus modulating the temperatures inside the polar vortex. Years with above average wave activity will result in vortices with warmer temperatures, which then, will have smaller areas of Polar Stratospheric Clouds within which chemical reactions destroy ozone. Additionally, years with greater wave activity also have shorter polar vortex life times. Conversely, years with quiet wave activity will lead to vortices with colder temperatures and larger areas of PSC clouds and greater ozone destruction and tend to have prolonged life times. Examination of the atmospheric wave structure also shows how much the polar vortex was off center and away from the South Pole. Figure 13 shows a time history of the altitude dependence of the amplitude of wave-1 for 2008. Very little wave activity is present during the austral winter with an especially calm period from mid-August through mid-September. The wave activity from mid-September onward is confined to the upper stratosphere. A reexamination of the meridional eddy heat flux at 100 hPa in Figure 4a confirms that at the lower stratospheric levels eddy heat flux due to meridional wave activity was very light throughout 2008.

Ozone amounts in the lower stratosphere are closely coupled to temperatures through dynamics and photochemistry. Extremely low stratospheric temperatures (lower than -78(C) over the Antarctic region contribute to depletion of ozone, in that low temperatures lead to the presence of polar stratospheric clouds (PSCs). PSCs enhance the production and lifetime of reactive chlorine, leading to ozone depletion (WMO, 1999). Daily minimum temperatures at 50 hPa (approximately 19 km) over the south polar region, averaged from 65S to 90S are shown in Figure 14 . Climatologically, from May through October minimum temperatures in the south polar region are well below -78ºC. In 2008 these minimum temperatures were near-average until November when they became slightly colder than average coinciding with the prolonged lifetime of the winter circulation.

Figure 15 shows monthly average temperature anomalies for September, October, and November (SON) at 50 hPa for three latitude regions, 25ºS to 25ºN, 65ºS to 25ºS, and 90ºS to 65ºS. For the south polar region, 2008 temperatures were much colder than average in the mid-latitudes and high latitudes. This agrees well with the extended lifetime of the polar vortex. Colder temperatures coincide with lesser amounts of ozone, thus agreeing with the ozone anomalies shown in Figure 9 . The tropical latitudes continue to show a historic trend of warm temperatures in the 1980s to cooler temperatures in the 1990’s and 2000’s. However, superimposed upon this overall trend is the variability of temperatures in the tropics due to the QBO. The relatively warmer temperatures for 2008 coincide with the positive tropical ozone anomalies shown in Figure 9 .

Figure 16 illustrates the direct relationship between the persistence of the Ozone Hole and the persistence of the SH polar vortex. In years when the winter polar vortex persisted later into the season, the duration of the Ozone Hole also tended to be extended. For the year 2008, the persistence of the SH polar vortex in the lower stratosphere extended longer than any previous year back to 1979. The persistence of the Ozone Hole also to the middle of December was also the longest on record.

During the Austral Spring, as the sun progresses towards higher solar zenith angles, ultraviolet(UV) radiation at the surface underneath the ozone hole can reach sub-tropical or tropical conditions with UV Indices greater than 10. UV radiation conditions over the ice and snow are higher due to reflection off of the bright surface. Antarctica also has surface elevations that are almost 3000 m. UV conditions are also greater at higher elevations because there is lesser scattering by air molecules. Additionally, at the south polar latitudes, the length of day is very long. Putting all of these effects together result in daily UV dosages that are equivalent to being in the tropics. Figure 17a and 17b show a situation in which the ozone hole passed over the Antarctic Peninsula. Cloud conditions were minimal such that very high daily UV dosages were estimated to be over this peninsula. Observations by the National Science Foundation UV station at Palmer Station verified that daily UV dosages were extremely high (greater than 5000 Joules/m2) and well above the envelope of observations for past years.

Figure 18a and 18b show another situation later in the Austral Spring when the ozone hole passed over the Antarctic Peninsula again. Cloud conditions were again such that very high daily UV dosages were estimated to be over this peninsula. Observations by the Palmer Station NSF UV station verified that daily UV dosages were extremely high (greater than 5000 Joules/m2) and well above the envelope of observations for past years.

Very low ozone values were again observed over Antarctica in the Winter/Spring of 2008. Ozone depletion greater than 45% was observed over Antarctica, compared to total ozone amounts observed in the early 1980's. Vertical soundings over the South Pole during September and October 2008 showed several days of near-complete destruction of ozone at altitudes between 13 and 21 km. Lower stratosphere temperatures in the winter of 2008 over the Antarctic region were near average levels. Distinguishing characteristics of the 2008 ozone hole were that the date when ozone in the south polar region depleted to the ozone hole threshold was much later than most years in the past two decades. This was because ozone levels in the south polar region were anomalously higher before the ozone depletion began. Once the threshold had been reached the ozone hole area grew rapidly to the fifth largest single day size since 1979. The south polar circulation throughout the winter and spring was not significantly perturbed. This resulted in the polar circulation and the ozone hole staying intact well into December.

Observations of chlorofluorocarbons and of stratospheric hydrogen chloride support the view that international actions are reducing the use and release of ozone depleting substances (WMO, 1999; Anderson et al., 2000). However, chemicals already in the atmosphere are expected to continue to impact the atmospheric ozone amounts for many decades to come. The Antarctic Ozone Hole is expected to continue for decades. Antarctic ozone abundances are projected to return to pre-1980 levels around 2060-2075, roughly 10-25 years later than estimated in the 2002 Assessment. The projection of this later return is primarily due to a better representation of the time evolution of ozone-depleting gases in the Polar Regions. In the next two decades, the Antarctic Ozone Hole is not expected to improve significantly (WMO, 2007). Further, changing conditions (i.e. meteorological, solar, and volcanic aerosols) that modulate ozone can complicate the task of detecting the start of ozone layer recovery. The eruption of the Pinatubo volcano provided an example of such a complication in the 1990s. Based on an analysis of 22 years of South Pole ozone vertical profile measurements, Hofmann et al., (2009) suggested that, according to indicators such as the September ozone loss rate at 14-21 km and ozone loss at the upper limits of the ozone hole (22-24 km), the beginning of recovery of the Antarctic Ozone Hole had not yet begun and may not be detected for some time. An intriguing aspect of recent observations of the Antarctic stratosphere had been the apparent trend towards a later breakup of the vortex in years since 1990, relative to the 1980s. The size and duration and size of the 2008 Ozone Hole is attributed in part to meteorological conditions. A full explanation of such meteorological anomalies is not yet available. Continued monitoring and measurements, including total ozone and its vertical profile, are essential to achieving the understanding needed to identify ozone recovery.

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