Antarctic Ozone Bulletin No 1 / 2006
Map of total ozone (Dobson units) for 22 August 2006 made at the World Ozone and UV Data Centre (WOUDC), hosted by Environment Canada. The map shows that some ozone depletion has started along the edge of polar vortex. In this box on the cover, future Antarctic Ozone Bulletins will highlight results from different WMO partners on a rotating basis.
Executive summary During the May-August 2006 time period, temperatures averaged over the 60-90°S region have been colder than the 1979-2005 average and on certain days colder than for any year on that date in the 1979-2005 period. The vortex area has been slightly larger than the 1986 - 2005 average since mid May. Since the beginning of August, the vortex area has risen more rapidly than usual, and by mid August the vortex area at ~17 km altitude was as large as the maximum for the 1986 - 2005 period. The area of the region where temperatures are low enough for the existence of polar stratospheric clouds (PSC type I or NAT), has been around the 1986 - 2005 average from mid May until mid July. Since mid July the PSC area at ~17 km altitude has been higher than the 20-year average and also higher than in the previous three winters. On several days it has been close to or even above the largest daily PSC areas since 1986. At the altitude of ~18 km the vortex is now almost entirely depleted of HCl, one of the reservoir gases that can be transformed to active chlorine. In the sunlit collar along the vortex edge there is 1.0 - 1.7 ppb of active chlorine (ClO), and some first signs of ozone depletion is visible. As the sun returns to Antarctica after the polar night, it is expected that the ozone destruction will speed up. It is still too early to give a definitive statement about the development of this year's ozone hole and the degree of ozone loss that will occur. This will, to a large extent, depend on the meteorological conditions. WMO and the scientific community will use ozone observations from the ground, from balloons and from satellites together with meteorological data to keep a close eye on the development during the coming weeks and months. 24 Aug. 2006
G l o b a l A t m o s p h e r e Watch
Background information
ECMWF
The meteorological conditions in the Antarctic stratosphere found during the austral winter (June-August) set the stage for the annually recurring ozone hole. Low temperatures lead to the formation of clouds in the stratosphere, so-called polar stratospheric clouds (PSCs).
Analysis of PV (10-6 Km2/kgs) 1 = 500 K 17 Aug 2006 12UT Day number 229
The amount of water vapour in the stratosphere is very low, only 5 out of one million air molecules are water molecules. This means that under normal conditions there are no clouds in the stratosphere. However, when the temperature drops below -78°C, clouds that consist of a mixture of water and nitric acid start to form. These clouds are called PSCs of type I. On the surface of particles in the cloud, chemical reactions occur that transform passive and innocuous halogen compounds (e.g. HCl and HBr) into so-called active chlorine and bromine species (e.g. ClO and BrO). These active forms of chlorine and bromine cause rapid ozone loss in sun-lit conditions through catalytic cycles where one molecule of ClO can destroy thousands of ozone molecules before it is passivated through the reaction with nitrogen dioxide (NO2).
Plotted at NILU by t106glob
Figure 1. Polar orthographic map of potential vorticity at the potential temperature level of 500 K (ca. 20 km) over the south polar region for 17 August 2006. The polar vortex is relatively concentric around the South Pole and its area is relatively large in comparison to recent years. The plot is based on data from the European Centre for Medium range Weather Forecasts (ECMWF).
hence the ozone hole later in the season. For more background information see: http://www.wmo.int/web/ arep/O3_summaries/O3_summaries_afischer.html.
When temperatures drop below -85°C, clouds that consist of pure water ice will form. These ice clouds are called PSCs of type II. Particles in both cloud types can grow so large that they no longer float in the air but fall out of the stratosphere. In doing so they bring nitric acid with them. Nitric acid is a reservoir that liberates NO2 under sunlit conditions. If NO2 is physically removed from the stratosphere (a process called denitrification), active chlorine and bromine can destroy many more ozone molecules before they are passivated. The formation of ice clouds will lead to more severe ozone loss than that caused by PSC type I alone since halogen species are more effectively activated on the surfaces of the larger ice particles.
Meteorological conditions Temperatures Meteorological data from the European Centre for Medium Range Weather Forecasts (ECMWF) in Reading, UK and the National Center for Environmental Prediction (NCEP) in Maryland, USA, show that stratospheric temperatures over Antarctica have been below the PSC type I threshold of -78°C since mid May and below the PSC type II threshold of – 85°C since late May/early June, as shown in Figure 2. This figure also shows that the daily minimum temperatures at the potential temperature level of 500 K (~19 km) have been relatively close to the 1996-2004 average, with the exception of the month of July where the minimum temperatures were somewhat above this average. Data from NCEP, made available through the Ozonewatch web page of NASA (see section on Acknowledgements and links at the end of the Bulletin), show that the mean temperature at 70 hPa in the 60-90°S region has been at or below the 1979-2005 average in May and June and well below this average in July and
The Antarctic polar vortex is a large low-pressure system where high velocity winds (polar jet) in the stratosphere circle the Antarctic continent. Figure 1 depicts the vortex on 17 August 2006. The region poleward of the polar jet includes the lowest temperatures and the largest ozone losses that occur anywhere in the world. During early August, information on meteorological parameters and measurements from ground stations, balloon sondes and satellites of ozone and other constituents can provide some insight into the development of the polar vortex and
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Area [106 km2]
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Figure 2. Time series of daily minimum temperatures at the 500 K isentropic level south of 30°S. The thick red curve shows 2006 (until 22 August). The blue line shows 2005. The average of the 1996-2004 period is shown for comparison in grey. The thin grey lines represent the highest and lowest daily minimum temperatures in the 1996-2004 time period. The two horizontal green lines at 195 and 188 K show the thresholds for formation of PSCs of type I and type II, respectively. The plot is based on data from ECMWF.
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Figure 3. Time series of the area where temperatures are low enough for the formation of PSCs of type I at the 450 K isentropic level. This corresponds to an altitude of approximately 17 km. The thick red curve shows 2006 (until 18 August). The blue, green and cyan curves represent 2005, 2004 and 2003, respectively. The average of the 1986-2005 period is shown for comparison in grey. The two thin grey lines show the maximum and minimum PSC area during the 1986-2005 time period for each date. The plot is based on data from NOAA's Climate Prediction Center.
August. On some days the temperature averaged over this region has been colder than the coldest observed in the 1979 to 2005 time period. A similar development is also seen at the 30 and 50 hPa levels. 35
PSC Area
30
Since 17 July, temperatures low enough for PSC type I formation have covered an area of more than 25 million square kilometres, or about 85-90% of the vortex area. The PSC area oscillated around the 1986-2005 mean until mid July and has been significantly higher than this 20-year mean since then. Since mid July it has also been higher than in the previous three winters and on several days close to or even above the largest PSC areas since 1986 (see Figure 3). The area with temperatures low enough for the existence of PSCs is directly linked to the amount of ozone loss that will occur later in the season, but the degree of ozone loss depends also on other factors, such as the amount of water vapour and HNO3.
Area [106km2]
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Figure 4. Time series of the area of the south polar vortex at the isentropic level of 450 K (~17 km). The area is defined as the region where potential vorticity is less than -32�10-6 Km2/kgs. The thick red curve shows 2006 (until 18 August). The blue, green and cyan curves represent 2005, 2004 and 2003, respectively. The average of the 1986-2005 period is shown for comparison in grey. The two thin grey lines show the maximum and minimum vortex area during the 1986-2005 time period for each date. It can be seen that the vortex area in mid August is as high as it has ever been before during the 1986-2005 time period. The plot is based on data from NOAA's Climate Prediction Center.
Based upon the historical meteorological record it is expected that the extent and frequency of PSC occurrence will level off and begin to decrease now as the sun rises over Antarctica, although the vortex will gradually increase in size throughout most of August.
Vortex size
HCl
Figure 1 shows a map of potential vorticity at the 500 K potential temperature level (~ 20 km). This picture indicates how isolated the polar air mass is from air masses outside the polar vortex. Yellow, orange and red colours depict regions where the air is particularly well isolated from the surroundings. Presently the vortex is relatively circular, stable and centred over the pole.
17 Aug 2005
ppbv 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Figure 4 shows the daily geographical extent of the south polar vortex at the isentropic level of 450 K (~ 17 km) and it can be seen that the size of the vortex has been slightly higher than the 1986-2005 average since mid May. Since the beginning of August, the vortex area has risen more rapidly than usual and by mid August the vortex area was as large as the maximum for the 1986-2005 period. It should be pointed out that vortex size gives no direct indication on the degree of ozone loss that might occur later in the season.
HCl
17 Aug 2006
Figure 6. Mixing ratio of HCl at the isentropic level of 490 K. 17 August 2005 and 2006 are shown for comparison. It can be seen that the degree of HCl removal is nearly equal for these two dates. These maps are made at NASA's Jet Propulsion Laboratory and based on data from the Aura-MLS satellite instrument. The white contours indicate isolines of scaled potential vorticity.
ppbv 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Ozone observations
tex in mid-August have total ozone column values below 225 DU (see map on the cover). Independent analyses and forecasts carried out by the Royal Netherlands Meteorological Institute also show that the ozone hole of 2006 is about to form and that the minimum total ozone columns in mid-August are around average for this time of the year when compared to the last nine years (see Figure 5).
Most of Antarctica still remains under winter darkness, so the average rate of ozone loss there remains relatively low. However, total ozone column maps synthesised by the World Ozone and UV Data Centre at Environment Canada, using surface-based WMO/GAW network observations and satellite data, show that some areas in the sunlit parts of the vor-
Ozone soundings carried out at the German GAW station at Neumayer (70.65°S, 8.26°W) also show some first signs of ozone depletion.
Minimum Ozone Column in the Southern Hemisphere 250
Chemical activation of the vortex
Dobson Units
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100 GOME / SCIAMACHY Assimilated Ozone KNMI / ESA 23 Aug 2006
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The south polar vortex is now activated and primed for ozone depletion. As soon as the sun comes back after the polar winter, ozone depletion will set in.
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Figure 6 shows the extent of removal of hydrochloric acid (HCl), which is one of the reservoirs for active chlorine, for 17 August in 2005 and 2006. As can be seen from the figure, HCl is almost completely removed inside the vortex and the degree of removal is similar for 2005 and 2006. Removal of HCl is an indicator for chemical activation of the vortex. Another indicator for vortex activation is the amount of chlorine monoxide (ClO). It should be noted, however, that ClO dimerises and forms (ClO)2 in darkness. The dimer is easily cracked in the presence of sun-
31 Dec
Figure 5. Daily minimum total ozone columns in the Southern Hemisphere as observed by GOME and SCIAMACHY from 1997 to now. The black dots show the observations for 2006. The open circles are forecasts for the next few days. The minimum ozone columns are near the average for the 1997-2005 time period. The plot is provided by the Netherlands Meteorological Institute (KNMI).
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SZA=94o
17 Aug 2005
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ClO
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Figure 7. Mixing ratio of ClO at 490 K. 17 August 2005 and 2006 are shown for comparison. It can be seen that there are substantial amounts of ClO in a collar along the vortex edge. In 2005, the mixing ratio of ClO was somewhat higher than in 2006. These maps are made at NASA's Jet Propulsion Laboratory and based on data from Aura-MLS. The white contours indicate isolines of scaled potential vorticity. The thick black contour line shows where the solar zenith angle is 94º.
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Ozone Hole Area w.r.t. 220 DU in the Southern Hemisphere GOME / SCIAMACHY Assimilated Ozone KNMI / ESA 23 Aug 2006
25 million square km
ClO
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Figure 8. Area (millions of km ) where the total ozone column is less than 220 Dobson units. All the years from 1997 to 2006 (black dots) are shown. It can be seen that the 2006 ozone hole so far is smaller than the ozone hole in 2005 (green curve). The small open circles are forecasts and should be used with caution. This plot is produced by KNMI in collaboration and is based on data from the GOME and SCIAMACHY satellite instruments.
ppbv
0.15 0.45 0.75 1.05 1.35 1.65
Figure 9 shows the ozone mixing ratio at the 490 K isentropic level as observed with the Aura-MLS satellite instrument. It can be seen that ozone is depleted in a collar around the Antarctic continent. The degree of depletion on 17 August 2006 is somewhat less than on the same date in 2005. This can be explained by the position of the vortex relative to the sunlit areas.
light. ClO will therefore be present in the sunlit parts of the vortex, whereas the dark areas will be filled with (ClO)2, which is not observed by Aura-MLS. Figure 7 shows the amount of ClO on 17 August 2005 and 2006. On both dates one can see an area of elevated ClO that forms a collar along the vortex edge. This collar constitutes the sunlit part of the vortex. It can be seen from Figure 7 that the amount of ClO in certain regions was somewhat higher on 17 August 2005 than at the same date in 2006. This is related to the position of the vortex with respect to the sunlight.
O3
17 Aug 2005
Ozone hole Temperatures are still sufficiently low within the polar vortex to maintain the chemical processes required for the formation of the annually recurring Antarctic ozone hole (see Figure 2). The ozone hole usually does not reach its maximum size until mid- to late September, but already by mid-August, one can see the first signs of ozone destruction, as shown in Figure 5 and on the cover map.
ppmv 1.6 1.9 2.2 2.5 2.8 3.1 3.4
O3
Figure 8 shows the area of the region where total ozone is less than 220 DU as deduced from the GOME and SCIAMACHY satellite instruments. This graph shows that the area of the ozone hole is still relatively small.
17 Aug 2006
ppmv 1.6 1.9 2.2 2.5 2.8 3.1 3.4
Figure 9. Mixing ratio of ozone at 490 K. 17 August 2005 and 2006 are shown for comparison. It can be seen that there is a region where ozone is depleted along the edge of the vortex. In 2005, the degree of ozone destruction was somewhat higher on 17 August than on the same date in 2006. These maps are made at NASA's Jet Propulsion Laboratory and based on data from Aura-MLS. The white contours indicate isolines of scaled potential vorticity.
As the sun rises over Antarctica during the coming weeks, the ozone hole is expected to grow both in depth and horizontal extent. The amount of ozone loss will be dependent upon prevailing meteorological conditions in the stratosphere, particularly during September and October.
sis).
Satellite ozone data are provided by NASA (http:// ozonewatch.gsfc.nasa.gov), NOAA/TOVS (http://www. cpc.ncep.noaa.gov/products/stratosphere/tovsto/), NOAA/ SBUV/2 (http://www.cpc.ncep.noaa.gov/products/stratosphere/sbuv2to/) and ESA/Sciamachy (http://envisat. esa.int). Satellite data on ozone, ClO, HCl and a number of other relevant parameters from the MLS instrument on the Aura satellite can be found here: http://mls.jpl.nasa.gov/plots/mls/mls_plot_locator.php.
The situation with annually recurring Antarctic ozone holes is expected to continue as long as the stratosphere contains an excess of ozone depleting substances. As stated in the recently published Executive Summary of the 2006 edition of the WMO/UNEP Scientific Assessment of Ozone Depletion, severe ozone holes are expected to form during the next couple of decades.
Potential vorticity and temperature data are provided by the European Centre for Medium Range Weather Forecasts (ECMWF) and their daily T106 meteorological fields are analysed and mapped by the Norwegian Institute for Air Research (NILU) Kjeller, Norway, to provide vortex extent, PSC area and extreme temperature information. Meteorological data from the US National Center for Environmental Prediction (NCEP) are also used to assess the extent of PSC temperatures and the size of the polar vortex (http://www.cpc.ncep.noaa.gov/products/stratosphere/polar/polar.shtml). NCEP meteorological analyses and climatological data for a number of parameters of relevance to ozone depletion can also be acquired through the Ozonewatch web site at NASA (http:// ozonewatch.gsfc.nasa.gov/meteorology/index.html).
In August, ozone loss is limited and the sun is still low in the sky, so the intensity of ultraviolet (UV) radiation in areas usually affected by the ozone hole is modest. UV levels will be reported in future ozone bulletins.
Distribution of the bulletins The Secretariat of the World Meteorological Organization (WMO) distributes Bulletins providing current Antarctic ozone hole conditions beginning around 20 August of each year. The Bulletins are available through the Global Atmosphere Watch programme web page at http://www.wmo.ch/web/arep/ozone.html. In addition to the National Meteorological Services, the information in these Bulletins is made available to the national bodies representing their countries with UNEP and that support or implement the Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol.
Ozone data analyses and maps are prepared by the World Ozone and UV Data Centre at Environment Canada (http://exp-studies.tor.ec.gc.ca/cgi-bin/selectMap) and by the Royal Netherlands Meteorological Institute (http://www.temis.nl/protocols/O3global.html). UV data are provided by the U.S. National Science Foundation’s (NSF) UV Monitoring Network (http:// www.biospherical.com/nsf). The Executive Summary of the 2006 WMO/UNEP Scientific Assessment of Ozone Depletion can be found here: http://www.wmo.int/web/arep/gaw/gaw_home.
Acknowledgements and links These Bulletins use provisional data from the WMO Global Atmosphere Watch (GAW) stations operated within or near Antarctica by: Argentina (Comodoro Rivadavia, San Martin, Ushuaia), Argentina/Finland (Marambio), Argentina/Italy/Spain (Belgrano), Australia (Macquarie Island and Davis), China/Australia (Zhong Shan), France (Dumont D’Urville and Kerguelen Is), Germany (Neumayer), Japan (Syowa), New Zealand (Arrival Heights), Russia (Mirny and Novolazarevskaja), Ukraine (Vernadsky), UK (Halley, Rothera) and USA (McMurdo, South Pole). More detailed information on these sites can be found at the GAWSIS web site (http://www.empa.ch/gaw/gaw-
html
Questions regarding the scientific content of this Bulletin should be addressed to Geir O. Braathen, mailto:
[email protected], tel: +41 22 730 8235. The next Antarctic Ozone Bulletin is planned for 6 September 2006. End of WMO Antarctic Ozone Bulletin 1/2006.
