An improved measure of ozone depletion in the Antarctic stratosphere

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL.???, XXXX, DOI: /, An improved measure of ozone depletion in the Antarctic stratosphere P. E. Huck, 1 S. Tilmes, 2 G. E. Bodeker, 3 W. J. Randel, 2 and A. J. McDonald, 1 P. E. Huck, Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand. (peh26@student.canterbury.ac.nz) S. Tilmes, National Center of Atmospheric Research, Boulder, Colorado, USA. (tilmes@ucar.edu) G. E. Bodeker, National Institute of Water and Atmospheric Research, Omakau, Central Otago, New Zealand. (g.bodeker@niwa.co.nz) W. J. Randel, National Center of Atmospheric Research, Boulder, Colorado, USA. (randel@ucar.edu) A. J. McDonald, Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand. (adrian.mcdonald@canterbury.ac.nz) 1 Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand. 2 National Center of Atmospheric Research, Boulder, Colorado, USA.

2 X - 2 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION Abstract. Two common indices that have been used to quantify Antarctic ozone depletion are ozone mass deficit and ozone hole area. However, as currently defined, neither of these measures are robust with respect to capturing the true extent of the chemical loss of ozone within the Antarctic vortex. Therefore, in this study, new definitions of these indices have been developed. The 220 Dobson Unit (DU) value currently used as the threshold for ozone depletion has been replaced with a new background ozone value based on pre-1980 measurements (unperturbed background). Negative deviations from this background have been considered over three different regions viz.: within the polar vortex, poleward of 62 S (the average latitude of the vortex edge) and poleward of 40 S (the most equatorward excursion of the vortex edge). An estimate of the error in these indices when avoiding the complicating factor of calculating the vortex edge is presented. By considering deviations from the unperturbed background rather than from the constant 220 DU threshold, variability in ozone loss due to transport can be better separated from variability due to chemistry. The improved representativeness of these indices has been demonstrated by comparing vortex average total ozone loss (defined using the new parameters) with partial col- 3 National Institute of Water and Atmospheric Research, Omakau, Central Otago, New Zealand.

3 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 3 umn accumulated chemical ozone loss calculated with the tracer-tracer correlation method for and in greater detail for 1996 and High correlations between the two measures are observed for all years except 2002 and 2004 suggesting that in years of high dynamical activity, these new parameters may need to be derived in a more advanced way.

4 X - 4 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION 1. Introduction Ozone destruction over Antarctica has been of scientific interest since the discovery of the ozone hole in 1985 [Farman et al., 1985]. Initially the severity of Antarctic ozone depletion was quantified using minimum total column ozone values over Antarctica. However, later studies [Stolarski et al., 1990] showed that in some years (e.g and 1989) while minimum ozone values were similar, the development of the ozone hole area was quite different. The ozone hole area, introduced by [Stolarski et al., 1990], and defined as the area within the 220 Dobson Unit (DU) contour, has become another standard measure of the severity of Antarctic ozone depletion. More recently, a third measure, ozone mass deficit, that combines the size and depth of the ozone hole has been used [Uchino et al., 1999]. This metric is calculated by integrating air masses depleted in ozone over latitudes poleward of 40 S. In this case air masses depleted in ozone are identified as those with less than 220 Dobson Units of total column ozone. This threshold has been generally acknowledged because it is lower than all pre-ozone hole values (before 1980) and the 220 DU contour lies in the region of steep ozone gradients surrounding the ozone hole. All three measures mentioned above (minimum total column ozone, ozone hole area (OHA) and ozone mass deficit (OMD)) have been used to describe and study inter- and intra-annual variations in ozone depletion [WMO, 2003]. The Antarctic annual minimum value in total column ozone over Antarctica is insensitive to dynamical disturbance of the vortex and has shown little change through the 1990s during the annual Antarctic vortex period (AVP: 18 July-30 November) [WMO, 2003]. The minimum appears usually in late September or early October and has remained es-

5 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 5 sentially unchanged at about 100 DU since 1993 after the considerable decrease during the 1980s and early 1990s. Equivalent effective stratospheric chlorine (EESC) [Daniel et al., 1995], a measure of the concentration in ozone depleting substances (ODSs) in the stratosphere, peaks in 1997 and the Antarctic equivalent, EEASC, as late as 2001 [Newman et al., 2006]. The insensitivity of annual minimum ozone values to changing EEASC suggests that its use in detecting a response of Antarctic ozone depletion to reductions in ODSs is limited. Similar saturation effects are observed in OHA. OHA is more sensitive to changes in the vortex collar region than minimum total ozone. The spring-time OHA increased rapidly during the 1980s and more gradually during the 1990s [WMO, 2003] while EEASC steadily increased. From 1995 to 2005 the OHA also shows a lack of response to EEASC [Newman et al., 2006]. Unlike AVP mean minimum total column ozone and ozone hole area, the AVP mean OMD shows significant growth during the 1990s as discussed in Bodeker et al. [2005]. They suggest that AVP mean OMD might be a more suitable measure to use when investigating the recovery of the Antarctic ozone hole than minimum total column ozone or ozone hole area. The long-term trend in ozone depletion is mainly controlled by chemical processes and the inter-annual variability can be explained by variability in temperatures and planetary wave activity [Huck et al., 2005]. However, when expanding these studies to intra-annual variability, more detailed processes need to be considered which can be neglected when averaging over the AVP. The start date and the rate of ozone destruction depend on temperatures and on sunlight returning to the polar regions after the darkness of polar winter. Furthermore, ozone depletion is controlled indirectly by planetary wave activity displacing air masses out of the polar darkness and therefore causing earlier ozone deple-

6 X - 6 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION tion in some regions [Solomon et al., 1993; Austin and Butchart, 2003]. By transporting heat, waves directly affect polar temperatures and therefore the occurrence of polar stratospheric clouds (PSCs) which control chlorine activation and therefore ozone destruction [Schoeberl and Hartmann, 1991]. To quantify intra-annual variability in ozone depletion a measure sensitive to all of these processes is necessary. If the OMD and OHA definitions are based on a 220 DU threshold, as was done in Huck et al. [2005] and earlier studies, the identified onset of ozone depletion occurs well after sunlight returns to the polar regions, which is inconsistent with observations and theory, and only part of the true depletion is captured (see Figure 1; figure details presented in Section 2.1). This suggests that the first step towards improving the OMD and OHA metrics is to define a better background value that describes the column ozone for chemically unperturbed conditions (pre-ozone hole conditions). Since the 220 DU contour has previously provided the unperturbed background ozone as well as the limiting contour for the Antarctic vortex, calculation of a new contour for defining the region within which ozone depletion occurs becomes necessary when using a chemically unperturbed background ozone. To test whether these new parameters for OMD and OHA calculation are likely to lead to more representative measures of Antarctic ozone depletion, vortex average total column ozone loss is derived using the unperturbed background ozone. This measure is compared with the accumulated chemical loss in partial column ozone derived using tracer-tracer correlations. With the tracer-tracer correlation method the impact of dynamical changes on the column ozone are separated from the chemical ozone loss [Proffitt et al., 1990; Müller et al., 2001].

7 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 7 The different approaches used to calculate the unperturbed background ozone and the three spatial integration limits considered are introduced in Section 2.1. In Section 2.2 the application of the new unperturbed backgrounds and spatial integration limits to OMD and OHA is described. In Section 2.3 the new method is tested by investigating whether these changes result in better tracking of Antarctic ozone loss calculated with more complex and sophisticated techniques such as the tracer-tracer correlation method. The results are presented in Section 3, in particular OMD in Section 3.1 and the chemical ozone loss in Section 3.2. The results are summarized and conclusions are drawn in Section Methods 2.1. Derivation of unperturbed background ozone To estimate the annual variation in total column ozone over Antarctica in the absence of stratospheric halogens (referred to as the unperturbed background), data from 1979 from the NIWA combined total column ozone data base [Bodeker et al., 2005] have been used. Although there are some indications of ozone destruction due to halogen chemistry as early as 1979 [Solomon et al., 2005], ozone levels before 1980 are considered to be preozone hole [Newman et al., 2006]. However, factors other than ODSs may have affected ozone in 1979 (e.g. the QBO) and corrections made to account for these factors are discussed in Section 2.2. Ideally the Antarctic ozone depletion index should be sensitive only to ozone depletion that occurs within the Antarctic vortex. Therefore, our first approach is to use the inner vortex edge, derived using the Nash et al. [1996] method, to define the region over which the ozone loss is calculated (the spatial integration limit). NCEP/NCAR reanalyses

8 X - 8 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION potential vorticity (PV) values were transformed into equivalent latitude coordinates. The vortex edge is located at the steepest PV gradient multiplied by the total wind speed. The inner and outer edge of the polar vortex are defined by the maximum and minimum in the second derivative of PV with respect to equivalent latitude. The inner vortex edge is used in these studies to avoid counting any mixed air in the vortex boundary region which would affect the value of ozone depletion. The vortex edge has been calculated for an altitude range between approximately km at potential temperature levels of 450, 550 and 650 K. The innermost poleward edge of the three potential temperature levels has been assigned as the border between vortex air and lower latitude air masses. It should be noted that the vortex edge might be shifted or twisted at altitudes above 650 K and therefore does not match the derived vortex edge between K. This effect could possibly occur at the end of winter or during a disturbed vortex (e.g. in 2002 [Newman and Nash, 2005]). The entire column ozone would be affected and it could result in an underestimation of OMD and OHA. Given the difficulty in realistically simulating the vortex edge in climate models, to ensure the utility of this method, two additional spatial integration limits are considered, viz.: 1) the 62 S average latitude of the vortex edge as suggested by Bodeker et al. [2002], and 2) 40 S, equatorward of the largest excursions of the polar vortex. In Figure 2 all three limiting contours as well as the 220 DU contour are displayed with an example latitudelongitude ozone distribution in the background. It is obvious that when using the 220 DU contour as the spatial integration limit, although this restricts the area of ozone depletion to within the polar vortex it also omits much of the vortex region, particularly early in

9 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 9 the season. Use of the 40 S limiting contour includes much of the ridge of ozone rich air in mid-latitudes. From the data which are available on a 1 by 1.25 latitude-longitude grid, daily area weighted means of total column ozone have been calculated over the three regions defined above i.e. the inner vortex edge, 62 S, and 40 S. To eliminate day-to-day variability in these time series, a 4th order Fourier function was fit to the data. The resulting unperturbed backgrounds are displayed in Figure 1. Over the first few months of the year, when halogen related ozone loss over Antarctica is likely to be negligible, the close correspondence between the inner vortex averaged ozone and the ozone time series from 2003 suggests that the vortex average ozone is likely to be the most suitable choice for the chemically unperturbed background ozone. Furthermore, the choice of the vortex average ozone provides the most conservative estimate of ozone loss and is less likely to include air masses that have not undergone heterogeneous ozone depletion chemistry. However, by simply choosing 62 S, a reliable result can be obtained until the first week of September. Later in the year, larger deviations occur. By considering large parts of the southern hemisphere and choosing the limiting contour at 40 S, an overestimation of ozone loss is to be expected Potential Activation Parameter, Ozone Mass Deficit and Ozone Hole Area In theory, ozone depletion begins as soon as sunlight reaches areas of activated chlorine. Chlorine is activated during polar winter when heterogeneous chemistry occurs on the surfaces of PSCs. The greater the illumination of the polar region, the greater the area of chlorine activation and ozone depletion. This process continues until the seasonal

10 X - 10 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION temperature increase driven by radiation and dynamics warms the polar region enough for the PSCs to disappear. Activated chlorine then returns to inactive reservoir species. The warming up of the polar regions also causes a weakening of the polar vortex until the final warming event breaks the structure apart. A potential activation parameter (PAP) has been derived based on this theory. PAP is calculated by integrating the hours of sunlight over PSC areas and multiplied by EESC. It provides an estimate of the start date and occurrence of the maximum of ozone depletion. In addition to calculating the daily OMD using the method described in Huck et al. [2005], three new OMD quantities have been calculated by replacing the 220 DU threshold with the three unperturbed backgrounds and by integrating the depleted ozone mass over the same three geographical regions used to define the unperturbed backgrounds. Daily values of these indices were calculated from the NIWA combined total column ozone data base [Bodeker et al., 2005] from 1980 to When used as integration limits for OHA, the three unperturbed backgrounds result in three new OHA quantities which are compared with the traditional 220 DU based quantity. The same data base has been used to calculate daily OHA by adding the area of each cell with an ozone value below the unperturbed background to a daily total, if it is within the spatial integration limit. OMD and OHA values have only been calculated during the Antarctic vortex season and are considered to be zero outside that period. Shortly after the vortex forms, in the absence of other drivers of ozone variability, the OMD and OHA values should be zero. However, the derived chemically unperturbed background ozone does not account for year-to-year ozone fluctuations caused by the quasi-biennial oscillation (QBO), solar cycle or volcanic eruptions etc. with the result that OMD and OHA values are sometimes

11 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 11 non-zero during this period. To correct for this artifact, the average May value for OMD and OHA for each year has been subtracted from the data. On average the OMD is corrected by 7 % of the annual maximum value and the OHA by 23 %. Future work will use a regression model incorporating basis functions to account for these sources of ozone variability and thus calculate an unperturbed background that adapts to the unique dynamical conditions of each year Comparison with tracer-tracer correlation derived chemical ozone loss To test whether the newly defined OMD and OHA indices better track the true Antarctic ozone depletion, spatially averaged ozone loss over the three regions defined above, was compared with ozone loss within the Antarctic vortex calculated with a tracer-tracer correlation method. The two methods for calculating Antarctic ozone depletion are completely independent. Daily vortex average total column ozone loss was calculated for by subtracting the measured ozone column from the unperturbed background. In addition, chemical ozone loss was derived with the tracer-tracer correlation method for [Tilmes et al., 2006a]. This method requires measurements of an ozone profile and simultaneous measurements of a passive tracer which can be used to track the diabatic descent of air parcels within the vortex. Halogen Occultation Experiment (HALOE) measurements were used for while Improved Limb Atmospheric Spectrometer (ILAS and ILAS-II) measurements were used for more detailed comparisons in 1996 and 2003, respectively. For the accumulated ozone loss derived from HALOE data, CH 4 and/or HF were used as the passive tracer while for the accumulated ozone loss derived from ILAS-II, N 2 O was used as the passive tracer. HALOE provides discontinuous measurements of O 3, CH 4 and HF [Russell III et al., 1993] which have been averaged

12 X - 12 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION over periods between 1 and 20 days and result in one to four data points per year. ILAS provided daily measurements of O 3 and N 2 O inside the polar vortex from 30 October to 13 December 1996 [Suzuki et al., 1995; Sasano et al., 1999] and ILAS-II from 2 April to 24 October 2003 [Nakajima et al., 2005]. While HALOE provides more years for comparison of the tracer-tracer method with the total column ozone loss derived here ( ), ILAS and ILAS-II provide better temporal coverage during the years for which data are available (1996 and 2003). For the tracer-tracer correlation method an early winter reference function is derived for chemically unperturbed conditions in an established vortex. Deviations from this reference function are used to identify chemical ozone loss. This reference function was calculated using O 3 and N 2 O measurements in June from ILAS and ILAS-II. The relationship between ozone and N 2 O does not change within the confined borders of the polar vortex unless ozone reacts chemically. Accumulated chemical ozone loss was calculated by integrating the tracer-tracer correlation derived ozone loss profiles between 350 K and 550 K for HALOE and between 350 K and 600 K for ILAS and ILAS-II. These altitude ranges have been identified as the region where the halogen catalyzed polar ozone loss takes place [Tilmes et al., 2006b]. Therefore the integrated values represent the total amount of chemically destroyed ozone in the polar vortex. Mixing of air from outside to inside the vortex could potentially result in an underestimation of chemical ozone loss [Müller et al., 2005]. 3. Results 3.1. Ozone Mass Deficit and Ozone Hole Area

13 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 13 For the three different unperturbed backgrounds, calculated over three different regions (poleward of the inner vortex edge, poleward of 62 S and poleward of 40 S), new OMD and OHA time series were calculated as described in Section 2.2. The three new OMD indices are plotted in Figure 3 for The think black line represents OMD integrated within the inner vortex edge (OMD 1 ), the grey line represents OMD integrated poleward of 62 S (OMD 2 ) and the dashed line OMD integrated poleward of 40 S (OMD 3 ). In comparison the old measure of OMD using 220 DU both as the unperturbed background and as the spatial integration limit is plotted as a thick black line. Additionally, the grey area represents the PAP for While the old OMD starts about two weeks later than all three measures of the new OMD, the seasonal start date of the new OMDs is now very close to the start date of the PAP (i.e. 21 May 2003, day 141) which is consistent with observations and theory. The timing of the maxima of the new OMDs and PAP are similar (i.e. 30 September 2003) as expected. OMD 1 terminates at the same time as the vortex breaks up (the time when no edge can be found using the criterion of Nash et al. [1996]) while OMD 2 and OMD 3 remain positive to the end of the year. OMD 1 and OMD 2 track each other closely until shortly after the seasonal maximum after which OMD 2 overestimates the ozone depletion compared to OMD 1. This leads to an overestimation of the seasonal total of OMD 2 by 29 % compared to OMD 1. The seasonal evolution of OMD 3 is similar to OMD 1 and OMD 2 but overestimated the seasonal total by 102 % compared to OMD 1. Overall, considering the years (not all shown), more depleted mass is accounted for in the new OMDs compared to the old OMD. This confirms that it is likely that the old OMD measure omits some of the depleted ozone mass. When comparing the three new OMDs over 25 years,

14 X - 14 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION an average overestimation of ozone depletion of 28 % for OMD 2 compared to OMD 1 and 141 % for OMD 3 compared to OMD 1 is observed. A similar comparison of the OHA measures conducted for 2003 (Figure 4), indicates that only the OHA calculated within the contour defined by the unperturbed background from within the inner vortex edge (i.e. OHA 1 ) is realistic. Comparing the old OHA with OHA 1, the seasonal evolution of the two measures is somewhat different with OHA 1 showing an earlier start compared to the original OHA but not reaching the maximum of the original OHA. This is a result of the correction applied to OHA 1 early in the season to account for inter-annual differences in dynamics (see Section 2.2). Without that correction the maximum of OHA 1 coincides with the maximum of the original OHA. Because the seasonal start date for OHA 1 is similar to that for PAP (and earlier than if the 220 DU contour is used), OHA 1 is considered to be a better measure of the ozone hole area than the method using the 220 DU contour Vortex Average Chemical Ozone Loss Two measures for vortex average chemical ozone loss have been derived as described in Section 2. The two measures are 1), the total column ozone loss derived by subtracting an unperturbed background ozone from column measurements within the vortex, and 2) the accumulated chemical ozone loss derived with the tracer-tracer correlation method. The two methods have been compared from 1992 to 2004 (Figure 5). To reduce uncertainties in the accumulated chemical ozone loss, up to 20 profiles inside the vortex have been averaged for one data point. Due to the orbit of the HALOE instrument this results in 1 to 4 data points per year. Except for 2002 and 2004 (dynamical very perturbed years) the two methods show excellent agreement, well within the 1σ uncertainties in the accumulated

15 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 15 ozone loss calculated with the tracer-tracer correlation method. This excellent agreement is in spite of the fact that the two measures cover different altitude ranges (suggesting that ozone loss above or below the region diagnosed with the tracer-tracer correlation method is negligible) and that the tracer-tracer correlation measurement samples only a few locations within the vortex (suggesting that the air within the core of the vortex is well mixed). The large discrepancy in 2002 is not surprising since in that year a mid-winter major warming [Hoppel et al., 2003; Sinnhuber et al., 2003] caused a significant influx of ozone rich air into the vortex. Unlike the tracer-tracer correlation method which is specific to chemical ozone loss, the total column ozone loss method considers the combination of chemical ozone loss and dynamical resupply. Therefore, in years (or periods of the year) when there is significant dynamical resupply of ozone, the total column ozone loss is expected to underestimate the true chemical loss of ozone. The underestimation of the ozone loss in 2002 and 2004 using the total column ozone loss method suggests that dynamical resupply of ozone to the Antarctic vortex may have become more prevalent in recent years. While no major stratospheric warming was observed in 2004, the dynamical state of the stratosphere was unusual in that year [Hoppel et al., 2005]. In Figure 6 the relationship between the two different vortex ozone depletion measures is displayed as a scatter diagram. This highlights that, except for 2002 and 2004, the two methods agree well (correlation coefficient = 0.64, 99.9 % significance) with the total column ozone loss method showing a slight underestimation of the ozone loss as expected since it considers both the effects of chemical loss and dynamical resupply.

16 X - 16 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION A day-by-day comparison of total column ozone loss and accumulated chemical ozone loss for 30 October to 13 December 1996 and 1 June to 20 October 2003 is displayed in Figure 7 and Figure 8, respectively. The accumulated chemical ozone loss for these periods is based on ILAS and ILAS-II observations. In this comparison, the three different backgrounds, resulting from integration over the three regions described in Section 2.1 are considered for the calculation of total column ozone loss. In 1996 ILAS data are only available for late in the season. Very high correlations between the accumulated chemical ozone loss and the total column ozone loss are observed while the vortex is still strong and well defined (Figure 7). Deviations between the accumulated chemical ozone loss and total column ozone loss start to occur from 6 December 1996 (day 340) onwards. Both, total column ozone loss within the inner vortex boundary and total column ozone loss averaged poleward of 62 S show a day-to-day variability of the same order as the accumulated chemical ozone loss. The large offset between the accumulated chemical ozone loss and total column ozone loss averaged poleward of 40 S indicates that the column ozone loss poleward of 40 S is not suitable as a measure of Antarctic ozone depletion. In 2003 ILAS-II data are available for the whole Antarctic vortex period. Very high correlations between accumulated chemical ozone loss and vortex average total column ozone loss are observed from the start of the ozone hole until the minimum is reached. The day-to-day variabilities of accumulated chemical ozone loss and total column ozone loss are of the same magnitude for the vortex average as well as for the average poleward of 62 S. Comparing the accumulated chemical ozone loss and the vortex average total column ozone loss, some deviation occurs after the minimum value is reached. Total column ozone loss reaches its minimum at about 20 DU higher than the accumulated

17 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 17 chemical ozone loss. The date when the minimum is reached is in both cases around 9 October The differences could be due to different altitude ranges considered for the two measures, filaments from lower latitudes intruding into the vortex and/or mixing of ozone rich air from the top of the vortex. Since the tracer-tracer correlation requires an impenetrable vortex edge, filaments from lower latitudes and mixing of ozone rich air from the top of the vortex are not accounted for. In Figure 2 it can be seen that there is a slight increase in total ozone in the vortex core after about 4 October 2003 (day 277). This increase is most probably due to mixing of polar air masses with ozone rich air from lower latitudes at high altitudes. 4. Discussion and Conclusions New definitions for the Antarctic ozone hole area (OHA) and Antarctic ozone mass deficit (OMD) have been presented and discussed above. The algorithm used to calculate these indices retains the simplicity of earlier methods (e.g. those that use 220 DU as both the unperturbed background and spatial integration limit) but has the advantage of better representing the full extent of Antarctic ozone depletion. This was demonstrated by comparing total column ozone loss, calculated using the unperturbed background and spatial integration limits used for the new OHA and OMD indices, with accumulated chemical ozone loss from a more complex and sophisticated tracer-tracer correlation technique. Except for dynamically perturbed years (e.g and 2004), the two methods showed excellent agreement. As noted above, because the total column ozone loss method (which uses the same assumptions as for the OHA and OMD calculations) does not distinguish between chemical and dynamical induced changes in ozone, the newly proposed Antarctic

18 X - 18 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION ozone depletion indices will only be representative of chemical depletion of ozone in years when dynamical contributions to ozone variability within the vortex are small. The new method for calculating OHA and OMD requires only total column ozone fields. These are more readily available than the ozone and passive tracer profiles required for the tracer-tracer correlation method, and therefore accurate measures of Antarctic ozone depletion can be obtained over longer time periods. Thus the recommendation is that the former method for calculating OMD should be replaced with the new method that integrates depleted ozone mass poleward of the inner vortex edge from an unperturbed background derived during a period before significant Antarctic ozone depletion began. We acknowledge that this method is somewhat more complicated than the former method in that it requires calculation of the dynamical vortex edge. We have shown, however, that if the additional complication of calculating the dynamical vortex edge is avoided and the OMD is integrated poleward of 62 S (the average latitude of the vortex edge), the ozone mass deficit is overestimated on average ( ) by only 28 %. Figure 9 is an updated version of Figure 1 in Huck et al. [2005] showing the inter-annual variability and long-term trend in ozone depletion with OMD 1. While the long-term trend follows the chlorine loading of the stratosphere, strong inter-annual variability makes the recovery of the Antarctic ozone hole uncertain. The new OHA, summation of the area where ozone is below the unperturbed background, derived poleward of the inner vortex edge, is 10% below the old OHA summed within the 220 DU contour. While this new version of OHA (and the new version of OMD) shows closer correspondence with the timing of the return of sunlight to the vortex and the onset of ozone depletion, it has not been demonstrated conclusively that the new OHA

19 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 19 is sufficiently improved on the former measure (using the 220 DU contour) to provide a replacement. Further improvement of the derivation of the unperturbed background (e.g. through the use of a regression model to remove the effects of ozone depleting substances) would be needed to improve the calculation of the OHA. A further advantage of using the unperturbed background is that the effect of the annually repeating cycle in dynamics on ozone is better captured than when a constant background is used. This leads to better separation of climatological mean transport effects and chemistry effects on the ozone depletion metric. However, inter-annual variability due to the quasi-biennial oscillation, solar cycle or volcanic eruptions is not currently included in the unperturbed background ozone. Future work will focus on deriving an unperturbed background specific to the dynamics of that year. In the interim, to correct for these effects, an offset, derived in May when the vortex has formed but before ozone depletion has started, is subtracted from the Antarctic ozone loss measures. These new definitions of OHA and OMD should also aid chemistry-climate model intercomparisons of these quantities since at present inter-model mean offsets in ozone cause modelled OHA and OMD to be biased. If the new definitions for OHA and OMD are used, each model would derive its own internally consistent unperturbed background for these calculations. Acknowledgments. P. Huck is supported through funding by the DAAD (Doktorandenstipendium). NCEP Reanalysis data were provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA, from their web site at We gratefully acknowledge all members of the HALOE team at NASA Langley and all members of the science team of the Improved Limb Atmospheric

20 X - 20 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION Spectrometer (ILAS and ILAS-II) led by Dr. Y. Sasano and Dr. H. Nakajima for processing ILAS (V6.0) and ILAS-II (V1.4) data. ILAS and ILAS-II were developed by the Ministry of the Environment, Japan (MOE) and was aboard the ADEOS and ADEOS-II satellite launched by the Japan Aerospace Exploration Agency (JAXA). ILAS and ILAS-II data were processed at the ILAS and ILAS-II Data Handling Facility, National Institute for Environmental Studies (NIES). References Austin, J., and N. Butchart, Coupled chemistry-climate model simulations for the period : Ozone depletion and the start of the ozone recovery, Q. J. R. Meteorol. Soc., 129, , Bodeker, G. E., H. Struthers, and B. J. Connor, Dynamical containment of Antarctic ozone depletion, Geophysical Research Letters, 29 (7), Bodeker, G. E., H. Shiona, and H. Eskes, Indicators of Antarctic ozone depletion, Atmospheric Chemistry and Physics, 5, , Daniel, J., S. Solomon, and D. Albritton, On the evaluation of halocarbon radiative forcing and global warming potentials, J. Geophys. Res., 100 (D1), , doi: /94jd02,516, Farman, J. C., R. J. Murgatroyd, A. M. Silnickas, and B. A. Thrush, Ozone photochemistry in the Antarctic stratosphere in summer, Q. J. R. Meteorol. Soc., 111, , Hoppel, K., R. Bavilacqua, D. R. Allen, G. E. Nedoluha, and C. Randall, POAM III observations of the anomalous 2002 Antarctic ozone hole, Geophys. Res. Lett., 30 (7),

21 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X , doi: /2003gl016,899, Hoppel, K., G. Nedoluha, M. Fromm, D. Allen, R. Bevilacqua, J. Alfred, B. Johnson, and G. Konig-Langlo, Reduced ozone loss at the upper edge of the Antarctic ozone hole during , Geophysical Research Letters, 32 (20), l20816, Huck, P. E., A. J. McDonald, G. E. Bodeker, and H. Struthers, Interannual variability in Antarctic ozone depletion controlled by planetary waves and polar temperature, Geophysical Research Letters, 32 (13), doi: /2005gl022,943, Müller, R., U. Schmidt, A. Engel, D. S. McKenna, and M. H. Proffitt, The O 3 /N 2 O relation from balloon-borne observations as a measure of Arctic ozone loss in 1991/92, Quarterly Journal of the Royal Meteorological Society, 127 (574), , part B, Müller, R., S. Tilmes, P. Konopka, J. U. Grooß, and H. J. Jost, Impact of mixing and chemical change on ozone-tracer relations in the polar vortex, Atmospheric Chemistry and Physics, 5, , Nakajima, H., et al., Characteristics and performance of the Improved Limb Atmospheric Spectrometer-II (ILAS-II) onboard the ADEOS-II satellite, Journal of Atmospheric Research, submitted, Nash, E. R., P. A. Newman, J. E. Rosenfield, and M. R. Schoeberl, An objective determination of the polar vortex using Ertel s potential vorticity, Journal of Geophysical Research-Atmospheres, 101 (D5), , Newman, P. A., and E. R. Nash, The unusual southern hemisphere stratosphere winter of 2002, Journal of the Atmospheric Sciences, 62 (3), , 2005.

22 X - 22 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION Newman, P. A., E. R. Nash, S. R. Kawa, S. A. Montzka, and S. M. Schauffler, When will the Antarctic ozone hole recover?, Geophysical Research Letters, Proffitt, M. H., J. J. Margitan, K. K. Kelly, M. Loewenstein, J. R. Podolske, and K. R. Chan, Ozone loss in the Arctic polar vortex inferred from high-altitude aircraft measurements, Nature, 347 (6288), 31 36, Russell III, J. M., L. Gordley, J. Park, S. R. Drayson, A. F. Tuck, J. Harries, R. J. Cicerone, P. J. Crutzen, and J. E. Frederick, The Halogen Occultation Experiment, Journal of Geophysical Research, 98, 10,777 10,797, Sasano, Y., M. Suzuki, T. Yokota, and H. Kanzawa, Improved limb atmospheric spectrometer (ILAS) for stratospheric ozone layer measurements by solar occultation technique, Journal of Geophysical Research, 26, , doi: /1998gl900,276, Schoeberl, M. R., and D. L. Hartmann, The dynamics of the stratospheric polar vortex and is relation to springtime ozone depletions, Science, 251, 46 52, Sinnhuber, B.-M., M. Weber, A. Amankwah, and J. Burrows, Total ozone during the unusual Antarctic winter of 2002, Geophys. Res. Lett., 30 (11), 1580, doi: /2002gl016,798, Solomon, S., J. Smith, R. Sanders, L. Perliski, H. Miller, G. Mount, J. Keys, and A. Schmeltekopf, Visible and near-ultraviolet spectroscopy at McMurdo station, Antarctica 8. observations of nighttime NO 2 and NO 3 from April to October 1991, Journal of Geophysical Research, 98 (D1), , Solomon, S., R. W. Portmann, T. Sasaki, D. J. Hofmann, and D. W. J. Thompson, Four decades of ozonesonde measurements over Antarctica, Journal of Geophysical Research- Atmospheres, 110 (D21), d21311, 2005.

23 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 23 Stolarski, R., M. Schoeberl, P. Newman, R. McPeters, and A. Krueger, The 1989 Antarctic ozone hole as observed by TOMS, Geophysical Research Letters, 17 (9), , Suzuki, M. A., A. Matsuzaki, T. Ishigaki, N. Kimura, N. Araki, T. Yokota, and Y. Sasano, ILAS, the improved limb atmospheric spectrometer, on the advanced Earth observing satellite, IEICE TRANS Commun, E78-B,12, , Tilmes, S., R. Müller, A. Engel, M. Rex, and J. M. Russel III, Chemical ozone loss in the Arctic and Antarctic stratosphere between 1992 and 2005, Geophysical Research Letters, under review, 2006a. Tilmes, S., R. Müller, J.-U. Grooß, R. Spand, T. Sugita, H. Nakajima, and Y. Sasano, Chemical ozone loss and related processes in the Antarctic winter 2003 based on ILAS-II observations, Journal of Geophysical Research, 111 (D11S12), doi:/ /2005jd006,260, 2006b. Uchino, O., R. D. Bojkov, D. S. Balis, K. Akagi, M. Hayashi, and R. Kajihara, Essential charactersistics of the Antarctic-spring ozone decline: Update to 1998, Geophysical Research Letters, 26 (10), , WMO, Scientific Assessment of ozone depletion: 2002, World Meteorological Organisation Global Ozone Research and Monitoring Project Report No. 47, 2003.

24 X - 24 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION Figure Captions Figure 1: The three different unperturbed background signals (thin black line - vortex mean ozone; grey line - mean ozone between 62 S and 90 S; dashed line - mean ozone between 40 S and 90 S), together with the 2003 vortex mean ozone (thick black line). The 220 DU threshold, commonly used in calculations of OMD and OHA, is shown with a dotted line. The shaded area indicates ozone loss that would not be incorporated in any ozone depletion index based on a 220 DU threshold. Figure 2: Ozone distribution over the southern hemisphere and limiting contours in 2003 (background: ozone distribution; thin black line: 220 DU contour; thick black line: inner vortex edge; thin white line: 62 S; thick white line: 40 S). Figure 3: Comparison of old and new OMD measure and potential activation parameter in 2003 (shaded area: PAP; thick black line: old OMD; thin black line: OMD 1 ; grey line: OMD 2 ; dashed line: OMD 3 ). Figure 4: Comparison of old and new OHA measure in 2003 (thick black line: old OHA; thin black line: OHA 1 ; grey line: OHA 2 ; dashed line: OHA 3 ). Figure 5: HALOE chemical ozone loss (black stars) in comparison with vortex average total column ozone loss (grey line) for Uncertainties in calculating the accumulated chemical ozone loss (error bars on black stars) result from the estimation of the

25 HUCK ET AL.: IMPROVED MEASURE FOR ANTARCTIC OZONE DEPLETION X - 25 reference function as well as averaging over the profiles inside the vortex. Figure 6: Comparison of vortex average chemical ozone losses derived with two different methods for Different symbols stand for different years and the dashed line represents the bisecting line. Figure 7: Comparison chemical ozone loss derived with tracer-tracer-correlations and total column ozone loss derived with the new method are displayed for 30 October 1996 (day 304) to 13 December 1996 (day 347) (Thick black line: ILAS ozone loss; dotted lines: uncertainties on ILAS ozone loss; thin black line: total column ozone loss (inside vortex); grey line: total column ozone loss (poleward of 62 S); dashed line: total column ozone loss (poleward of 40 S)). Figure 8: Comparison chemical ozone loss derived with tracer-tracer-correlations and total column ozone loss for 2003 (Thick black line: ILAS-II ozone loss; dotted lines: uncertainties on ILAS-II ozone loss; thin black line: total column ozone loss (inside vortex); grey line: total column ozone loss (poleward of 62 S); dashed line: total column ozone loss (poleward of 40 S)). Figure 9: Annual mean OMD plotted against the left ordinate (black) and EESC plotted against the right ordinate (grey) showing the inter-annual variability and longterm trend of ozone depletion.