SUPPLEMENTARY INFORMATION

Transcription

1 doi: /nature Supplementary Discussion Temperature/Vortex Diagnostics The fields shown in Supplementary Figures 1 and 2 provide details of the patterns of temperature and vortex evolution underlying the key results shown in Figure 1 and discussed in the text. Supplementary Figure 1a shows that minimum temperatures at 460 K were lower in 2010/2011 than those in 1996/1997 throughout nearly the entire winter. Comparison with the Antarctic envelope shows that late March/April temperatures in 2010/2011, and to a lesser degree in 1996/1997, matched those seen in some of the warmer Antarctic winters. The patterns at other levels throughout the lower stratosphere are similar. Time series such as Supplementary Figure 1a are used to quantify the number of days with T<T act at levels throughout the lower stratosphere (Supplementary Figure 1b). At levels from 430 to 490 K, there were more days in 2010/2011 with T<T act than in any previous Arctic winter, and at 550 K more than any other year except 1996/1997. The coldest overall previous winters (that is, those with largest V psc ), especially 1999/2000 and 2004/2005, had many days with T<T act over most of the lower stratospheric levels, whereas more moderately cold years (e.g., 1996/1997, 2007/2008) had large values at fewer levels. Supplementary Figure 1c summarizes these statistics, showing the sum of the number of days with T<T act at seven levels (approximately equally spaced in altitude) from 390 to 550 K. Over these levels, there were nearly 100 more days with T<T act in 2010/2011 than in any previous Arctic winter. While the long persistence and unusually deep region of T<T act, and the corresponding large V psc (Figure 1c), are the primary factors differentiating the 2010/2011 meteorology from that in previous Arctic winters and leading to the large ozone loss, several other aspects of vortex/temperature evolution have secondary effects on interannual variability in the amount of chlorine activation, denitrification and ozone loss. The strength of the vortex is important in that it indicates the degree to which the cold vortex air is confined within the region where the chemical reactions that activate chlorine and destroy ozone take place. Potential vorticity (PV) gradients are a good measure of strength of the barrier to transport at the vortex edge 1. As measured by maximum PV gradients (Figure 1a shows them at 460 K, and very similar patterns are seen at all levels throughout the lower stratosphere), the vortex was extraordinarily strong from January through March 2011, in contrast to the previous years with large ozone loss, when the vortex strength was unremarkable. That relatively large ozone loss occurred in years with relatively weak polar vortices demonstrates that an exceptionally strong vortex is not required in order to have 1

2 large chemical loss. However, a strong vortex is not only likely to be more persistent, maintaining the isolation of polar-processed air conducive to ozone destruction, but also is less permeable, inhibiting in-mixing of extra-vortex air that can dilute the signature of chemical loss. Some other aspects of vortex and temperature evolution relevant to ozone loss, related to the positions of the vortex and cold region, are illustrated in Supplementary Figure 3. The degree to which the polar vortex and the cold region are concentric has implications for denitrification and chlorine activation, with a cold region that is more concentric with the vortex leading to greater denitrification since longer continuous exposure to cold conditions promotes formation and sedimentation of large PSC particles 2, 3. Supplementary Figure 3a shows as a rough measurement of the concentricity of the vortex and cold regions: the distance between the center of the vortex and the center of the cold region (where a short distance corresponds to a vortex and cold region that are more concentric). As expected given its more quiescent and pole-centered vortex, the Antarctic vortex and cold regions are consistently more concentric than those in the Arctic. The Arctic values show much interannual and day-to-day variability. The winters with vortices/cold regions that are very non-concentric (extremely large values) before early March are the most disturbed, warmest years, typically ones with strong stratospheric sudden warmings; in March and April increasing non-concentricity is associated with the vortex breakdown. All of the cold years highlighted in Supplementary Figure 3a show more concentricity (smaller values) on average than warmer winters, but significant differences exist between even those winters. As shown in previous studies 3, 1996 (blue line) had very non-concentric periods compared to 2000 (green line). The degree of concentricity in 2011, while not unusual, was consistently high throughout the winter. Characteristics of wave propagation related to vertical vortex structure affect the longevity and permeability of the stratospheric vortex in several ways 4, 5. An upright polar vortex is typically more stable (thus likely to be longer-lived) than one that tilts with height. Vortices that decrease in area with height are less stable than those that are constant or increase in area with height; increasing area with height is associated with greater stability than constancy only when the windspeeds do not also increase with height. In addition, a strong vortex with little vertical wind shear (that is, with neither position nor size changing rapidly with altitude) is associated with less mixing into the vortex, since vertical wind shears are implicated in the differential advection that results in mixing that brings extravortex air into the vortex in lamination events 6, 7. Supplementary Figure 2f through j provides a view of how the vortex size varies with altitude in the five cold winters we focus on here. In all the cold winters, the vortex area increases with height, indicating relative stability. Since, however, the winds also increase with height, a larger increase with height may 2

3 not necessarily indicate greater stability. In 2010/2011, the vertical change in vortex size was less, especially after early February, than in any of the previous cold winters studied. A rough measure of the relative verticality of the vortex is given in Supplementary Figure 3b, showing the distance between the centers of the cold regions at 460 and 550 K. As with other such diagnostics related to vortex position, there is large variability within each winter. Overall, however, 2010/2011 and 1996/1997 show less tilt with height of the vortex than the other cold Arctic winters (all of which show relatively low values compared to warmer winters). Thus, while the vortex position and structure in 2010/2011 do not alone stand out as extreme, these diagnostics are consistent with the unusually strong, cold, long-lived vortex in that winter, and, in combination with the prolonged cold period extending into spring, favor less mixing with extravortex air, greater denitrification, more complete and persistent chlorine activation, and hence larger chemical ozone loss. Effects of Denitrification on Ozone Loss in 1996/1997 and 2010/2011 Supplementary Figure 4 shows the results of photochemical box model calculations aimed at assessing the effect of different degrees of denitrification and initial ozone values on chemical loss in spring. As described in the Methods section, initial ozone and HNO 3 values were chosen to approximately match the vortex-averaged values observed on 1 March in 1997 and All other aspects of the initialization (e.g., temperature, latitude, active chlorine amounts) were the same. For both initial ozone values, lower HNO 3 leads to much more rapid chemical loss, with about 0.6 ppmv more loss (the difference between dashed and solid lines of the same color) in March and early April in each of the denitrified cases than in the corresponding cases with high HNO 3. By early April, modeled ozone was 1.3 ppmv lower in the case with low initial ozone and low initial HNO 3 (closely matching conditions on 1 March 2011) than in the case with high initial ozone and high initial HNO 3 (closely matching conditions on 1 March 1997); within the uncertainties, this agrees well with the difference of 1.5 ppmv derived from ozonesonde observations on 460 K spring equivalent potential temperature near the end of March (the last date for which sufficient observations were available for the vortex average calculation, the last point on the red curve in Figure 4c and Supplementary Figure 4). That the difference in ozone loss between high and low HNO 3 cases is nearly the same whether initial ozone is high or low indicates that the initial degree of denitrification (thus having occurred prior to 1 March) is the primary factor controlling determining those loss rates. Note that in the high HNO 3 cases (solid black and grey lines), the ozone loss slows and ceases much sooner than that in the low HNO 3 cases (dashed black and grey lines). Less HNO 3 in 2011 led to less NO 2 (from the photolysis of HNO 3 and its reaction with OH) and 3

4 consequently to slower chlorine deactivation via ClO + NO 2 + M ClONO 2 + M. The persistence of activated chlorine kept ozone destruction rates high through March, resulting in more ozone loss during March and early April The net difference in ozone loss between 1997 and 2011 thus arose via nearly equal contributions from early winter loss of ppmv in 2011 that did not occur in 1997, and about ppmv larger loss in March and April 2011 as a consequence of denitrification. For extreme loss such as that seen in 2011 to occur requires a prolonged continuous cold period in early winter, both to activate chlorine so that loss begins early and to promote formation of PSC particles large enough to fall out and permanently remove HNO 3 from the range of altitudes over which ozone loss occurs before temperatures begin to rise in spring. Because the strong, cold vortex in 2011 persisted into April, the delayed chlorine deactivation resulting from denitrification allowed rapid ozone loss much later into spring than has heretofore been observed. The results of these calculations emphasize that very severe chemical ozone loss in the Arctic requires a prolonged period with a large fraction of the vortex having T<T act ; that cold period must both begin early in winter and persist late into spring. When these conditions are met it follows that V psc (as a fraction of the vortex) for the winter will be large, supporting the previously reported close correlation between V psc and the degree of chemical ozone loss 8. Column Ozone and Dynamical Effects It is difficult, if not impossible, to assess the amount of Arctic chemical loss from total column ozone alone, because of the dominating influence of dynamical effects on total ozone 9, 10. Several dynamical processes influence total ozone variations on day-to-day or seasonal timescales. The largest dynamical contributions to interannual ozone variability over seasonal timescales are the resupply of ozone transported downward in the polar vortex by the residual circulation, and mixing across the vortex edge in the lower stratosphere due to wave motions. Previous studies have shown that interannual differences in replenishment from above typically account for half or more of the variability in winter/spring Arctic ozone 10, 11. In particular, relatively weak resupply and chemical loss contributed approximately equally to low springtime total ozone in Dynamical resupply varies interannually from 60 to 150 DU 10. Since the diabatic descent that replenishes vortex ozone in the lower stratosphere is stronger when temperatures are higher (because, to first order, the diabatic cooling accompanying that descent drives temperatures towards radiative equilibrium) 12, replenishment is generally smaller in cold winters, but because of the lingering cold, was particularly weak in spring 1997 and Horizontal mixing also has a strong influence on total ozone in spring, with less such mixing accounting for much of the observed deficit in March total ozone following cold winters 13. Though 4

5 both 1997 and 2011 were anomalously cold in spring, the polar vortex was stronger, suggesting greater inhibition of mixing, in late winter 2011 than in 1997 (Figure 1a). In addition to the processes discussed above, day-to-day dynamical variations related to weather systems in the upper troposphere strongly affect total ozone 9, 14, 15. Low total ozone is strongly correlated with a cold lower stratosphere, an elevated tropopause, and transport of low ozone from low latitudes near the level of the tropopause 9, 15, 16. While the relative contributions of each of these processes varies, in the most extreme cases of ozone mini-holes, they can combine to produce dynamically-induced ozone deficits of nearly 200 DU for one to several days. Under the extreme conditions related to ozone mini-holes (which were common in 1997, including one in mid-march 17 ), a 5 K difference in lower stratospheric temperature can be associated with over 50 DU lower total ozone 15. Temporary decreases in minimum column ozone (Supplementary Figure 5) in late February 2011 and after mid-march 1997 are associated with mini-holes. Ozone mixing ratios are approximately constant at the tropopause and drop precipitously from above to below it. An elevated tropopause is thus associated with lower total ozone since the higher values just above the tropopause that most affect the column are lifted to lower pressures, where their contribution to the column is smaller. Supplementary Figure 6 indicates substantial differences in late winter/spring tropopause altitude and temperature between 1997, 2000, 2005, and The tropopause was highest and coldest in 1997 from early March through mid-april, with a large temporary increase (decrease) in altitude (temperature) after mid-march associated with the mini-hole at that time. Lower/warmer tropopauses in 2000 and 2005 are consistent with earlier springtime warming in those years. Previous studies have associated a 1-km increase in tropopause altitude (approximately the difference between the values for 1997 and 2005 shown in Supplementary Figure 6) with 15 DU lower total ozone 14. Because of these dynamical effects, which, though individually modest, can in aggregate result in large total ozone changes, the time series of total ozone (fractional area of the vortex with total ozone below 275 DU in Figure 5, vortex ozone deficit and minimum vortex total ozone in Supplementary Figure 5) shown here are difficult to interpret in terms of relative amounts of chemical loss. The minimum, as a local diagnostic, is especially strongly affected by variations related to tropospheric weather and tropopause changes, as seen in the strong signatures of mini-hole events. The processes described above related to lower stratospheric temperatures and tropopause variations produce low-ozone regions that are strongly spatially correlated with temperature, but are only correlated with the vortex when the vortex and cold regions are concentric. Thus, low 5

6 total ozone that is strongly correlated with the vortex in circumstances in which temperature is not (such as in Figure 5b, d) is a clear indication that chemical loss is a predominant factor. In the Antarctic, the amount and area of chemical loss are so much larger, and the dynamical effects so much smaller, that the chemical loss signature always strongly dominates over the dynamical signature in total ozone. 6

7 2 Supplementary Figures 1 7 Minimum Temperature (K) # Days T < T act Total Days T < T act (a) Minimum Temperature Dec 1 Jan 1 Feb 1 Mar 1 Apr (b) Days Below Activation T (c) Sum of Days Below Activation T K 490K 460K 430K 390K Supplementary Figure 1. Polar winter temperature statistics. (a) Minimum 460 K temperatures poleward of 40 latitude. Light (dark) shading shows the range of values for 1979 through 2010 for the Arctic (Antarctic). The thin horizontal line shows the approximate chlorine activation threshold temperature(t act, the temperature below which PSCs and other cold aerosol particles form, reactions on the surfaces of which activate chlorine). (b) Number of days with temperature less than T act for the 32 years of the MERRA reanalysis. Individual bars for a year show values for (left to right) 390, 430, 460, 490 and 550 K potential temperature levels. (c) Total numbers of days with T<T act summed over seven levels spanning the lower stratosphere, the five levels shown in (b) as well as 410 and 520 K. Red, orange, green, purple and blue lines/bars/arrows highlight values for the 2010/2011, 2004/2005, 1999/2000, 1996/1997, and 1995/1996 Arctic winters, respectively. 7

8 doi: /nature10556 (a) 1996 (b) 1997 (g) 1997 (c) 2000 (h) 2000 (d) 2005 (i) 2005 (e) 2011 (j) Dec Jan 1 Feb 1 Mar 1 Apr Area T Tact (fraction of hemisphere) (o) 2011 (n) 2005 (m) 2000 (l) 1997 (k) 1996 Potential Temperature (K) (f) Dec Jan 1 Feb 1 Mar 1 Apr Vortex Area (fraction of hemisphere) 1 Dec Jan 1 Feb 1 Mar 1 Apr Area T Tact (fraction of vortex) Supplementary Figure 2. Area of vortex and cold region. (a) through (e) Area north of 40, expressed as fraction of a hemisphere, with T<Tact in 1995/1996, 1996/1997, 1999/2000, 2004/2005, and 2010/2011, respectively. (f) through (j) As in (a) through (e) but showing the area of the polar vortex. The polar vortex is considered to be undefined if the equivalent latitude (defined in Methods section) of its edge is greater than (k) through (o) As in (a) through (e) but expressed as fraction of the vortex area shown in (f) through (j). T act and vortex definitions are given in the Methods section. The area fields shown in panels (a) through (e) are integrated over the vertical range shown by the dashed horizontal lines, averaged over 16 December through 15 April, and divided by the volume of the vortex integrated from the fields shown in panels (f) through (j) to get the V psc shown in Figure 1c. The solid horizontal line indicates the 460 K level shown in Figure 1a and Supplementary Figure 1a. 8

9 8 (a) Cold Center to Vortex Center Distance Distance (1000 km) Distance (1000 km) (b) 460K to 550K Cold Center Distance Jan Feb Mar Apr Supplementary Figure 3. Vortex and cold region positions. (a) Distance between the center of the vortex (defined as the maximum in equivalent latitude) and center of the cold region (location of minimum temperature). (b) Distance between the center of the cold region at 460K and at 550K, indicating the vertical tilt of the vortex. Colored lines are as in Supplementary Figure 1. Values for all other Arctic (all Antarctic) winters are shown as light grey (dark grey) lines; individual years are shown rather than just the envelope to highlight extreme and typical values. 9

10 3.0 Model Sensitivity to Denitrification 2.5 Ozone (ppmv) ppbv HNO 3 6 ppbv HNO 3 1 Mar 11 Mar 21Mar 1 Apr 11 Apr Supplementary Figure 4. Sensitivity of chemical ozone loss to denitrification. See Methods for detailed description of models and runs. Grey (black) lines show runs for a spring equivalent potential temperature of 460 K initialized on 1 March with 3 (2.2) ppmv of ozone, the approximate value in 1997 (2011). Solid (dashed) lines in each case show initialization with 10 (6) ppbv of HNO 3, as in 1997 (2011). Purple (red) lines show the ozone values at 460 K spring equivalent potential tempeature of 460 K in 1997 (2011) from Figure 4c, with error bars indicating the one sigma uncertainties based on the scatter of individual ozonesonde measurements. 10

11 Vortex O 3 Deficit / DU 1 Aug 1 Sep 1 Oct 50 (a) Vortex O 3 Deficit Minimum Column Ozone (DU) 350 (b) Minimum Vortex Total O Feb 1 Mar 1 Apr Supplementary Figure 5. Deficit and Minimum in Column Ozone. (a) Time series of the vortexaveraged deficit in column ozone relative to a reference state with little chemical loss. (b) Minimum in column ozone in the vortex region. Colors and shading are as in Supplementary Figure 1a. 11

12 Tropopause Alt (km) (a) Tropopause Altitude Under Vortex (a) Tropopause Temperature Under Vortex Tropopause T (K) Feb 1 Mar 1 Apr Supplementary Figure 6. Tropopause Height and Temperature in Cold Arctic Winters. Time series of tropopause (a) altitude and (b) temperature in the region underneath the stratospheric vortex (defined as in the Methods section by spv at 460K) during February through late April 1997, 2000, 2005 and 2011 (purple, green, orange and red, respectively). The tropopause is defined here by the PV contour, a commonly used definition

13 550 etheta (K) Ozone (ppmv) Supplementary Figure 7. Ozonesonde values in early/late winter. All ozonesonde profiles in the vortex used in the spring equivalent potential temperature calculations for 1 through 15 January (blue symbols) and 26 March through 5 April (red symbols). Dots show individual measurements, black profiles the mean in each period, and horizontal bars the 1-σ scatter at each level. Minimum ozone values were observed during the latter period. Vertical coordinate is spring equivalent potential temperature, so the difference between early and late profiles is the amount of chemical loss, as shown in Figure 4b. 13

14 3 Additional References 1. Manney, G. L., Zurek, R. W., O Neill, A. & Swinbank, R. On the motion of air through the stratospheric polar vortex. J. Atmos. Sci. 51, (1994). 2. Mann, G. W., Davies, S., Carslaw, K. S., Chipperfield, M. P. & Kettleborough, J. Polar vortex concentricity as a controlling factor in Arctic denitrification. J. Geophys. Res. 107, 4663 doi: /2002jd (2002). 3. Manney, G. L. et al. Lower stratospheric temperature differences between meteorological analyses in two cold Arctic winters and their impact on polar processing studies. J. Geophys. Res. 108, 8328, doi: /2001jd (2003). 4. Waugh, D. W. & Dritschel, D. G. The dependence of Rossby wave breaking on the vertical structure of the polar vortex. J. Atmos. Sci. 56, (1999). 5. Polvani, L. M. & Saravanan, R. The three-dimensional structure of breaking Rossby waves in the polar wintertime stratosphere. J. Atmos. Sci. 57, (2000). 6. Pierce, R. B., Fairlie, T. D., Grose, W. L., Swinbank, R. & O Neill, A. Mixing processes within the polar night jet. J. Atmos. Sci. 51, (1994). 7. Appenzeller, C. & Holton, J. R. Tracer lamination in the stratosphere: A global climatology. J. Geophys. Res. 102, 13,555 13,569 (1997). 8. Tilmes, S., Müller, R., Engel, A., Rex, M. & III, J. M. R. Chemical ozone loss in the Arctic and Antarctic stratosphere between 1992 and Geophys. Res. Lett. 33, L20812, doi: /2006gl026925, 2006 (2006). 9. Petzoldt, K. The role of dynamics in total ozone deviations from their long-term mean over the Northern Hemisphere. Ann. Geophys. 17, (1999). 10. Tegtmeier, S., Rex, M., Wohltmann, I. & Krüger, K. Relative importance of dynamical and chemical contributions to Arctic wintertime ozone. Geophys. Res. Lett. 35 (2008). 11. Chipperfield, M. P. & Jones, R. L. Relative influences of atmospheric chemistry and transport on Arctic ozone trends. Nature, (1999). 12. Shine, K. P. The middle atmosphere in the absence of dynamic heat fluxes. Q. J. R. Meteorol. Soc. 113, (1987). 14

15 13. Salby, M. L. & Callaghan, P. F. On the wintertime increase of Arctic ozone: Relationship to changes in the polar vortex. J. Geophys. Res. 112, D06116, doi: /2006jd (2007). 14. Steinbrecht, W., Claude, H., Köhler, U. & Hoinka, K. P. Correlations between tropopause height and total ozone: Implications for long-term changes. J. Geophys. Res. 103, 19,183 19,192 (1998). 15. Hood, L. L., Soukharev, B. E., Fromm, M. & McCormack, J. P. Origin of extreme ozone minima at middle to high northern latitudes. J. Geophys. Res. 106, 20,925 20,940 (2001). 16. Allen, D. R. & Nakamura, N. Dynamical reconstruction of the record low column ozone over Europe on 30 November Geophys. Res. Lett. 29, 1362, doi: /2002gl (2002). 17. Orsolini, Y. J., Stephenson, R. B. & Doblas-Reyes, F. J. Storm track signature in total ozone during northern hemisphere winter. Geophys. Res. Lett. 25, (1998). 18. Manney, G. L., Zurek, R. W., Gelman, M. E., Miller, A. J. & Nagatani, R. The anomalous Arctic lower stratospheric polar vortex of Geophys. Res. Lett. 21, (1994). 19. Homeyer, C., Bowman, K. P. & Pan, L. L. Extratropical tropopause transition layer characteristics from high-resolution sounding data. J. Geophys. Res. 115, D13108, doi: /2009jd (2010). 15