Ozone signatures of climate patterns over the Euro-Atlantic sector in the spring

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1 Q. J. R. Meteorol. Soc. (2003), 129, pp doi: /qj Ozone signatures of climate patterns over the Euro-Atlantic sector in the spring By Y. J. ORSOLINI 1 and F. J. DOBLAS-REYES 2 1 Norwegian Institute for Air Research, Kjeller, Norway 2 European Centre for Medium-Range Weather Forecasts, Reading, UK (Received 1 August 2002; revised 12 June 2003) SUMMARY The in uence of low-frequency tropospheric dynamics upon column ozone interannual variability is estimated in the spring season over the Euro-Atlantic sector. This dynamical variability of tropospheric origin is examined in terms of leading climate patterns, derived from an empirical orthogonal function analysis of the National Center for Environmental Prediction 500 mb geopotential height analyses. In order to ngerprint the spatial and temporal ozone signatures of these patterns, the Total Ozone Mapping Spectrometer satellite observations of column ozone during the last two decades are used. The geographical ozone signatures of these climate patterns are extracted by linear regression and their time evolution reconstructed. Geopotential anomalies associated with four leading patterns of variability (the North Atlantic Oscillation (NAO), the Scandinavian pattern, the east Atlantic pattern and the European blocking pattern) induce column ozone anomalies in the range of 5 15 Dobson Units per standard deviation of the pattern index. The impact of these combined Euro-Atlantic climate patterns upon the regional ozone trends and interannual variability is also estimated. In order to reconstruct the regional ozone trend and variability, it proved important to incorporate several patterns, in addition to the dominant NAO. A hitherto little-known lifting of the Arctic tropopause by the polar night jet is also shown to imprint upon the ozone signatures of some patterns. KEYWORDS: Euro-Atlantic variability Ozone trends Stratosphere 1. INTRODUCTION Column ozone observations from both ground-based and satellite instruments reveal a springtime decrease over the northern hemisphere high and middle latitudes in recent decades (see the review by Staehelin et al. 2001). However, there is a considerable interannual and decade-to-decade variability superimposed on the weak trend, which amounts to around Dobson Units (DU) per decade in midlatitudes. Recent quantitative estimations put the dynamical contribution to the midlatitude trend, i.e. the contribution induced by changes in the atmospheric circulation, at about 30% (Knudsen and Andersen 2001). The midlatitude ozone trend is also known to exhibit longitudinal asymmetries that might arise from these dynamical or chemical in uences (Entzian and Peters 1999; Hood et al. 1999; Knudsen and Andersen 2001). It has long been recognized that upper-troposphere dynamics contribute to ozone variability, from the time-scale of synoptic weather systems (e.g. Orsolini et al. 1998, 2001) to monthly or interannual time-scales. In ozone trend studies based on multidecadal time series of ground-based observations, the dynamical contribution is accounted for, including tropospheric geopotential height, tropopause height, or climate pattern indices as explanatory variables. For example, Appenzeller et al. (2000) analysed 50 years of ground-based column ozone observations at Arosa, Switzerland. They concluded that 50% of the ozone trend at Arosa in winter is caused by a modulation of column ozone induced by the North Atlantic Oscillation (NAO). Others (Bronnimann et al. 2000; Steinbrecht et al. 2001) chose a more general approach in their analysis, and included in their ozone regression the indices pertaining to a series of climate patterns, such as the NAO, the Scandinavian pattern or the east Atlantic pattern. While the ozone signatures of the NAO and the related Arctic Oscillation (AO) have been deduced Corresponding author: Norwegian Institute for Air Research, PO Box 100, Instituttveien 18, N-2007 Kjeller, Norway. orsolini@nitu.no c Royal Meteorological Society,

2 3252 Y. J. ORSOLINI and F. J. DOBLAS-REYES from Total Ozone Mapping Spectrometer (TOMS) satellite observations (Thompson and Wallace 2000; Braesicke et al. 2003), it is yet unclear which patterns are relevant for an ozone trend analysis at a given location, because the geographical signatures of main patterns of variability upon ozone have not been identi ed. Mindful that individual climate patterns, such as the NAO, represent only a fraction of the total tropospheric geopotential variance, our goal in this paper is to analyse satellite ozone data for correlation with a series of major climate patterns over the Euro-Atlantic sector, which will be systematically derived. Our purpose is to ngerprint the temporal and spatial signatures of climate patterns, and more precisely (i) to identify the signatures upon column ozone of the main tropospheric climate patterns, especially the geographical character of these signatures, and (ii) to understand the pattern contributions to ozone variability and trend in association with meteorological phenomena, i.e. going beyond the use of local tropopause height or other dynamical proxies as predictors. We focus on the spring season, when chemical ozone loss can be high, and it is then of special interest to link the contribution of dynamics to the ozone trend. We next calculate the leading climate patterns over the Euro-Atlantic sector (section 2), and their ozone signatures (section 3). The role of the polar night jet in modulating ozone anomalies is discussed in section 4. We present the impact of the climate pattern signatures on ozone interannual variability and trends in section 5, and our main conclusions in section LEADING CLIMATE PATTERNS OVER THE EURO-ATLANTIC SECTOR Following a common approach in climate research (Barnston and Livesey 1987), climate patterns over the Euro-Atlantic sector were determined through an Empirical Orthogonal Function (EOF) analysis. Our methodology follows closely Pavan et al. (2000a,b), who calculated the leading wintertime EOFs over the Euro-Atlantic sector (20 N 90 N and 90 W 60 E). Global EOFs and their correlations with column ozone were also investigated by Ambrizzi et al. (1998). We analysed the 500 mb geopotential height in the spring over the years , as deduced from the National Center for Environmental Prediction (NCEP) re-analyses (Kalnay et al. 1996). While we recognize that the 100 or 200 mb geopotential height is better correlated with column ozone (e.g. Knudsen and Andersen 2001), the 500 mb mid-troposphere height was chosen to select patterns that in uence the tropopause level and the ozone layer, and yet are clearly identi able as of tropospheric origin. In this study, spring means the two months of March and April. Constraining the domain of the EOFs to the Euro-Atlantic sector, rather than calculating global EOFs, improves the robustness of the patterns (Ambaum et al. 2001). The EOFs were calculated using monthly-mean gridded data (2 5 latitude by 2 5 longitude). We obtained four leading EOFs which explain individually more than 5%, and collectively 65% of the sectorial variance (Table 1): the NAO, the Scandinavian pattern (SCAND), the east Atlantic pattern (E-ATL), and the European blocking pattern (EU-BLOCK). The hemispheric covariances of the 500 mb geopotential with the principal component (PC) time series are shown in Fig. 1, and their normalized springaveraged PCs are shown in Fig. 2 for the years and which are of interest for our ozone study. The patterns are very similar to the wintertime patterns identi ed by Pavan et al. (2000a), although the spring patterns appear in a slightly different order; the SCAND pattern ranks second in springtime while fourth in winter.

3 OZONE SIGNATURES OVER THE EURO-ATLANTIC SECTOR IN SPRING 3253 TABLE 1. LEADING EMPIRICAL ORTHOGONAL FUNCTIONS (EOFS) OF 500 MB GEOPOTENTIAL HEIGHT OVER THE EURO-ATLANTIC SECTOR IN SPRIN G, SHOWING VARIANCE EXPLAINED AND CORRELATION COEFFICIENTS WITH THE POLAR NIGHT JET INDEX EOF Pattern Variance explained (%) Correlation 1 NAO SCAND E-ATL EU-BLOCK Figure 1. Leading four Empirical Orthogonal Function patterns over the Euro-Atlantic of the 500 mb geopotential in spring (March April), deduced from NCEP re-analyses (units of gpm): (a) North Atlantic Oscillation, (b) Scandinavian, (c) east Atlantic and (d) European blocking patterns. Negative phases are shown.

4 3254 Y. J. ORSOLINI and F. J. DOBLAS-REYES d d d d Figure 2. Timeseries of spring-averaged principal components (PCs) from rst four EOFs of 500 mb geopotential (thick line-full squares): (a) PC1 (North Atlantic Oscillation pattern) with NAO index (nao), (b) PC2 (Scandinavian pattern), (c) PC3 (east Atlantic pattern) and (d) PC4 (European blocking pattern) with European blocking index (eu block). All graphs also show polar night jet index (dashed). See text for details. (a) The NAO pattern The rst pattern (Fig. 1(a)),with its centres of action displaying a characteristic meridional dipole structure over the mid and north Atlantic, is clearly identi ed as the NAO (Pavan et al. 2000a). Its PC (Fig. 2(a)) shows a strong similarity with the NAO index deduced from surface pressure difference between Iceland and Gibraltar, a standard NAO index provided by the Climate Unit at the University of East Anglia. This EOF-1 has far-reaching linkages into east Asia and the north Paci c, indicating that the NAO phenomenon is of planetary scale (Perlwitz and Graf 1995; Limpasuvan and Hartmann 1999). (b) The SCAND pattern The second leading EOF for spring (EOF-2, Fig. 1(b)) has a main centre of action over northern Scandinavia and two adjacent opposite centres over Siberia and the eastern Atlantic. It resembles the wintertime EOF-4 (Scandinavian) of Pavan et al. (2000a), and the Eurasian type-1 pattern of Barnston and Livesey (1987). In its negative phase, EOF-2 corresponds to blockings over Scandinavia, a synoptic situation which indeed is more frequent in spring than in winter.

5 OZONE SIGNATURES OVER THE EURO-ATLANTIC SECTOR IN SPRING 3255 (c) The E-ATL pattern Our EOF-3 (Fig. 1(c)) resembles the east Atlantic pattern (E-ATL) of Pavan et al. (2000a). Its main centre of action lies over the eastern Atlantic, part of a wave train of four centres stretching eastwards from the mid-atlantic across to eastern Siberia. Note the extreme E-ATL pattern in the spring of 1997 (Fig. 2(c)). (d) The EU-BLOCK pattern Our EOF-4 closely resembles the wintertime EOF-3 of Pavan et al. (2000a) and the European blocking pattern of Doblas-Reyes et al. (2002). It is the most spatially con- ned of the four leading patterns. Its positive phase corresponds to localized blockings over the eastern Atlantic and British Isles, a frequent situation in the spring (Doblas- Reyes et al. 2002). The opposite phase indicates strengthened ridging over central and southern Europe. Such European blockings are associated with closed persistent anticyclonic circulations. A blocking index based on the procedure of Tibaldi and Molteni (1990) and NCEP data for March April, is shown in Fig. 2(d). The index measures occurrences over the European sector (10 W 50 E) of blocking days, characterized by westerlies north of 60 N and by easterlies between 40 N and 60 N. The good correspondence between the PC-4 and that index further corroborates that the EOF-4 is associated with European blocking phenomena. (e) Discussion While the NAO is characterized by a dipole oriented meridionally, the SCAND and E-ATL patterns display an elongated series of centres, tilted in longitude. It is also worth noting that a pattern such as the EU-BLOCK can contribute strongly to local variability, while it ranks as a high order pattern when averaged over the entire Euro- Atlantic sector, The NAO had a marked increase in the early and mid-nineties, before changing sign at the end of the decade (Fig. 2(a)). Other patterns show hints of decadal changes: the increasing SCAND during the nineties, and the decreasing EU-BLOCK from the eighties to mid-nineties. 3. OZONE SIGNATURES OF LEADING CLIMATE PATTERNS In order to derive the ozone signatures of these leading climate patterns, TOMS satellite observations were analysed covering the years 1979 to Hemispheric regression maps of monthly mean ozone anomalies in spring against the PCs of the leading EOFs were calculated. The TOMS data consist of measurements made aboard the Nimbus-7 (1979 to 1993), Meteor (1994) and Earth Probe (1997 to 2000) satellites. No TOMS observations were made in spring 1995 and Ozone data are not available in the polar night, hence we discarded data north of 70 N, which is near the terminator at the beginning of March. The ozone anomalies were obtained by subtracting the 20-year average for each calendar month. Regression maps (Fig. 3) were derived using normalized PCs so that the maps indicate ozone anomalies (in DUs) associated with one standard deviation of the corresponding climate pattern index. (a) The NAO pattern The NAO is associated in its positive phase with an enhancement of ozone by about 10 DU over Labrador southern Greenland and a lowering of ozone over Europe and over a large portion of the northern hemisphere, especially northern and eastern Asia (Fig. 3(a)). The ozone regression map shows good agreement with the results of

6 3256 Y. J. ORSOLINI and F. J. DOBLAS-REYES Figure 3. Regression maps of ozone (TOMS data ) against the 4 leading EOF patterns of 500 mb geopotential. Plotted are ozone anomalies (in DU) associated with one standard deviation of the corresponding pattern. Contour steps are 5 DU, with yellow and red corresponding to strongest ozone increase and blue-purple to largest ozone decrease. (a) EOF-1 (North Atlantic oscillation), (b) EOF-2 (Scandinavian), (c) EOF-3 (east Atlantic) and (d) EOF-4 (European blocking) patterns. Thompson and Wallace (2000), who regressed the TOMS March ozone against the AO for the years 1979 to In its negative phase, the NAO is characterized by more prominent blockings over Labrador southern Greenland (Thompson and Wallace 2002), which is consistent with the lowering of ozone in that region. (b) The SCAND pattern The ozone regression map for the SCAND pattern (Fig. 3(b)) shows negative anomalies (in excess of 10 DU) over western Europe and the British Isles, and over northern Asia. We do not nd clear geographical links to all the geopotential anomalies characterising this pattern. For example, Fig. 3(b) shows no signature of lowered ozone

7 OZONE SIGNATURES OVER THE EURO-ATLANTIC SECTOR IN SPRING 3257 above Scandinavia in the negative phase of this pattern. We will come back to this point in the next section, devoted to the role of the stratospheric polar vortex. (c) The E-ATL pattern The succession of centres of action alternating in sign across the Atlantic, Europe and Asia, that are characteristic of the E-ATL pattern, are clearly mirrored in ozone anomalies of the order of 5 10 DU (Fig. 3(c)). (d) The EU-BLOCK pattern The centres of action of EOF-4 over mid-atlantic, west of the British Isles and over southern and central Europe are mirrored in ozone, with amplitudes near 5 10 DU. The more dominant negative phase of the EU-BLOCK from the late seventies to the mid-nineties accentuated the ozone decrease over central Europe, reducing spring ozone by about 5 15 DU during the strong events in 1989 and (e) Discussion In summary, the leading patterns over the Euro-Atlantic sector have noticeable signatures on ozone, in the range of 5 15 DU per standard deviation. In the spring of 1997, the E-ATL pattern was anomalously large and negative, contributing to a strong reduction of ozone over Europe. Ridging over Europe (Fig. 3(c)) in the negative phase of this pattern is consistent with the repeated anticyclonic intrusions of ozonepoor subtropical air observed by Orsolini et al. (1998) in spring One might note that our method does not discriminate between vertical and horizontal ozone advection generating the anomalies. It is also worth nothing the far-reaching anomalies linked to the NAO over the northern Paci c. An ozone anomaly with a main wide and elongated centre from northern Asia to the Arctic (90 E 150 E) and a secondary centre over northern Europe is common to Figs. 3(a), (b) and (c). This anomaly is largely outside the Euro-Atlantic sector and it appears with a different sign in the E-ATL signature. This anomaly is not clearly connected to the localized geopotential anomalies of the corresponding patterns. We now proceed to show that this anomaly can be understood as a consequence of the lifting of the high-latitude tropopause by an enhanced polar night jet (PNJ), a mechanism studied by Ambaum and Hoskins (2002). 4. THE STRATOSPHERIC POLAR NIGHT JET AND THE ARCTIC TROPOPAUSE An enhanced PNJ acts as potential vorticity positive anomaly that deforms upwards the isentropic surfaces underneath, hence lifting the tropopause and lowering column ozone. Ambaum and Hoskins (2002) demonstrated this effect on the tropopause height using analysed elds, and this pattern is also apparent in the Thompson and Wallace (2000) March ozone regressions. This tropopause height modulation is not a zonally symmetric pattern, but is concentrated on northern Asia, northern Europe and the Arctic. We constructed a PNJ anomaly index analogous to the one of Kuroda and Kodera (1999), by taking zonal-mean zonal wind at 60 N and 50 mb. The monthly mean PNJ index has been derived from NCEP data for the years of interest, 1979 to 2000, and is shown in Fig. 2(a). Correlations of the PNJ index with our leading Euro-Atlantic EOFs are also shown in Table 1. A strong PNJ is found when the NAO or the SCAND index is positive, or the E-ATL is negative. There is no correlation with the EU-BLOCK index. In fact it

8 3258 Y. J. ORSOLINI and F. J. DOBLAS-REYES Figure 4. Regression map of ozone against the polar night jet (PNJ) index; ozone anomalies (in DU) associated with one standard deviation of the PNJ index. is the SCAND pattern which explains the most variance in the PNJ index variability in a multiple linear regression (not shown), and we speculate that this relates to the high latitude of the tropospheric geopotential perturbations associated with the SCAND pattern. A case study by Kristjansson (1986) demonstrated that a Scandinavian blocking episode in autumn 1979 was associated with a distorted polar vortex at 50 mb. A regression map of ozone versus the PNJ index (Fig. 4), indeed shows a signature nearly identical to the wintertime tropopause height signature of Ambaum and Hoskins (2002). This ozone signature hence superimposes positively or negatively onto the climate pattern signatures that are induced by the 500 mb geopotential pattern anomalies (Fig. 3). Ambaum and Hoskins (2002) calculated that a mean 8% drop in tropopause pressure was associated with an anomaly of one standard deviation in the polar vortex strength. Assuming linearity with column ozone, that would translate into a 25 DU amplitude. This mechanism implies that ozone decreases over large sectors of northern Asia and northern Europe when the PNJ index is positive, i.e. when the zonal-mean PNJ is strong. This occurs when the NAO or SCAND indices are positive, as shown on the regression maps of Fig. 3. Conversely, during Scandinavian blockings (negative SCAND, Fig. 1(b)), the large positive ozone anomaly over northern Asia and northern Europe associated with the PNJ patterns tends to cancel the regional negative ozone anomaly. The E-ATL index on the other hand negatively correlates with the PNJ index, hence ozone decreases over northern Asia and northern Europe during the E-ATL negative phase, such as in March April 1997.

9 OZONE SIGNATURES OVER THE EURO-ATLANTIC SECTOR IN SPRING 3259 Figure 5. Spring ozone over western Europe (40 N 60 N, 15 W 15 E) from TOMS data (thick line) for years and reconstructed spring ozone with leading pattern anomalies successively added to the climatological mean: the NAO only (thin), plus the SCAND (dot), plus the E-ATL (dot-dash) and EU-BLOCK (long dash). The EU-BLOCK contribution brings little change over that region. 5. DYNAMICAL CONTRIBUTIONS TO OZONE INTERANNUAL VARIABILITY AND TRENDS In section 3, we have seen that the leading Euro-Atlantic patterns induce anomalies in the range of 5 15 DU, numbers comparable to the midlatitude springtime ozone trend per decade (Staehelin et al. 2001). We use these leading patterns to reconstruct the dynamically-induced spring ozone anomalies over the northern hemisphere in the years Figure 5 shows the spring ozone over western Europe, as derived from TOMS observations. When the NAO-induced ozone anomaly is added to the mean climatological ozone and subsequently the SCAND, the E-ATL, and the EU-BLOCK contributions, the reconstructed year-to-year variations in spring ozone resemble the curve derived from TOMS observations. The correlation coef cient increases from 0.42 (one EOF) to 0.54 (two EOFs), and then to 0.66 (three EOFs), but is not changed by the fourth EOF (EU-BLOCK). Using this procedure, we next estimate the amount of interannual variability and trend that is explained by the four combined patterns of tropospheric origin over the northern hemisphere. The ozone satellite record spans only about 20 years, and there is considerable interannual and decade-to-decade variability. Despite these caveats, it is quite common to investigate the linear trend in spring ozone. Knudsen and Andersen (2001) indicate that 35% of the trend is of dynamical origin over the northern midlatitudes in April May. Hood et al. (1999) estimated it at 25 to 40% in spring. Steinbrecht et al. (2001) estimated it to be about 30% in February, based on an analysis of the 33-year total ozone record from Hohenpeissenberg (Germany). The ratio of the reconstructed trend to the observed trend (Fig. 6(a)) displays large regional variations, with a maximum contribution as high as 70% over the Iberian Peninsula. Naturally, it is over the Euro-Atlantic sector that a larger portion of the trend is explained. The explained trend is nevertheless signi cant over central and northern Asia, where some patterns have a strong in uence (section 4). A smaller fraction of the negative ozone trend is explained over northern Europe, where dilution of ozonedepleted polar air is expected to be a strong contributor to the trend. The series of regional maxima and minima in the dynamical ozone trend across the Euro-Atlantic sector (e.g. Knudsen and Andersen 2001) can hence be attributed to the four abovementioned climate patterns.

10 3260 Y. J. ORSOLINI and F. J. DOBLAS-REYES Figure 6. Comparison of reconstructed spring ozone time series (derived from anomalies induced by the four leading EOF patterns superimposed on the climatological mean) against TOMS observations from : (a) ratio of trends and (b) ratio of year-to-year variances.

11 OZONE SIGNATURES OVER THE EURO-ATLANTIC SECTOR IN SPRING 3261 In order to roughly eliminate the decadal variability and the trend, we formed difference time series, subtracting ozone anomalies from consecutive years. The year-toyear variance was then calculated for both the reconstructed time series and the TOMS observations. Figure 6(b) shows the ratio of these two variances. These results indicate that the dynamical variability induced by the patterns may explain about half of the year-to-year variability in ozone over certain parts of Europe and Asia. Little yearto-year variability is explained over northern Europe however. Joint examination of Figs. 3 and 6(b) shows that while the NAO is important, one needs to consider the additional patterns. For example, the EU-BLOCK pattern is important for the central Europe region. 6. CONCLUSIONS The novelty of our approach has been to use a method widely used in climate research, namely empirical orthogonal function analysis, to investigate meteorological factors in uencing the ozone column variability and trend. We have also relied on the 20-year record of satellite column ozone observations by the TOMS instrument, rather that on local station observations. The ozone statistical signatures of climate patterns originating in the troposphere obtained in this study can readily be understood in terms of geopotential anomalies in the troposphere, and can be quanti ed (Figs. 3 and 4). Four leading patterns induce regional ozone anomalies of similar amplitude. It proves important to incorporate several patterns, in addition to the NAO pattern. Higher order patterns might prove dominant in given years (e.g. the E-ATL pattern in the spring 1997). More localized patterns, such as the EU-BLOCK, also in uence the regional ozone trend. The ozone signature of a hitherto little-known lifting of the Arctic and Eurasian tropopause by an enhanced polar night jet (Ambaum and Hoskins, 2002) has also been quanti ed. A signi cant part of the springtime ozone trend over given regions of the Euro- Atlantic sector has been accounted for by dynamical trends in a series of climate patterns. The ozone record is quite short (about 20 years) and contains strong, even extreme, anomalies. Linear ts are hence not statistically robust. Nevertheless, our results suggest that the low-frequency dynamical variability of tropospheric origin induces marked regional ozone trends. The remaining part of the trend is in all likelihood explained by a series of phenomena that are known to in uence ozone: the quasibiennial oscillation, the volcanic aerosols, the solar cycle, interannual variations in the meridional mean circulation, and the pronounced chemical ozone depletion in the nineties. The latter is most certainly a key forcing of the interannual ozone variability over middle and high latitudes. In addition, our analysis is restricted to the four leading Euro-Atlantic patterns which explain 65% of the variance; hence, not all the dynamical contribution is explained by these four patterns. The relative importance (Corti et al. 1999) of the shape of the main patterns of climate variability are likely to change in an evolving atmosphere due to compositional modi cations of anthropogenic origin. Understanding the ozone impact of such climate patterns in a changing atmosphere is necessary to assess future local ozone trends. To this end, mechanisms forcing these climate patterns need to be better understood. Ozone was treated in this study as responding passively to the low-frequency dynamical variability originating in the troposphere. The role of atmospheric ozone depletion on long-term trend in springtime climate patterns (e.g. the NAO) in the northern hemisphere is still an open question.

12 3262 Y. J. ORSOLINI and F. J. DOBLAS-REYES ACKNOWLEDGEMENTS The lead author was supported by the European Union (EU) Commission through project EVK and by a grant of the Norwegian Supercomputing Programme, the second author by the EU Commission through project EVK We acknowledge the National Aeronautics and Space Administration for provision of TOMS data and the Climate Unit at the University of East Anglia for the NAO index. We also thank Dr. G. Manney for providing the polar night jet index, Dr. V. Pavan for the blocking index, and Drs M.J. Casado and M.A. Pastor. REFERENCES Appenzeller, C., Weiss, A. K. and Staehelin, J North Atlantic Oscillation modulates total ozone winter trends. Geophys. Res. Lett., 27, Ambaum, M., Hoskins, B. J. and Stephenson, D. B Arctic Oscillation or North Atlantic Oscillation? J. Climate, 14, Ambaum, M. and Hoskins, B. J The NAO stratosphere troposphere connection. J. Climate, 15, Ambrizzi, T., Kayano, M. T. and Stephenson, D. B A comparison of global tropospheric teleconnections using observed satellite and general circulation model total ozone column data for Clim. Dyn., 14, Barnston, A. G. and Livesey, B. E Classi cation, seasonality and persistence of low-frequency atmospheric circulation patterns. Mon. Weather Rev., 115, Braesicke, P., Jrrar, A., Hadjinicolaou, P. and Pyle, J. Bronnimann, S., Luterbacher, J., Schmutz, C. and Wanner, H Variability of total ozone due to the NAO as represented in two different model systems. Meteorol. Z., 12, Variability of total ozone at Arosa, Switzerland, since 1931 related to atmospheric circulation indices. Geophys. Res. Lett., 27, Corti, S., Molteni, F. and Palmer, T. N Signature of recent climate change in frequencies of natural atmospheric circulation regimes. Nature, 398, Doblas-Reyes, F. J., Casado, M. J. and Pastor, M. A Sensitivity of the northern hemisphere blocking frequency to the detection index. J. Geophys. Res., 107, ACL 6, 1 22 Entzian, G. and Peters, D Very low zonally asymmetric ozone values in March 1997 above the North Atlantic European region, induced by dynamical processes. Ann. Geophys., 17, Hood, L., Rossi, S. and Beulen, M Trends in lower stratospheric zonal winds, Rossby wave breaking behavior and column ozone at northern mid-latitudes J. Geophys. Res., 104, Kalnay, E., Kanamitsu, M., Kirtler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetma, A., Reynolds, R., Jenne, R. and Joseph, D The NCEP NCAR 40-year reanalysis project. Bull. Am. Meteorol. Soc., 77, Knudsen, B. M. and Andersen, S. B Longitudinal variations in springtime ozone trends. Nature, 413, Kristjansson, J. E Budgets of kinetic energy and enthalpy in a Scandinavian blocking. Master Thesis, Geophysics Institute, Univ. of Bergen, Norway Kuroda, Y. and Kodera, K Role of planetary waves in the stratosphere troposphere coupled variability in the northern hemisphere. Geophys. Res. Lett., 26, Limpasuvan, V. and Hartmann, D Eddies and the annual modes of climate variability. Geophys. Res. Lett., 26, Orsolini, Y. J., Stephenson, D. B. and Doblas-Reyes, F. J Storm track signature in total ozone during northern hemisphere winter. Geophys. Res. Lett., 25, Orsolini, Y. J. and Limpasuvan, V 2001 The North Atlantic Oscillation and the occurrences of ozone miniholes. Geophys. Res. Lett., 28,

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