An extreme anomaly in stratospheric ozone over Europe in

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L08101, doi: /2004gl019611, 2004 An extreme anomaly in stratospheric ozone over Europe in S. Brönnimann 1 Lunar and Planetary Laboratory, University of Arizona, Tucson, USA J. Luterbacher NCCR Climate and Institute of Geography, University of Bern, Switzerland J. Staehelin Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland T. M. Svendby Department of Physics, University of Oslo, Norway Received 29 January 2004; revised 4 March 2004; accepted 18 March 2004; published 16 April [1] Reevaluated historical total ozone data reveal extraordinarily high values over several European sites in , concurrent with extreme climatic anomalies at the Earth s surface. Using historical radiosonde data, reconstructed upper-level fields, and total ozone data from Arosa (Switzerland), Dombås, and Tromsø (Norway), this unusual case of stratosphere-troposphere coupling is analyzed. At Arosa, numerous strong total ozone peaks in all seasons were due to unusually frequent upper troughs over central Europe and related ozone redistribution in the lower stratosphere. At the Norwegian sites, high winter total ozone was most likely caused by major stratospheric warmings in Jan./Feb. 1940, Feb./Mar. 1941, and Feb Results demonstrate that the dynamically driven interannual variability of total ozone can be much larger than that estimated based on the past years. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere constituent transport and chemistry (3334); 1610 Global Change: Atmosphere (0315, 0325); 1620 Global Change: Climate dynamics (3309); 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342). Citation: Brönnimann, S., J. Luterbacher, J. Staehelin, and T. M. Svendby (2004), An extreme anomaly in stratospheric ozone over Europe in , Geophys. Res. Lett., 31, L08101, doi: /2004gl Introduction [2] Understanding the natural variability of total ozone and its relation to large-scale atmospheric circulation is considered important for assessing past ozone trends and for detecting the expected recovery of the ozone layer [WMO, 2003]. In this context, it is interesting to note that extremely high total ozone values were observed at several European sites in the early 1940s [Götz, 1951], together with extraordinarily cold temperatures at the Earth s surface. 1 Now at Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland. Copyright 2004 by the American Geophysical Union /04/2004GL This anomaly has largely been forgotten (except by Labitzke and van Loon [1999]), but in the light of recent research on ozone and atmospheric dynamics [WMO, 2003] a detailed study seems worthwhile. Moreover, it could contribute to a better understanding of stratosphere-troposphere coupling and its relation to climate variability. [3] In this paper we report on this unprecedented total ozone anomaly over Europe and address the role of atmospheric dynamics using historical radiosonde data and reconstructed upper-level fields. A future paper deals with the large-scale circulation during this period. 2. Data [4] We used daily total ozone data from Arosa, Switzerland (47 N, 10 E), since 1926 [Staehelin et al., 1998] and Dombås, Norway (62 N, 9 E), [Svendby, 2003] and monthly mean values from Tromsø, Norway (70 N, 19 E), [London et al., 1962] (converted to Bass-Paur scale; see Figure 3 for locations). Historical upper-air data were taken from Brönnimann [2003], additional data were digitized for Bergen, Trondheim, Kristiansand, Narvik (Norway), Pajala, Riksgränsen (Sweden), Lerwick, Penzance, Larkhill (UK), Helgoland (Germany), and Swinoujscie (Poland), Dec to Feb Processing and quality tests were performed as in Brönnimann [2003] (see Temperature data from Jungfraujoch, Switzerland (3580 m asl) were provided by MeteoSwiss. In addition, we used statistically reconstructed monthly fields of geopotential height (GPH) and temperature up to 100 hpa, N, [Brönnimann and Luterbacher, 2004] and 300 hpa GPH over Europe, [Schmutz et al., 2001]. [5] All data were expressed as anomalies from the mean annual cycle (for daily data the first two harmonics were extracted) based on all available data in For this purpose Dombås total ozone was supplemented with data from Uppsala (Sweden, 320 km east of Dombås), [Rindert, 1976] and Oslo (Norway, 250 km south), [Svendby and Dahlback, 2002] and the Tromsø series with data from the WOUDC data base. The reference for all L of5

2 Figure 1. Monthly anomalies of total ozone at Arosa (purple), local 300 hpa GPH (green), and temperature at Jungfraujoch (blue), smoothed with a 2-year moving average. upper-air data was NCEP/NCAR Reanalysis [Kistler et al., 2001]. 3. Results 3.1. Low-Frequency Variability [6] The anomaly is illustrated by long series of total ozone and meteorological variables from Arosa. In order to focus on variations longer than the Quasi-Biennial Oscillation, we smoothed the monthly anomalies with a 2-year moving average (Figure 1). Total ozone is affected by stratospheric ozone depletion after around The most outstanding feature, however, is the peak in , which was unprecedented in amplitude. In the same years temperature at nearby Jungfraujoch reached record low values. The winters 1939/40, 1940/41, and 1941/42 were among the coldest in Europe in the 20th century. The total ozone series has been homogenized carefully; observational artifacts can be excluded. Chemical explanations are unlikely. Rather, the high total ozone must be explained by transport processes and hence an anomalous stratospheric circulation that was most likely related to the climate anomaly. This is supported by reconstructed local 300 hpa GPH, which reached record low values in At midlatitudes 300 hpa GPH is negatively correlated to total ozone in all seasons due to ozone redistribution in the lower stratosphere accompanying a change in planetary wave structure [Brönnimann et al., 2000]. [7] Strong total ozone peaks also appear at Dombås and Tromsø, accompanied by record low surface air temperatures. However, local 300 hpa GPH was not anomalous. Simultaneous total ozone peaks at Tromsø and Arosa are not necessarily expected; the two monthly series are nearly uncorrelated in later periods. In the Arctic total ozone is strongly influenced by the dynamics of the stratospheric polar vortex in winter [Labitzke and van Loon, 1999] whereas the correlation with local 300 hpa GPH is weak. Yet, there may be an indirect relation between ozone in the Arctic and at midlatitudes in that the polar vortex is connected with the zonal circulation of the midlatitude troposphere [Hartmann et al., 2000; Limpasuvan et al., 2004] High-Frequency Variability [8] The reevaluated radiosonde data allow to study the vertical structure of the anomaly on a day-to-day scale. Total ozone anomalies at Arosa, Dombås, and Tromsø are compared to standardized anomalies of temperature and GPH from nearby sites (Figure 2). At Arosa, the total ozone anomaly was characterized by a strong persistence and by sudden and short spikes. Although more pronounced in winter, the latter were also observed in summer, which is unusual. The highest values ever observed at Arosa in May, June, August, October, and November were within the displayed period. Unfortunately, the series from Dombås does not provide winter data and the one from Tromsø is only available as monthly means. Nevertheless, a pronounced total ozone peak is evident at Dombås in late Figure 2. Daily anomalies of total ozone at Arosa, Dombås, and Tromsø and standardized daily anomalies of GPH and temperature at Freiburg i. B. (supplemented with data from Trier), Kjeller, and Tromsø (Pajala, Narvik, Riksgränsen), Dec to Mar. 1942, smoothed with a Gaussian filter (s = 3 days). Blue curves show more heavily filtered total ozone anomalies (s = 15 days) or monthly mean data (Tromsø, 10 daily values must be available). 25% of the filter weights must be non-missing. Blue and orange arrows denote upper-trough events and suggested MMWs. 2of5

3 Figure 3. Anomalies of 200 hpa temperature (top) and 400 hpa GPH (bottom) from radiosonde data for Jan (left), 25 Oct. 7 Nov (middle), and 6 19 Jan (right). At least 7 values must be available per 14-day period. A, D, and T mark Arosa, Dombås, and Tromsø, respectively. Feb and extremely high monthly anomalies at Tromsø in each winter. The latter are not due to the effect of ozone depletion in the reference period; they appear in similar strength also with respect to [9] Total ozone spikes can have different dynamical causes. At midlatitudes they can be due to advection of polar ozone-rich air in the lower stratosphere, horizontal convergence and descending motion above an upper-tropospheric trough, as indicated by warmings at 100 hpa, cold anomalies in the troposphere, and low GPH at all levels [Hakim, 2003]. At high latitudes, ozone spikes in winter can be related to major midwinter warmings (MMWs) in the middle stratosphere [Labitzke and van Loon, 1999]. Thereby the polar vortex collapses and strong descent in the polar stratosphere causes high total ozone. MMWs propagate downward and often, but not always, reach the 100 hpa level [Limpasuvan et al., 2004]. They are accompanied by rapid poleward transport of ozone in the middle stratosphere, which may cause total ozone spikes at midlatitudes over Europe [Braun and Dütsch, 1984]. [10] In the case of Arosa, many spikes can be classified as upper-trough events (marked in Figure 2), accompanied by a warm lower stratosphere and cold-air advection in the free troposphere. Upper troughs were frequent; opposite events (subtropical intrusions with low total ozone) were almost absent. At Dombås and Tromsø, the relation between total ozone and upper-air data is less obvious. Several ozone peaks at Dombås seem to have been caused by upper troughs. High total ozone in Feb at Tromsø could have been related to a MMW. In the following, examples for upper-trough events are presented and possible MMWs are examined Upper Troughs [11] Figure 3 shows 400 hpa GPH and 200 hpa temperature anomalies from radiosonde data for three persistent upper-trough situations over Europe, each averaged for 14 days. All of them were accompanied by ozone spikes at Arosa. Cold spells were observed in the troposphere at the beginning of all three cases and the cold air remained almost stationary over central Europe for at least 2 weeks. The 400 hpa GPH data show strongest negative deviations in all cases over central Europe (eastern Germany). The anomalies disappear, and eventually reverse, towards western and northern Europe. The 200 hpa temperature was higher than normal over central Europe, in agreement with the lower stratospheric circulation above an upper-trough system. Again, temperature anomalies disappeared or changed sign north of 60 N. [12] The total ozone anomalies can largely be explained by ozone redistribution related to the upper-trough circulation (in fact, stratospheric ozone enters the troposphere in such situations, which also occur in summer [Stohl et al., 2003; Zanis et al., 2003]). Arosa, where strong anomalies were observed (average for the 3 periods: 73 DU), is situated near the center of the upper-tropospheric anomaly. Total ozone also showed a peak at Dombås in Oct. 1941, but the anomaly (14-day average: 24 DU) was not as strong as the one in Arosa Stratospheric Warmings [13] MMWs are defined based on daily data from the middle stratosphere, which are not available for the 1940s. However, analyzing data since 1952 [Labitzke and Naujokat, 2000] we find that monthly 100 hpa temperature Figure 4. Reconstructed temperature anomalies in Feb (top), Feb (middle), and Feb (bottom) at 100 hpa (left) and vertical cross-sections along 10 E (middle). Light shaded areas denote low reconstruction skill (RE < 0.2 [Brönnimann and Luterbacher, 2004]). Radiosonde data at 100 hpa (Alaska: 150 hpa) are given in dashed (data used for reconstruction) or solid circles (independent data). Right: radiosonde data at sites within 280 km of the cross-section (>280 km north of 65 N) on three specific days. Yellow lines mark the cross-section. 3of5

4 anomalies >4.5 C averaged over the polar region (70 90 N) in January or February almost exclusively occur with MMWs in the same or previous month. Also, MMWs are accompanied by a weak polar vortex at 100 hpa. Hence, reconstructed fields and radiosonde data provide indirect evidence for MMWs. [14] The 100 hpa GPH fields show a very weak polar vortex in Jan. 1940, Mar. 1941, and Feb. 1942, somewhat less so in Jan. 1939, Feb. 1940, and Feb Very high temperatures in polar regions are found in Feb. 1942, Feb [Brönnimann and Luterbacher, 2004], to a lesser extent in Feb. 1940, Mar. 1941, and Jan Both together suggest MMWs in Jan./Feb. 1940, Feb./Mar. 1941, and Feb (Figure 4; Jan was probably a Canadian warming during which the vortex does not collapse [Labitzke and Naujokat, 2000]). Unfortunately, the reconstruction skill for 100 hpa temperature is low over the polar region (light shaded areas). However, warm arctic temperatures (except in Feb. 1940) and the main features at midlatitudes are confirmed by independent (i.e., not used for reconstruction) radiosonde data. The vertical structure is shown in meridional cross-sections at 10 E. Temperatures were generally low up to 100 hpa south of 65 N and the warmings were confined to the polar areas. The 1941 and 1942 warmings affected the troposphere. [15] The daily radiosonde data allow a more detailed view. For the 1940s warming sparse data from Pajala (67 N), the only direct evidence, suggest that it started in late January and lasted until early February (Figure 4 top right, 25 Jan. 1940). The timing around the turn of the month would make this case difficult to detect (or reconstruct) in monthly means. The 1941 warming was probably confined to polar latitudes and Alaska, only occasionally reaching the European Arctic. Sporadic warmings (17 Feb. over Narvik, 19 Feb. over Trondheim and Bergen, 23 Feb. over Narvik and Riksgränsen) occurred with otherwise low 100 hpa temperatures over Europe. On 28 Feb. a strong warming at 100 and 200 hpa started north of 55 N, continuing into early March (Figure 4, 1 Mar. 1941). The 1942 case is best documented with upper-air data. The polar vortex was already disturbed in Jan and 100 hpa temperatures at Tromsø show several weaker warmings. The main warming started around 22 Feb. (Figure 4), with a sudden temperature increase of 10 C at Tromsø, intensified and continued until early March (Figure 2). In all three cases the 100 hpa data from the Scandinavian Arctic were not used for reconstructing the fields shown in Figure 4 and hence independently confirm the warmings. [16] The suggested MMWs in Jan./Feb. 1940, Feb./Mar. 1941, and Feb are in excellent agreement with total ozone anomalies at Tromsø in the same months and a strong peak at Dombås in Feb./Mar In fact, based only on Tromsø total ozone, Labitzke and van Loon [1999] suggested major warmings in Jan and Feb The Arosa data also show ozone peaks in these months, but daily upper-level fields are necessary to analyze whether they were related to the warmings. 4. Discussion and Conclusions [17] The upper-level circulation over Europe in was characterized by unusually frequent upper troughs over central Europe (almost no subtropical intrusions) and three suggested MMWs over the Arctic (for comparison: during the past 50 years, MMWs occurred every third winter on average). Since MMWs are preceded by increased planetary wave activity [Limpasuvan et al., 2004], the two mechanisms may have been related to each other via a change in planetary wave structure. This change was accompanied by very low surface air temperatures over Europe. [18] This circulation explains the numerous ozone spikes. However, total ozone anomalies at Arosa and Tromsø also show a strong persistence (Figure 2, blue lines); they remained high well into summer or even autumn. This persistence was not caused by ozone depletion in the reference period but also appears with respect to As dynamically caused ozone anomalies are not strictly reversible due to mixing, a memory-effect is possible. In addition, increased net poleward transport of tropical ozone in the middle stratosphere could have contributed, as is suggested by frequent MMWs [Braun and Dütsch, 1984; Limpasuvan et al., 2004]. [19] At Arosa and probably also Tromsø (in the unhomogenized record and TOMS data), the total ozone anomaly was far outside the variability found in the past years, which were previously used to characterize ozone variability [WMO, 2003]. This is important with respect to assessing future long-term trends. A strong increase in total ozone at middle and highlatitudes, even over several years, does not necessarily imply recovery, all the more as major warmings not only dynamically affect ozone, but also temporarily slow down arctic chemical ozone loss. The results show that the natural variability of total ozone over Europe is very large even at interannual time scales, related to a similarly strong interannual variability of troposphere-stratosphere coupling. [20] Acknowledgments. SB was funded by the Swiss National Science Foundation and the Holderbank-Foundation. JL was funded by the Swiss National Science Foundation (NCCR Climate). References Braun, W., and H. U. Dütsch (1984), The origin of ozone-rich air in the middle stratosphere over Europe at the end of January 1979, J. Atmos. Chem., 2, Brönnimann, S. (2003), A historical upper-air data set for the period, Int. J. Climatol., 23, Brönnimann, S., and J. 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