During recent years ozone depletion has been detected not only over polar regions but also over mid-latitude areas. This study analyzed daily total ozone (TO) data from three south-western European locations in order to detect

long-time TO trends by means of a filtering technique. Correlation analysis with atmospheric circulation patterns was carried out to explain the decreasing trends observed. Results appear to show a strong correlation between TO decrease and the North Atlantic Oscillation and Arctic Oscillation Indices throughout recent decades. On the other hand, the trends also indicate that, at least during the last ten years, TO variations cannot be explained solely by natural atmospheric cycles over the studied area

Casale, G.R.

Monge-Sanz, B.M.

Palmieri, S.

Siani, A.M.

English

During recent years ozone depletion has been detected not only over polar regions but also over mid-latitude areas. This study analyzed daily total ozone (TO) data from three south-western European locations in order to detect long-time TO trends by means of a filtering technique. Correlation analysis with atmospheric circulation patterns was carried out to explain the decreasing trends observed. Results appear to show a strong correlation between TO decrease and the North Atlantic Oscillation and Arctic Oscillation Indices throughout recent decades. On the other hand, the trends also indicate that, at least during the last ten years, TO variations cannot be explained solely by natural atmospheric cycles over the studied area

B. N. Monge Sanz

S. Palmieri

CASALE, Giuseppe Rocco

SIANI, Anna Maria

Archivio della ricerca- Università di Roma La Sapienza

An investigation on total ozone over western Mediterranean

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 26 C, N. 1 Gennaio-Febbraio 2003An investigation on total ozone over western Mediterranean(∗)B. M. Monge-Sanz(1), G. R. Casale(2), S. Palmieri(2) and A. M. Siani(2)(1) Dpto. Ingenier´ıa Electro´nica y Comunicaciones, Facultad de CienciasUniversidad de Zaragoza - Campus Pza. San Francisco, 50009 Zaragoza, Spain(2) Dipartimento di Fisica, Universita` di Roma “La Sapienza”P.le A. Moro 2, 00185 Rome, Italy(ricevuto il 10 Dicembre 2001; revisionato l’1 Agosto 2002; approvato il 10 Settembre 2002)Summary. — During recent years ozone depletion has been detected not only overpolar regions but also over mid-latitude areas. This study analyzed daily totalozone (TO) data from three south-western European locations in order to detectlong-time TO trends by means of a filtering technique. Correlation analysis withatmospheric circulation patterns was carried out to explain the decreasing trendsobserved. Results appear to show a strong correlation between TO decrease and theNorth Atlantic Oscillation and Arctic Oscillation Indices throughout recent decades.On the other hand, the trends also indicate that, at least during the last ten years,TO variations cannot be explained solely by natural atmospheric cycles over thestudied area.PACS 92.70.Gt – Climate dynamics.PACS 92.60.Bh – General circulation.PACS 95.75.Wx – Time series analysis, time variability.1. – IntroductionIn the last three decades, total ozone has been decreasing in many places, includingover Europe. Currently, one of the main subjects of interests is to understand to whatextent this decline is due to human activity or rather to natural climatic causes and/oratmospheric circulation variability.At middle latitudes ozone variations are mainly due to dynamical processes at differenttime scales which are interpretable in terms of horizontal and vertical transport as wellas to the intensity of positive or negative sources. At southern Mediterranean latitudeshorizontal stratospheric advection very likely plays a major role [1]. Vice versa, north ofthe polar jet, stratosphere-troposphere ozone downward flux is active and this shall beaccounted for in offering possible interpretations. The authors do not aim at proposing an(∗) The authors of this paper have agreed to not receive the proofs for correction.c© Societa` Italiana di Fisica 5354 B. M. MONGE-SANZ, G. R. CASALE, S. PALMIERI and A. M. SIANIoverall mechanism able to interpret any ozone change by means of a horizontal advectionscheme.Short-term variations may be associated with weather conditions: in connection withhigh-pressure systems, the tropopause raises pushing up air out of column with thereduction of ozone [2, 3]. In western Europe an increase of ozone amount is often foundin the rear side of cyclones at the passage of a cold front, while a minimum may be foundin the south-west side of a high-pressure area close to the warm front [4]. In addition,atmospheric circulation can influence interannual ozone variability and long-term trends.Investigations on ozone trends in terms of the North Atlantic Oscillation (NAO) and theArctic Oscillation (AO) Indices were carried out by several researchers [5-7]. It has beensuggested that a) the predominance of the positive phase of the NAO in the last decadesis closely related with this decreasing tendency of ozone and b) the NAO influence on TOfeatures in Europe is expected to be higher than that of the Quasi Biennial Oscillation(QBO) or that of the solar activity.The paper has an observational and descriptive character confined and concentratedon an area (south-western Mediterranean) not often analysed, without any ambition topropose or suggest new physical mechanisms.Long-time trends of total ozone (TO) were estimated for three western Mediterraneanlocations by filtering the long-time ozone series collected at the three stations by meansof Dobson solar spectrophotometer.The relationship between ozone behaviour and circulation indices such as North At-lantic Oscillation index (NAO) and Arctic Oscillation index (AO) was also examined inorder to explore any possible linkage between ozone and atmospheric circulation. Trends,as well as correlation, were computed on yearly basis and for winter season when thestrongest coupling between troposphere and stratosphere takes place.2. – Ozone and NAO index trends2.1. Filtering technique. – According to Rao and Zurbenko methodology, described inEskridge et al. [8] and Milanchus et al. [9], any geophysical variableX(t) can be separatedinto several components as follows:X(t) = e(t) + S(t) + w(t) + T (t) + ε(t) ,(1)where w(t) is the meso-synoptic scale component (weather pattern related), S(t) is theseasonal component and e(t) is the long-time scale signal. T (t) and ε(t) represent turbu-lence and instrumental error, respectively. These two components were negligible in thecase in which the average monthly data are used.With reference to cycles, w(t) may well be the high-frequency component, S(t) themiddle-frequency term (periods from 3 weeks to 1 year), and e(t) is the low-frequencyone, with a periodicity of at least one year.To detect any possible trends, the lowest frequencies have to be isolated. The KZ(Kolmogorov-Zurbenko) filtering technique consists of an iterative moving average effec-tive in removing frequencies higher than a specific threshold. To remove the periodicityshorter than P months, a filter KZ (D, N) was used in order to satisfy the followingcriterion:D√N  P,(2)where D indicates the temporal window width and N is the number of iterations.AN INVESTIGATION ON TOTAL OZONE OVER WESTERN MEDITERRANEAN 553003053103153203253303353401/1/57 1/1/61 1/1/65 1/1/69 1/1/73 1/1/77 1/1/81 1/1/85 1/1/89 1/1/93 1/1/97dateozone (DU)Vigna di ValleEl ArenosilloLisbonFig. 1. – Filtered series by using the KZ(105,2).2.2. Total ozone long-term trend . – All total ozone (TO) data were extracted from theWorld Ozone and Ultraviolet Radiation Data Center (WOUDC) at Toronto (Canada).The analysed stations were Lisbon, El Arenosillo and Vigna di Valle (table I).A KZ (105,2) filter was applied to the monthly series in order to remove all periodicityshorter than 12.4 years. In addition to seasonal and annual fluctuations, other knowncycles, such as the Quasi Biennial Oscillation (QBO) and the 11-year solar cycle, wereeliminated. The resulting filtered series are shown in fig. 1.It should be noted that use of a different filter, KZ (120,1), saves the 11-year solarcycle, whose contribution to the total ozone series is as follows: 3.4% for Lisbon, 3.0%for El Arenosillo and 2.3% for Vigna di Valle.Ozone trends were subsequently estimated by using a linear regression analysis onthe long-term component series (fig. 1) beginning from the year in which the declineappeared to start (last column in table II). The % trend per decade with the uncertaintyis reported in table II. The correlation coefficient R (last column in table II) betweenthe filtered series, e(t), and the remaining series, TO(t)− e(t), indicates that the specificcomponents are independent.The decline in total ozone in most European areas started in the 70s; this is alsoconfirmed for Lisbon and Vigna di Valle series, (−2.6±1.2)% per decade and (−2.8±1.1)%per decade, respectively (table II).For El Arenosillo station the decreasing tendency started only in the early 80s al-though ozone observations at this station began in the mid 70s and the filter ignoredTable I. – Stations and period of the series.Station Country Latitude Longitude Height (m) PeriodLisbon Portugal 38.77 N 9.15 W 105 Sept. 60-Dec. 99El Arenosillo Spain 37.10 N 6.73 W 41 Jan. 76-Dec. 99Vigna di Valle Italy 42.08 N 12.22 E 262 July 57-Dec. 9956 B. M. MONGE-SANZ, G. R. CASALE, S. PALMIERI and A. M. SIANITable II. – Total ozone trends with the filter KZ (105,2).Station Estimated decline starting year Trend %/decade RLisbon 1975 −2.6± 1.2 0.041El Arenosillo 1984 −3.7± 1.7 −0.125Vigna di Valle 1973 −2.8± 1.1 −0.003Correlation coefficient R is given with p < 0.05.data from both ends of the series. Thus data for El Arenosillo cover a shorter periodthan those of the other two stations. The delay in the decline starting year is presumablydue the gap in data series and not to a physical process.Therefore the trend estimated with the filtered data (KZ (105,2)) may not be accurateand thus the decline might have started earlier.2.3. Ozone winter and NAO index trends . – Given that circulation patterns changenoticeably from winter to summer, a trend analysis limited to the cold period of the year(December to March) was performed. First, the series containing solely winter meanozone values were retrieved. Subsequently a KZ(12,1) filter was applied on yearly wintervalues to remove all periods not exceeding 12 years. Results in fig. 2 and table III indicatebehaviours quite similar to the monthly data although somewhat less smooth. The trendswere obtained by linear regression, starting from the year in which the decline started.The winter trend for Lisbon is slightly weaker than its annual trend (−2.4% per decadefor winter, −2.6% per decade for the annually), while the other two stations showed thatwinter trends were stronger than the yearly trends (table III). The values obtained arein agreement with those reported by WMO [3].290300310320330340350360195819601962196419661968197019721974197619781980198219841986198819901992199419961998yearozone (DU)Vigna KZ(12,1)El Arenosillo KZ(12,1)Lisboa KZ(12,1)Fig. 2. – Winter series by using the KZ(12,1). Diamond marker line for Vigna di Valle series,grey square marker line for the Lisbon one and triangle marker line for that of El Arenosillostation.AN INVESTIGATION ON TOTAL OZONE OVER WESTERN MEDITERRANEAN 57Table III. – Total ozone winter trends with the filter KZ (105,2).Station Time period Winter trend % per decadeLisbon 1969-1993 −2.4± 1.1El Arenosillo 1982-1993 −4.6± 2.1Vigna di Valle 1963-1993 −3.5± 1.9Ozone depletion, in addition to the generally accepted concept of being the conse-quence of the increase of chlorine concentration in the atmosphere, may be related tochanges in the general atmospheric circulation specially in the stratosphere as suggestedby a recent study [10]. Indeed large variations in the strength of the stratospheric circu-lation are followed by anomalous tropospheric regimes affecting NAO and AO.The North Atlantic Oscillation (NAO) is a major disturbance of the atmosphere overthe North Atlantic Ocean. It affects to a considerable extent the climate of the nearcontinents from Eastern North America to Western Europe [5]. The NAO is commonlydescribed by an index whose time behaviour gives information on temporal variations ofsurface pressure difference between a station in the Azores and a station in Iceland. Thusit is related to the changes in the strength of the dominant middle-latitude westerly windflow during winter. NAO’s characteristic is that it alternates yearly between positiveand negative index values. Depending on the phase of NAO, the atmospheric pressuredistribution changes, as does the tropopause pressure value. In high-pressure conditions,the tropopause raises producing a shrinking of vertical column and hence low ozonelevels. Conversely, low tropopause pressures are associated with horizontal convergence,air column stretching and high total ozone column.During the positive phase, the depression over Iceland is below its normal average,and the Azores pressure is higher than average reinforcing the westerly flow across theAtlantic towards Europe advecting ozone poor maritime air. Vice versa, during thenegative phase of the NAO, the westerly flow is weak or even replaced by currents withsouthward components and hence ozone rich air affects Europe. This determines ananticorrelation between NAO and ozone.Monthly averaged data from the National Center for Atmospheric Research (NCEP/NCAR) were used to derive mean NAO Index winter values from December to March.The NAO index time series was filtered by KZ (12,1) operator, similar to that appliedto the TO winter series. Figure 3 shows an increasing NAO trend beginning in 1970.No trend is detectable from 1958 to 1969; the mean value is −0.34, whereas a definitepositive tendency, with a mean value of +0.26, appears between 1970 and 2000, even ifin the graph only the period from 1958 to 1989 is considered.Comparison of the behaviours of TO and NAO indices (figs. 2 and 3) clearly indicatesthe anticorrelation of the two series and that the respective trends began around 1970.3. – Space differences in the ozone behaviourIn order to investigate on space difference in TO behaviour, a correlation analysiswith NAO index was carried out with significance level p < 0.05.Correlation coefficients on a monthly basis are shown in table IV. Major importanceis attributed to the coefficients in bold with explained variance > 15% (the explainedvariance is the percentage of the total ozone variance accounted for by a linear regression58 B. M. MONGE-SANZ, G. R. CASALE, S. PALMIERI and A. M. SIANI-0.6-0.4-0.200.20.40.60.819581959196019611962196319641965196619671968196919701971197219731974197519761977197819791980198119821983198419851986198719881989year NAO indexFig. 3. – NAO Index time series.model where NAO and AO are independent variables). Although the use of NAO Index isrecommended for winter only, coefficients were computed for every month. As expected,ozone and NAO index are anticorrelated. The stronger anticorrelation occurs in Februaryat the Iberian stations, while at Vigna di Valle the maximum is found in March evenif at the limit of significance. It is interesting to note that, for the Iberian stations,anticorrelation is also present in April and in January at the El Arenosillo station. AtVigna di Valle the anticorrrelation is very poor in all months.The differences between Lisbon and Vigna di Valle are remarkable. A possible in-terpretation is that the western Mediterranean area is more intensively affected by coldoutbreaks of ozone-rich air from northern latitudes with respect to the Atlantic coast.This phenomenon is more marked during low NAO index periods in winter.According to Thompson and Wallace [11] and Baldwin [12], the Arctic Oscillation(AO) is the circulation annular mode that carries information on the intensity of the arcticTable IV. – Correlation coefficients between TO mean values and those of NAO and AO indices.Lisbon El Arenosillo Vigna di ValleNAO AO NAO AO NAO AOJanuary −0.277 −0.299 −0.462 −0.388 −0.382 −0.570February −0.452 −0.412 −0.506 −0.390 −0.323 −0.291March −0.317 * −0.260 −0.365 −0.383 −0.514April −0.413 * −0.409 * * *May −0.288 * −0.318 * * *June −0.210 * * * * *July * * * * * *August −0.230 * * −0.249 * *September −0.338 −0.294 * * * *October −0.317 −0.201 * * * −0.273November −0.221 * −0.235 * −0.203 *December * * −0.266 0.382 * *AN INVESTIGATION ON TOTAL OZONE OVER WESTERN MEDITERRANEAN 59polar vortex. AO index data are available for the period 1958-2000 (NCEP/NCAR). Themonthly correlation between NAO and AO indices from 1959 to 2000 is high mainly inwinter (R = 0.780).As in the case of NAO index the correlation between TO and the AO index series wascomputed (table IV). Winter correlations are homogenous as far as sign is concerned atthe three stations. The fact that the Mediterranean area is more prone to cold outbreaksfrom higher latitudes than the Atlantic Iberian coast, presumably leads to a greater AOindex at Vigna di Valle.4. – Concluding remarksThe aim of the present study is to investigate long-time trends of total ozone (TO)and their relationship to atmospheric circulation in the western Mediterranean region.So far the total ozone in this area was not particularly studied in relation to circulationindices. The type of filter analysis used was effective in detecting trends in the long timeseries of total ozone and NAO index.Results show the existence of a remarkable ozone downward tendency over the westernMediterranean, with a decrease of about 3.0% per decade. The result agrees with recentstudies [3]. The cause of this trend is not completely understood so far. Continuityequation for atmospheric ozone indicates that total ozone time changes may be dueto three-dimensional transport and intensity of sources. Although the latter is oftenmentioned in the literature as the main cause of ozone depletion (chemical consequencesof chlorine increase) the effect of circulation changes, particularly within the stratosphere,requires further studies.Moreover, it is still unclear whether NAO positive trend is simply the result of naturalvariability or an effect of the global warming.∗ ∗ ∗The authors are grateful to the World Ozone and Ultraviolet Radiation Data Centre(WOUDC) for providing ozone data and to National Center for Atmospheric ResearchNCEP/NCAR for AO and NAO indexes data. This research was supported partly bythe EDUCE (European Database for UV Climatology and Evaluation) project and Fa-colta` 60% (1999). Moreover this study was carried out at the University of Rome “LaSapienza” in the framework of the Erasmus Programme. We also appreciate the con-structive criticism and suggestions from the anonymous reviewer. His suggestions havebeen incorporated into the paper.REFERENCES[1] Holton J. R., An Introduction to Dynamic Meteorology, 3rd edition (Academic PressInc.) 1992.[2] Dobson G. M. B. and Harrison D. N., Proc. R. Soc. London, Ser. A, 110 (1926) 660.[3] WMO Scientific Assessment of Ozone Depletion: 1998, Global Ozone and ResearchMonitoring Project Report No. 44. Geneva Switzerland, 1999.[4] Gotz F. W. P., Ozone in the atmosphere, Compendium of Meteorology, edited by T. F.Malone (American Meteorology Society, Boston, USA) 1951, pp. 275-291.[5] Appenzeller C., Weiss A. K. and Staehelin J., Geopys. Res. Lett., 27 (2000) 1131.[6] Bro¨nnimann, S., Luterbacher J., Schmutz C., Wanner H. and Staehelin J.,Geophys. Res. Lett., 27 (2000) 2213.60 B. M. MONGE-SANZ, G. R. CASALE, S. PALMIERI and A. M. SIANI[7] Weiss A. K. and Staehelin J., Proceedings of the Quadrennial Ozone Symposium-Sapporo2000, pp. 13-14.[8] Eskridge R. E., Ku J. Y., Rao S. T., Porter P. S. and Zurbenko I. G., Bull. Am.Meteorol. Soc., 78 (1997) 1473.[9] Milanchus M. L., Rao S. T. and Zurbenko I. G., J. Air & Waste Mgt. Assoc., 48(1998) 201.[10] Baldwin M. P. and Dunkerton T. J., Science, 294 (2001) 581.[11] Thompson D. W. J. and Wallace J. M., Geophys. Res. Lett., 25 (1998) 1297.[12] Baldwin M. P., SPARC Newsletter, 14 (2000) 10.

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An investigation on total ozone over western Mediterranean

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