Summer 2003 Anticyclone Case Study

Composition and Chemistry

European surface ozone in the extreme summer 2003


  • S. Solberg,

    1. Norwegian Institute for Air Research, Kjeller, Norway
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  • Ø. Hov,

    1. Norwegian Meteorological Institute, Oslo, Norway
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  • A. Søvde,

    1. Department of Geosciences, University of Oslo, Oslo, Norway
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  • I. S. A. Isaksen,

    1. Department of Geosciences, University of Oslo, Oslo, Norway
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  • P. Coddeville,

    1. Ecole des Mines de Douai, Douai, France
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  • H. De Backer,

    1. Royal Meteorological Institute of Belgium, Brussels, Belgium
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  • C. Forster,

    1. Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Weßling, Germany
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  • Y. Orsolini,

    1. Norwegian Institute for Air Research, Kjeller, Norway
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  • K. Uhse


[1] Measurements of ozone and other trace species in the European EMEP network in 2003 are presented. The European summer of 2003 was exceptionally warm, and the surface ozone data for central Europe show the highest values since the end of the 1980s. The 95th percentiles of daily maximum hourly ozone concentrations in 2003 were higher than the corresponding parameter measured in any previous year at many sites in France, Germany, Switzerland and Austria. In this paper we argue that a number of positive feedbacks between the weather conditions and ozone contributed to the elevated surface ozone. First, we calculated an extended residence time of air parcels in the atmospheric boundary layer for several sites in central Europe. Second, we show that it is likely that extensive forest fires on the Iberian Peninsula, resulting from the drought and heat, contributed to the peak ozone values in north Europe in August. Third, regional-scale model calculations indicate that enhanced levels of biogenic isoprene could have contributed up to 20% of the peak ozone concentrations. Measurements indicate elevated concentrations of isoprene compared to previous years. Sensitivity runs with a global chemical transport model showed that a reduction in the surface dry deposition due to drought and the elevated air temperature both could have contributed significantly to the enhanced ozone concentrations. Because of climate change, such heat waves may occur more frequently in the future and may gradually overshadow the effect of reduced emissions from anthropogenic sources of VOC and NOx in controlling surface ozone.

1. Introduction

[2] The European summer of 2003 was exceptionally warm, in particular in central Europe. On the basis of a temperature reconstruction of monthly (back to 1659) and seasonal (from 1500 to 1658) temperature fields for European land areas (25°W to 40°E, 35°N to 76°N) Luterbacher et al. [2004] concluded that the summer of 2003 was very likely warmer than any other summer back to 1500. Compared to the 1901–1995 average surface temperature, the summer of 2003 exceeded that average by about 2°C.

[3] The observations reported within EEA (European Environment Agency) show exceptionally long-lasting and spatially extensive episodes of high ozone concentrations, mainly in the first half of August, and covering the regions with the highest temperatures [Fiala et al., 2003]. For all monitoring sites the average number of hourly ozone exceedances above 180 μg m−3 was higher in the summer of 2003 than in any of the previous 12 years. Rural monitoring data from the EMEP (European Monitoring and Evaluation Program) network [Solberg et al., 2005] indicate that in central Europe an indicator like the 6-months AOT40 (Accumulated Ozone exposure over a Threshold of 40 ppb) was higher in 2003 than in any other year since 1990, and that the AOT40 values in 2003 were almost a factor 2 higher than the average during the 1990s. In Switzerland the 2003 summer mean of the daily ozone maxima exceeded the 1992–2002 summer mean of daily ozone maxima by more than 15 ppb, corresponding to 5 standard deviations of the 1992–2002 summer means, similar to the deviation in surface temperature [Ordonez et al., 2005]. In the UK the heat wave was studied through the TORCH campaign [Lee et al., 2006], one of the most highly instrumented to date, local ozone formation rates up to 17 ppb h−1 was found.

[4] The high levels of atmospheric pollutants had direct consequences for human health. In case studies of the number of deaths related to the 2003 heat wave in the United Kingdom and the Netherlands, Stedman [2003] estimated that for the first two weeks of August 2003 there were 2045 excess deaths over the 1998–2002 average, and between 423 and 769 of these were related to elevated ambient ozone and PM10 concentrations. In a similar study for the Netherlands, Fischer et al. [2003] found an excess of 1000–1400 deaths during the summer of 2003, and 400–600 were air pollution related (ozone and PM10). Trigo et al. [2005] have shown that in France the geographical pattern of the temperature anomaly matched particularly well the mortality rates.

[5] Schär et al. [2004] and Beniston [2004] carried out regional climate simulation calculations to investigate if summers like the one in 2003 could become more prevalent under future climate change. On the basis of the SRES A2 transient greenhouse gas scenario as specified by IPCC, Schär et al. [2004] found that for 2071–2100 conditions (SCEN) for a grid point in northern Switzerland for June–July–August (JJA) the JJA average is shifted 4.6°C toward warmer temperatures, and there is a pronounced widening of its statistical distribution with the standard deviation increasing by 102%. This widening is highly significant. Furthermore, they analyzed other models and greenhouse gas scenarios and found that all exhibit a substantial increased level of variability over large parts of Europe. Schär et al. [2004] conclude that in response to greenhouse gas forcing the year-to-year variability in European summer climate may increase, and that the unusual European summer of 2003 may be an example of what is to come. Beniston [2004, p. 1] draws a similar conclusion: “For many purposes the 2003 event can be used as an analogue of future summers in coming decades in climate impacts and policy studies.”

[6] In the following study we present surface ozone data from the EMEP network of rural background sites in Europe, as well as complementary measurements of isoprene and a number of ozone soundings. Ozone in the atmospheric boundary layer is controlled by the emissions of NOx and VOC and their photochemical transformations, which are temperature-dependent, the residence time of air parcels in the boundary layer over the source regions, the dry removal to the ground and the incoming UV and short-wave visible radiation as well as the horizontal and vertical advection and mixing.

[7] The link between climate change and regional air quality is a topic receiving increased attention the last years. It was a basis for, e.g., the ICARTT campaign recently reported [Fehsenfeld et al., 2006] and is the underlying focus of the study presented below. We propose that a number of positive feedback mechanisms linked to the heat waves contributed to the high surface ozone concentrations observed, and investigate their potential strengths through chemical transport model (CTM) calculations.

2. Meteorological Conditions

[8] The meteorological conditions during the European summer 2003 have been extensively described in several recent studies, and we will only highlight some key points. Over Europe, persistent anticyclonic anomalies, high temperatures and a series of intense heat waves characterized the summer [Schär et al., 2004; Stott et al., 2004]. In particular, several pronounced heat waves led to soaring ground temperatures in August. The hottest period was around the middle of August, during a highly anomalous blocking event centered over France [Grazzini et al., 2003; Black et al., 2004; Fink et al., 2004]. All-time temperature records then tumbled over much of Europe. Schär et al. [2004] showed that the temperatures in the anticyclone over central Europe were four standard deviations above normal.

[9] During the heat waves in June and August monthly temperature anomalies of 6–7°C compared to the 1961–1990 climatological average were observed at sites in Switzerland and south Germany. At Hohenpeißenberg, Germany, the period June–August was 5°C higher than the average since 1781 when temperature monitoring began. According to Fink et al. [2004] the elevated temperatures in August were not accompanied by increased convection and vertical mixing. On the contrary the August heat wave lead to a stabilization and subsidence, presumably because of the soil drought leading to low humidity in the lower troposphere. The central Europe anticyclonic anomaly was part of an extensive, quasi-stationary Rossby wave train, stretching from the western Atlantic across Europe and toward Siberia. These waves extended high into the stratosphere where they disturbed the easterly circumpolar circulation up to an altitude of 20–25 km [Orsolini and Nikulin, 2006].

[10] In addition, the extreme surface temperatures gave the heat wave devastating impact. These high temperatures resulted from a local radiative budget influenced by the lasting spring-to-summer dryness, by the low background soil moisture, and the clear skies during the anticyclonic conditions [Schär et al., 2004; Black et al., 2004]. Another point worth mentioning is that the Atlantic jet stream was displaced northward, resulting in a reduction in the passage of cyclones over central Europe.

[11] On the basis of the FLEXTRA three-dimensional air mass back trajectories [Stohl et al., 1995; Stohl and Seibert, 1998] we have estimated the residence times in the European planetary boundary layer (PBL) for air masses arriving at a number of monitoring sites in Europe as shown in Figure 1. This was done by computing the number of hours the trajectories stayed within a central European domain and below a vertical level which was set to 2.5 km above sea level. The domain was defined to be the area between 10°E, 30°W, 35°N and 55°N (Figure 1). The basis for the calculations were 7-d backward trajectories arriving every 6 h at 500 m above ground for the period 1996–2003 for the given sites.

[12] The results, presented in Figure 2, show that for Rigi in Switzerland and Peyrusse Vieille in southern France the residence time in the European PBL was particularly high in June and August. Also at Kosetice in the Czech Republic the residence time in the European PBL was high in June. At the other sites, further north (Mace Head and Waldhof) and east (Starina), the PBL residence times varied during summer, but without a clear perturbation. This is in line with the geographical distribution of the temperature anomalies which show the highest values in south and central France, Switzerland, southern Germany and northern Italy [Fink et al., 2004]. A lengthened residence time over surface emission areas is only one of several parameters necessary for more effective ozone formation. Additional parameters favoring ozone production include more solar radiation (little cloud cover), higher temperatures and slower mixing processes. As mentioned and further discussed below, such conditions persisted during the 2003 heat waves. Thus, the situation was particularly favorable for significant ozone formation for several weeks in south and central parts of Europe, whereas the regions to the north and east were more at the outskirts of the anticyclone and to a higher extent a receiver of photochemically processed air masses.

3. Surface Ozone

[13] Surface ozone measurements have been a part of the EMEP extended measurement activities since its third phase, and the monitoring started in the late 1980s [Dollard et al., 1995; Hjellbrekke and Solberg, 2004]. A total of 131 stations in 27 European countries reported data for 2003. The ozone monitoring sites are situated mainly in central, western and northern Europe and the network density is poorer in the eastern and Mediterranean parts of Europe. The stations are located in rural or remote areas, away from local emission sources, and thus representative of the regional concentration field.

[14] In many of the countries there are national or regional networks with a large number of ozone monitoring sites. The dedicated EMEP sites have been selected from these by the countries and by EMEP's Chemical Coordinating Centre (EMEP-CCC) at the Norwegian Institute for Air Research (NILU) on the basis of certain criteria of site location, as specified in detail by the EMEP manual [EMEP, 1995]. The most important criteria is that the sites should be regionally representative, thus located away from nearby emission sources, and, second, not located in certain topographic areas like valleys subject to inversion situations or on mountain tops or passes. The monitoring data are subject to a strict quality control procedure as defined in the EMEP manual, both in the individual countries and at the EMEP-CCC, before being accepted as valid data. Information about the ozone data quality, calibration and maintenance procedures was collected from the participants during 2000 [Aas et al., 2001]. The UV-absorption method was the only measurement method in use in 2003.

[15] An upper extreme value of ozone is a better indicator for photochemical episodes than, e.g., a mean value as the latter is to a larger extent controlled by the background values. The 95-percentile ozone concentrations in 2003 (based on daily maximum hourly values) are given in Figure 3. Also given is the ratio of the 95-percentiles in 2003 relative to the 95-percentiles observed during the period 1991–2002. Note that the number of years with ozone monitoring varies between the sites so that the ratios shown in Figure 3 do not refer to the same group of previous years for all sites. The ratios were only calculated for sites with ozone monitoring back to at least 1998. Figure 3 shows that the 95-percentile ozone concentration in 2003 exceeded 160–170 μg m−3 over a large region in central Europe extending from Austria in southeast across most of Germany to Belgium, the Netherlands and the southeast part of the UK. The 95-percentiles in 2003 exceeded the previous 95-percentiles in France and at several sites in Switzerland, Germany and Austria. Also in the most northern part of Scandinavia, record-breaking ozone values were observed around 20 April 2003 peaking at 85 ppb (170 μg m−3) at Esrange in northern Sweden as discussed in detail by Lindskog et al. [2007].

[16] In contrast, the 2003 peak values in UK were lower than the maximum values for previous years except for Wicken Fen, and except for Harwell where the 2003 peak value reached the same peak value, 246 μg m−3, as observed during 1991–2002. The reason for these regional differences is both that the main area of the European ozone plume in 2003 was located south of UK and also that most of the UK sites have continuous monitoring data back to 1991 (and before) compared to, e.g., the French sites which have a shorter monitoring history. It has been estimated that peak ozone concentrations at the EMEP stations in the UK declined about 30% in the period 1986–1999 [National Expert Group on Transboundary Air Pollution, 2001]. The main reason for this decline is believed to be the reduction in emissions of ozone precursors in Europe.

[17] The ten highest ozone concentrations observed in the EMEP network in 2003 are given in Table 1. The highest value of 296 μg m−3 was observed at Eupen in Belgium. Most of the 10 highest peak values occurred during the first half of August with a few exceptions. At Montelibretti (near Rome) in the south the peak value occurred in June and in UK the peak value at Harwell was seen in mid-July.

EupenBelgium6°00′E50°38′N2968 Aug 2003
MontelibrettiItaly12°38′E42°06′N28713 Jun 2003
DononFrance7°08′E48°30′N25411 Aug 2003
HarwellUK1°19′W51°34′N24615 Jul 2003
VreedepeelNetherlands5°51′E51°32′N2447 Aug 2003
SchmückeGermany10°46′E50°39′N24312 Aug 2003
VezinBelgium4°59′E50°30′N2398 Aug 2003
RevinFrance4°38'E49°54′N2398 Aug 2003
BassumGermany8°43′E52°51′N23812 Aug 2003
Lullington HeathUK0°11′E50°48′N23611 Aug 2003

[18] Figure 4 shows the monthly means of daily maximum (MDM) ozone values in 2003 for each of the months March–August relative to the highest MDMs during the years 1991–2002. The MDMs in June and particularly August were record high compared to the data back to 1991 over a large region in central Europe. At Payerne and Tänikon in Switzerland the MDM in August 2003 was approximately 15% higher than in any other August since 1991. However, the MDMs were also record high in other periods of 2003, like in March and April in parts of Scandinavia and the UK. This shows that Europe in 2003 experienced record-high ozone concentrations in several individual periods during the whole spring/summer period. In May and July 2003 the MDMs in central Europe and UK were lower than the maximum of the MDMs for previous years at most sites. In middle and northern Scandinavia, however, peak MDM values were observed in July.

[19] Figure 5 shows the seasonal cycle in ozone measured at 8 sites in the EMEP network in 2003 compared to the average seasonal cycle based on the 12-year period 1991–2002. The seasonal cycles were calculated as 30-d running median of the daily maximum of the hourly ozone concentrations. The results shown in Figure 5 indicate a most pronounced perturbation in the ozone level during June–August 2003 at the central European sites (Waldhof, St. Koloman, Schmücke and Payerne) with a deviation from the 12-year reference of the order of 40–50 μg m−3 at most. During these periods the running median values for 2003 occasionally exceeded the running 90-percentiles of the reference climatology. In the southern UK, represented by data from Yarner Wood, a marked period of enhanced ozone concentrations are seen in summer, although lower concentrations than those observed at the central European sites. Further north in the UK, e.g., at sites in Scotland, the 2003 data showed only a minor deviation from the reference. At Mace Head, on the west coast of Ireland, slightly elevated mean ozone levels are evident from the middle of May to the middle of August. In July and August 2003 Scandinavia experienced positive monthly temperature anomalies of around 2°C compared to the 1961–1990 climatological average but was located outside of the main area of the heat waves. The ozone measurements from Birkenes (Figure 5) only have a minor peak in August while at Rørvik in southwest Sweden slightly elevated levels are seen from June to the beginning of August.

The heatwave of 2003

More than 20,000 people died after a record-breaking heatwave left Europe sweltering in August 2003. The period of extreme heat is thought to be the warmest for up to 500 years, and many European countries experienced their highest temperatures on record.

Physical Impacts

Low river flows and lake levels

The River Danube in Serbia fell to its lowest level in 100 years. Bombs and tanks from World War 2, which had been submerged under water for decades, were revealed, causing a danger to people swimming in the rivers. Reservoirs and rivers used for public water supply and hydro-electric schemes either dried up or ran extremely low.

Forest fires

The lack of rainfall meant very dry conditions occurred over much of Europe. Forest fires broke out in many countries. In Portugal 215,000 hectares area of forest were destroyed. This is an area the same size as Luxembourg. It is estimated millions of tonnes of topsoil were eroded in the year after the fires as the protection of the forest cover was removed. This made river water quality poor when the ash and soil washed into rivers.

The satellite image shown above shows forest fires in southern Portugal and Spain in September 2003. The fires are shown by the red dots and smoke is in white.

Melting glaciers

Extreme snow and glacier-melt in the European Alps led to increased rock and ice falls in the mountains.

Effects of the heatwave

About 15,000 people died due to the heat in France, which led to a shortage of space to store dead bodies in mortuaries. Temporary mortuaries were set up in refrigeration lorries. There were also heat-related deaths in the UK (2,000), Portugal (2,100), Italy (3,100), Holland (1,500) and Germany (300).

Human effects

  • Heat-stroke - normally we sweat, and this keeps us cool on hot days. On very hot days our bodies may not be able to keep cool enough by sweating alone, and our core body temperature may rise. This can lead to headaches, dizziness and even death.
  • Dehydration - this is the loss of water from our bodies. It can cause tiredness and problems with breathing and heart rates.
  • Sunburn - damage to the skin which can be painful and may increase the risks of getting skin cancer.
  • Air pollution - it is thought that one third of the deaths caused by the heatwave in the UK were caused by poor air quality.
  • Drowning - some people drowned when trying to cool off in rivers and lakes.

We provide the Department of Health with heatwave warnings (Heat-Health Watch) to prepare the NHS, health professionals, carers and the general public for the effects of extreme heat. You can find out more about our Heat-Health Watch in the weather section of the Met Office website.

Summers as hot as 2003 could happen every other year by the year 2050 as a result of climate change due to human activities.

Environment and social effects

  • Water supplies - drinking water supplies were affected in some parts of the UK and hosepipe bans introduced.
  • Tourism - many parts of the UK reported increased levels of tourism as people decided to holiday in the UK while the weather was unusually dry and hot.
  • Agriculture - many chickens, pigs and cows died during the heat in Europe and crops failed in the dry conditions. This led to higher food prices. It is thought to have cost European farming 13.1 billion euros.
  • Transport - some railway tracks buckled in the heat. The London Underground became unbearable. Some road surfaces melted. Low river levels prevented some boats from sailing.
  • The London Eye closed on one day as it became too hot in the cabins.
  • Energy - two nuclear power plants to close down in Germany. These rely on water for cooling in the power generation process.

Immediate responses to the heatwave

  • France requested aid from the European Union to deal with the effects.
  • Public water supply shortages occurred in several countries, including the UK and Croatia, which led to a temporary ban on using hose pipes.
  • TV news, internet and newspapers informed the public on how to cope with the heat - drinking plenty of water, wearing cool clothing, and staying in the shade in the middle of the day.
  • Network Rail in the UK imposed speed restrictions for trains when the temperature was above 30 °C. This was to help avoid trains derailing when railway lines might have buckled
  • Workers around Europe altered their working hours. Some refuse collectors started earlier to pick up rapidly decomposing rubbish from the streets.

What happened to cause the heatwave?

Above is a weather chart for midday on 5 August 2003.

It shows an area of high pressure over most of Western Europe. Air is moving around the high in a clockwise direction, bringing a hot, dry tropical continental air mass to the UK at this time. This pattern occurred for much of the rest of the month. High pressure areas usually bring little cloud and warm conditions in summer.

You can find out more about Key to symbols and terms in the weather section of the Met Office website.

Satellite imagery

The satellite images below confirm there is very little cloud over most of Europe.

The image to the left shows an infrared satellite image of north-west Europe at 2 p.m. on 5 August. Infrared satellite images tell us information about the temperature of the Earth as seen from space. The white areas (cold) in this image show where there is cloud. In contrast, the darker areas over most of Europe show that the land surface is very warm in the afternoon sunshine where there has been little or no cloud cover for some time.

This visible satellite image is also for north-west Europe at 2 p.m. on 5 August. Visible satellite images like this one tell us information about the amount of sunlight that is being reflected from clouds or the ocean, land or sea-ice surfaces.

You can Using satellites to help create weather forecasts in the science section of the Met Office website.

Maximum temperatures

Many parts of Europe saw their temperature records broken during this summer, including the UK. A sweltering 38.5°C was recorded in Brogdale in Kent on 10 August 2003, a record high which still stands today.

European rainfall

Rainfall over much of Europe was below what is normally expected during the months of June, July and August. The long-lasting high pressure system tended to reduce the amount of rain that fell.

As a result of the European heatwave:

  • A joint Met Office/Department of Health project called the Heat-Health Watch now gives advanced warning of UK hot. weather. It operates every summer from 1 June to 15 September.
  • The French government has made efforts to improve its prevention, surveillance and alert system for people such as the elderly living alone.

Further information on the Met Office site

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