Recent trends in the frequencies of extreme values of daily maximum atmospheric pressure at ground level in the central zone of the Iberian Peninsula

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: (2008) Published online 15 October 2007 in Wiley InterScience ( Recent trends in the frequencies of extreme values of daily maximum atmospheric pressure at ground level in the central zone of the Iberian Peninsula J. L. Labajo, a, *Q.Martín, b A. L. Labajo, a A. Piorno, a M. Ortega c and C. Morales c a Departamento de Física General y de la Atmósfera. Universidad de Salamanca, Spain b Departamento de Estadística. Universidad de Salamanca, Spain c Departamento de Geografía. Universidad de Valladolid, Spain ABSTRACT: Using data sets on the daily data of maximum atmospheric pressure at ground level collected at 14 weather stations located in the central zone of the Iberian Peninsula [Spanish central plateau (SCP)], the series of daily maximum pressure anomalies at each station, together with the difference between the daily value and mean daily value for each day of the year for the period between 1961 and 2003, have been established. The regional series of such anomalies were constructed for the whole study zone and for two differentiated parts of the same. As thresholds for the extreme values of the anomaly series, the values corresponding to the P 05 and P 95 percentiles were used. The series of annual frequencies of days with anomaly values below and above the threshold values were constructed for each of the weather stations, together with the average series for the whole study zone and each of its two parts. The corresponding regional average series of seasonal frequencies were also constructed. From an analysis of the trend of the series of the annual frequency of extreme anomaly values in daily maximum pressure it may be deduced that the lowest values show a decreasing annual trend, while the highest ones show an increasing frequency. This indicates that between 1961 and 2003 the number of days per year on the SCP with the highest extreme atmospheric pressure values at ground level increased along the study period. In contrast the number of days per year with lower extreme values decreased. Additionally, analysis of the seasonal frequency series indicated that it was the winter that dictated such behaviour. Copyright 2007 Royal Meteorological Society KEY WORDS pressure; Iberian Peninsula; extreme values; frequency; trend Received 15 February 2007; Revised 27 July 2007; Accepted 6 September Introduction The results of studies carried out to date show that at global level a variation can be seen in the values of climate variables, suggesting the possibility of climate change due to human activity (IPCC, 2002, 2007). The increase in greenhouse gases, together with other factors such as deforestation or the pollution of water bodies, has led to the generation of alterations in the equilibrium of climate values in the different regions of the planet. In recent decades, investigation into these issues has mainly focused on the behaviour of the mean values of the climate elements (Kadioglu, 1997; Maugeri and Nanni, 1998; Pinto et al., 2001; García-Herrera et al., 2003), but as from the 1990s investigators became interested in establishing the causes responsible for the increase in mean global temperature or for the alterations in climatic equilibrium in different regions of the planet. In the third report on the assessment of climate change (IPCC, 2002), special emphasis was placed on the notion * Correspondence to: J. L. Labajo, Departamento de Física General y de la Atmósfera. Universidad de Salamanca, Spain. jll45@usal.es that possible alterations to the climate will become manifest not so much in the mean (normal) values of climate elements but instead in their extreme values. Accordingly, the contributors to that report stressed the need to study the behaviour of extreme events occurring in different climate elements. The suggestion made by the investigators of the IPCC sparked many works addressing the behaviour of such events, mainly as regards rainfall and temperature (Domonkos et al., 2003; Klein and Können, 2003; Shouraseni and Balling, 2004; Hellström, 2005, among others). In the summary elaborated by Working Group I for the fourth IPCC report, reference is made to the trend observed in the production of extreme events, mainly as regards precipitation and temperature (IPCC, 2007). Although it is one of the major elements of the climate, since it is determinant in the behaviour of the other elements, atmospheric pressure trends have been little studied. Specific studies in the field of climate and climate change addressing atmospheric pressure at sea level have usually considered the reconstruction of series (Bärring et al., 1999; Allan et al., 2002), their temporal changes (Zveryaev, 1999; Pinto et al., 2001; Copyright 2007 Royal Meteorological Society

2 1228 J. L. LABAJO ET AL. Polyakov et al., 2003), and their relationship to other climate variables (Kutiel and Paz, 1998; Gouirand and Moron, 2003; Zangvil et al., 2003). Additionally, recent years have seen an increase in regional studies aimed at confirming the results of global studies and establishing the differences in behaviour of climate variables in the different zones of the planet. In this work an analysis of the behaviour of the extreme values of daily maximum atmospheric pressure [maximum ground-level pressure (MAGLP)] is reported. The daily data series were obtained at weather stations belonging to the synoptic and climatological networks of the Spanish Institute of Meteorology (INM), located on the central zone of the Iberian Peninsula [(Spanish central plateau (SCP)]. 2. The study area The study zone is the central zone of the Iberian Peninsula, known as the SCP, since overall it configures an area with a high mean altitude. It has an extension of km 2 and its geographical position is determined by latitudes and N and longitudes 0 54 and W. The zone is demarcated by orographic systems along almost the whole of its periphery and it is crossed by a mountain range, in the SW NE direction, that divides it into two parts. The two parts present structural and geomorphological differences that are sufficient for them to be differentiated from each other: the northern zone (northern sub-plateau) and the southern zone (southern sub-plateau). The northern zone of the central plateau (northern sub-plateau) includes nearly all the northern part of the study zone. Its geographic position is defined by latitudes and N and longitudes 1 46 and 7 02 W. Its coverage is km 2. The maximum distance from east to west is more than 400 km, and from north to south more than 300 km. It is limited by mountain ranges that often surpass altitudes of 2000 m above sea level (a.s.l), except in the west, and the altitude of the zone is between 700 and 1000 m a.s.l. From the point of view of climate, these geomorphological characteristics favour a clear differentiation of the zone. The southern zone of the central plateau (southern sub-plateau) comprises its southern half. The geographic coordinates limiting the zone lay between latitudes and N and longitudes 0 54 and 5 24 W. The zone has an extension of km 2 and a mean altitude between 500 and 700 m a.s.l. It is limited by mountain ranges, but these are slightly less elevated than those delimiting the northern sub-plateau. All of the above imply that the climatic characteristics of the southern sub-plateau differ with respect to its northern counterpart. The climate of the SCP is Mediterranean (subcontinental) (Martín-Vide and Olcina, 2001). The two sub-plateaus have different climates, although in both cases the features of continentality are clear. On the northern sub-plateau, during the three winter months the mean monthly temperatureis less than 6 C. On the southern sub-plateau the winter is shorter and, in general, only two months have temperatures below 6.5 C, although in some zones minimum temperatures lower than 20 C have been recorded. Precipitation is scarce on both subplateaus, normally not surpassing 600 mm/year. 3. Data and methodology The data used in the present study, which were provided by the Spanish INM, refer to data on the maximum daily values of pressure obtained from continuous recording at 14 weather stations: 8 on the northern sub-plateau and 6 on the southern one. The weather stations chosen are located in Ávila, Burgos (Villafría), León (Virgen del Camino), Salamanca (Matacán), Segovia, Soria, Villanubla (Valladolid), Zamora, Los Llanos (Albacete), Ciudad Real, Cuenca, Madrid (Retiro), Barajas (Madrid) and Toledo (Figure 1). These weather stations belong to the synoptic and/or climatological networks of the INM in the regional communities of Castile-Leon, Madrid and Castile-La Mancha, all located on the SCP. Currently, the availability of the data from these stations only allows a relatively short interval of time to be considered ( ), although this is of the same length or even longer that those reported by others (Manton et al., 2001; Salinger and Griffiths, 2001; Hellström, 2005, among others). Additionally, these data series have gaps, sometimes many. Accordingly, so that the results obtained could be considered valid, the conditions that the data series had to be of the same length and that the percentage of gaps had to be less than 20% were imposed. Of the 14 stations considered here, only 3 had more than 10% of gaps, while for the other 11 the number of gaps in the series was less than 1% of cases. Regarding spatial distribution, the stations considered cover the study area sufficiently well (Figure 1). The number of data of each of the series, once the process of gap filling had been completed, was , corresponding to the time period of This interval can be considered sufficient for the results to be considered significant. The process followed to collect the frequency series of the occurrence of extreme values was as follows. The data series of daily MAGLP were analysed to determine the percentage of gaps and to detect any clearly erroneous data that might be present. Only the data that did not correspond to the same reference (sea level instead of ground level) were considered as erroneous. To accomplish this, an initial graphic analysis was performed, after which a series of logic filters was applied (The extreme maximum pressure values considered to define the outliers were 950 and 850 hpa for the northern sub-plateau and 975 and 875 hpa for the southern sub-plateau). The erroneous data were removed and the gaps thus generated, together with those existing previously, were filled: first from the original series and

3 TRENDS OF EXTREME VALUES OF PRESSURE IN THE IBERIAN PENINSULA 1229 Figure 1. Study zone. Location of weather stations used. then by multiple linear regression techniques. The values of the correlation coefficients between the series from the different stations were very high above 0.90, and in some cases reaching 0.99 such that the filling could be considered reliable. Once the series had been filled in, their homogeneity was checked with the Levene test (Levene, 1960). All the series proved to be homogeneous. With the daily data series, we constructed the series of daily maximum pressure anomalies [(anomaly of maximum ground-level pressure (AMAGLP)] for each station. To do so, we calculated the differences between the daily values of the working series and the corresponding mean daily values, obtained as an average between each of the 365 days of the year, of the original series. Also using these anomaly series, the regional series corresponding to maximum pressures on the SCP were constructed. To construct these series, those of the weather stations selected were used, employing the method for the inverse of distance for the averaged space (Jones and Hulme, 1996) on a grid between 38 and 43 N and 1 and 6 W for spatial averaging. The series from all the weather stations considered were used because each of them has its own peculiarities that should be taken into account in the regional series. These anomaly series were used as the basis for the development of the work. Also, bearing in mind the differences between the two sub-plateaus the regional series were constructed for each of them. These series were studied separately so that it would be possible to compare the results obtained with those obtained for the central zone of the Iberian Peninsula. The series of values of daily anomalies were considered instead of the series of daily maximum pressure values for two main reasons: first, because those values do not depend on the altitude of the weather stations, and hence can be compared directly with one another without having to introduce any kind of correction; second, because when anomaly data are used the annual periodicity of pressure is filtered. Also, the effect that undetected inhomogeneities might have on the results of the analysis is minimized. With the daily anomalies series (AMAGLP) from each of the stations and the regional ones for the whole study zone, the series of annual frequencies of extreme anomalies were constructed, which included the number of days in the year on which maximum daily pressure anomalies were above or below certain thresholds Determination of thresholds One way of determining the threshold values of the highest and lowest pressure anomalies is through the use of percentiles. This criterion is widely used by investigators both for the determination of class intervals in the variables data set (García-Herrera et al., 2003; Lana et al., 2003) and for the determination of the thresholds of extreme climate variable values (Suppiah and Hennessy, 1998; Brunetti et al., 2001; Manton et al., 2001; Salinger and Griffiths, 2001; Griffiths et al., 2003). The percentiles used to determine the thresholds of extreme values of climatic variables differ as a function of the variability of each of such variables. Normally, for the lowest extremes, the percentiles considered range between P 01 and P 10, and for the highest ones between P 90 and P 99. Here, the P 05 percentile was employed as a threshold of the lowest extreme values and P 95 as that of the highest extreme values. The reason why these two percentiles were chosen to determine the threshold values that would allow us to consider an AMAGLP value as extreme is based on the definition of the tri-mean statistical parameter. This is a mean established for the values of a series, excluding those that lie below the threshold established by P 05 and those surpassing that established by P 95, to eliminate the strong influence of the extreme values

4 1230 J. L. LABAJO ET AL. in the arithmetic mean. As a result, as the threshold of extreme values in the present series those values corresponding to the P 05 and P 95 percentiles of each series were taken. Applying this criterion to the AMAGLP series, the threshold values were determined below or above which the daily atmospheric pressure anomalies could be considered extreme values for the series from the stations at Ávila (Av), Burgos (Bu), León (Le), Salamanca (Sa), Segovia (Sg), Soria (So), Villanubla (Vn), Zamora (Za), Los Llanos (Lla), Ciudad Real (Cr), Cuenca (Cu), Madrid (Ret), Barajas (Bar) and Toledo (To), and for the partial and total regional series. Additionally, it seemed logical to determine which maximum atmospheric pressure values could be considered extreme in the study zone. Accordingly, the values corresponding to the P 05 and P 95 percentiles in the series of daily maximum pressure values were determined. The values obtained for the thresholds of extreme AMAGLP values, together with those of the MAGLP itself, are shown in Table I. The thresholds of the extreme pressure values for the partial and total regional series are not included because they were not significant. This was due to the fact that the pressures of the weather stations used to establish these thresholds did not refer to the same height above sea level. From the percentile values obtained, and using software specifically designed for the purpose, the number of cases per year in which the AMAGLP values were above and below the threshold determined by the P 95 and P 05 percentiles, respectively, was established. Thus, the annual frequency of anomaly of maximum ground level pressure (FAMAGLP) series was obtained. Table I. Threshold of extreme events, lowest and highest (P 05 and P 95 percentiles), of the AMAGLP anomaly series and of the daily pressure values (MAGLP) at each of the weather stations and in the study zone (hpa). Stations AMAGLP MAGLP P 05 P 95 P 05 P 95 Ávila Burgos León Salamanca Segovia Soria Villanubla (Valladolid) Zamora Los Llanos (Albacete) Ciudad Real Cuenca Barajas (Madrid) Madrid (Retiro) Toledo Northern sub-plateau Southern sub-plateau Central plateau Finally, the temporal behaviour of the annual frequencies of extreme values was analysed, initially establishing their temporal evolution graphically and then establishing the possible trend followed by them along the study period ( ), applying the Spearman method (Sneyers, 1975). 4. Results From the series of extreme events of maximum pressure anomalies (FAMAGLP) obtained at the weather stations included in this work, plots showing the temporal evolution of such extreme events were constructed. The plots, shown in Figures 2 6, show the corresponding trend lines. Analysis of these plots revealed that at nearly all the weather stations the observed trend was decreasing for the case of the lowest values (P 05 threshold), whereas in the case of the highest values (P 95 threshold), it was increasing. The slope of the trend lines is different in each of the plots, indicating that the confidence level associated with the trend observed in each case is different. From the plots shown in Figures 2 and 3, it may be seen that the variability of the FAMAGLP series is large. For the lowest extreme values (days with extremely low MAGLP values, P 05 threshold) the highest frequencies were recorded in 1963 and 1966 at nearly all the weather stations. For the highest extreme values (days with extremely high MAGLP values, P 95 threshold) a pronounced maximum was recorded at all the stations in In all cases, the lines shown on the plots point to the existence of trends along the study period. To check whether the trends observed were significant, at least for a confidence level of 95%, the Spearman test (Sneyers, 1975) was implemented in the FAMAGLP series obtained at each of the weather stations considered. The results are shown in Table II. The results in Table II indicate that the FAMAGLP series of the lowest extreme values (<P 05 ) from all the stations show an appreciable trend, with the exception of Burgos, Ciudad Real, Madrid (Retiro) and Toledo, where the confidence level was low. In general, at the remaining stations the series of annual frequencies of lowest extreme values showed decreasing trends at a confidence level greater than 95%, except in the case of León, where it was at the 90% level. The series corresponding to the stations in Burgos, Ciudad Real, Madrid (Retiro) and Toledo maintained the sign of the trend. Regarding the series of the highest extreme values, in most cases the trend of these was increasing at a confidence level greater than 95%. The exceptions were the series from León, for which the confidence level was greater than 90%, and Soria, where it was slightly lower than 90%. The series from Burgos, Ciudad Real, Madrid (Retiro) and Toledo followed a trend of opposite sign from that of the other weather stations, although its confidence level was very high (γ 0.95) and hence the trend was not significant.

5 TRENDS OF EXTREME VALUES OF PRESSURE IN THE IBERIAN PENINSULA 1231 Figure 2. Temporal evolution of the annual frequency of the lowest extreme events of AMAGLP (below P 05 ) at each of the weather stations of Castile-León. These results suggest a modification in the behaviour of the pressure field at the surface, frequency and intensity varying such that cyclonic or anti-cyclonic disturbances affected the Iberian Peninsula throughout the year. Once the behaviour of the individual maximum daily pressure series had been analysed, the behaviour of the regional series was checked, as above. From the thresholds obtained for the regional AMAGLP series of the whole study zone and its two differentiated parts, shown in Table I, the FAMAGLP series was compiled. For the SCP, one of the FAMAGLP series included the average number of days per year, for each year, in which the AMAGLP values were below the P 05 threshold ( 7.94), while the other one included the number of days per year with AMAGLP values above the P 95 threshold (6.75).

6 1232 J. L. LABAJO ET AL. Figure 3. The same as Figure 2, at each of the weather stations of Castile-La Mancha and Madrid. For the northern zone of the SCP (northern subplateau), the FAMAGLP series were established for a threshold of and 8.37, while for the southern part (southern sub-plateau) the thresholds were 9.19 and 8.0. The temporal behaviour of these regional series is shown in Figure 4. These plots also show the trend line. As in the case of the series from the individual weather stations, in the regional series a trend was seen in all cases. This trend was increasing for the series of frequencies of the highest values and was decreasing for those of the lowest values for the period. These observations are consistent with those reported by Pinto et al. (2001) for the mean annual pressure series, obtained from monthly mean pressure values for the period. The level of significance of the trends observed from analysis of the plots was established as in the previous cases by applying the Spearman test (Sneyers, 1975) to the FAMAGLP series. The results are shown in Table III. From these data, it is clear that on average the trends seen at the weather stations persisted throughout the study zone: a decreasing trend in the annual frequency of the lowest ground-level pressure (GLP) extreme values and an increasing trend in the case of the highest extreme values. On the northern sub-plateau, the trend of the annual frequency of the highest and lowest GLP values has a high level of confidence: more than 99%. On the southern sub-plateau, the confidence level of the trend of the frequency of lowest values is high (greater than 95%); however, that of the frequency of the highest values, although it maintains its sign, is considerably lower (less than 90%). Accordingly, the temporal behaviour of the frequency of the occurrence of the extreme values of AMAGLP along the year is somewhat clearer in the northern than in the southern zone of the SCP. This could be explained in terms of the notion that cyclonic systems arriving at the Iberian Peninsula from the west tend to affect the northern half more than the southern one. For the SCP, the annual frequency trends of the highest and lowest GLP values are significant, with a confidence level greater than 99% in the case of the lowest values and of 90% in the case of the highest values.

7 TRENDS OF EXTREME VALUES OF PRESSURE IN THE IBERIAN PENINSULA 1233 Figure 4. Temporal evolution of the annual frequency of the highest extreme events of AMAGLP (above P 95 ) at each of the weather stations of Castile-Leon. From the foregoing text it may be concluded that the temporal behaviour of the extreme GLP values, as regards the frequency of their annual occurrence, is better defined in the northern part than in the southern zone of the study area, and that the results obtained for the zone in question are fully representative of its behaviour there. Additionally, the annual frequency of the lowest extreme values in the AMAGLP observed in the SCP tended to decrease along the period between 1961 and 2003, whereas the annual frequency of the highest extreme values tended to increase. That is, a tendency to increase is seen for the number of days with very high maximum pressures, while for the number of days per year with very low maximum pressures, it tended to decrease, implying a possible increase in anti-cyclonic situations at the expense of a decrease in cyclonic events along the year. These results are in good agreement with those obtained in studies addressing the temporal behaviour

8 1234 J. L. LABAJO ET AL. Figure 5. The same as Figure 4, at each of the weather stations of Castile-La Mancha and Madrid. of other variables, in particular, the mean annual temperature of the air. A greater annual frequency of anticyclonic events implies an increase in solar radiation reaching the ground along the year, contributing to the increase detected for the air temperature in recent decades at both global (Jones and Moberg, 2003; IPCC, 2007) and regional scale (Forland and Hanssen-Bauer, 2001; Galán et al., 2001). Additionally, the literature consulted appears to be devoid of references to the extreme values of GLP, and there are considerable difficulties involved in obtaining daily data on maximum pressure observations. All this means that the results obtained in the present study cannot be referred to a larger area. Nevertheless, it would be possible to attempt to confirm their validity by comparing them with the behaviour of the North Atlantic Oscillation (NAO) index within a similar time interval. The Spearman correlation coefficient is 0.401, with a level of significance of 99%. These findings point to an increasing trend of the NAO index, which implies an increase in its positive values and a decrease in the negative ones along the time interval considered. This is in agreement with the trends of the annual frequencies of the extreme MAGLP values. Having established the temporal behaviour of the extreme AMAGLP values in the study zone, it became necessary to determine which seasons of the year most affected this kind of behaviour. To do so, for the annual ones the FAMAGLP series for spring (March, April, May), summer (June, July, August), autumn (September, October, November) and winter (December, January, February) were compiled. The trend for each of them was determined and the results are shown in Table IV. From the results (Table IV) it may be deduced that the mean behaviour of the extreme pressure values in winter in both the overall study zone and for each of its two parts governs their annual behaviour. In the rest of the seasonal periods no trend is seen and in some case such as the summer, for the highest extreme values the trend is seen to be the opposite of that observed at other times of the year. The difference in the behaviour of the MAGLP between the northern and southern sub-plateaus may be due to the existence in that period of the year of a thermal drop, mainly affecting the southern part of the Iberian Peninsula.

9 TRENDS OF EXTREME VALUES OF PRESSURE IN THE IBERIAN PENINSULA 1235 Figure 6. Temporal evolution of the regional annual frequency of the highest and lowest extreme events of AMAGLP. Table II. Trends of the FAMAGLP series of values lower or higher than the thresholds established with the P 05 and P 95 percentiles for the stations in the study zone. Stations FAMAGLP < P 05 FAMAGLP > P 95 r s α s Trend r s α s Trend Ávila c <10 4 D c I Burgos NT NT León a D a I Salamanca c D c I Segovia c D c I Soria b D NT Villanubla (Valladolid) c <10 4 D c <10 4 I Zamora b D c <10 4 I Los Llanos (Albacete) c <10 4 D c <10 4 I Ciudad Real NT NT Cuenca c <10 4 D a I Barajas (Madrid) c <10 4 D c <10 4 I Madrid (Retiro) NT NT Toledo NT NT For a level of confidence of 90% or greater: I, increasing; D, decreasing; NT, no significant trend. a Level of confidence 90% and 95% (α 0.10 y α 0.05). b Level of confidence 95% and 99% (α 0.05 y α 0.01). c Level of confidence 99% (α 0.01).

10 1236 J. L. LABAJO ET AL. Table III. Trends of the FAMAGLP series lower or higher than the thresholds defined by the P 05 yp 95 percentiles in the study zone. FAMAGLP < P 05 FAMAGLP > P 95 r s α s Trend r s α s Trend Northern sub-plateau c D c I Southern sub-plateau b D NT Central plateau c D a I For a level of confidence of 90% or greater: I, increasing; D, decreasing; NT, no significant trend. a Level of confidence 90% y 95% (α 0.10 y α 0.05). b Level of confidence 95% y 99% (α 0.05 y α 0.01). c Level of confidence 99% (α 0, 01). Table IV. Trends of seasonal AMAGLP frequencies greater than or lower than the P 05 yp 95 thresholds in the study zone. Zone Percentiles Variables Winter Spring Summer Autumn Northern zone of the central plateau FAMAGLP < P05 r s c α s Trend D NT NT NT FAMAGLP > P95 r s c α s trend I NT NT NT Southern zone of the central plateau FAMAGLP < P05 r s b α s Trend D NT NT NT FAMAGLP > P95 r s b a α s Trend I NT D NT Central plateau FAMAGLP < P05 r s c α s Trend D NT NT NT FAMAGLP > P95 r s b a α s Trend I NT D NT I, increasing; D, decreasing; NT, No significant trend. a Level of confidence 90% and 95% (α 0.10 y α 0.05). b Level of confidence 95% and 99% (α 0.05 y α 0.01). c Level of confidence 99% (α 0, 01). 5. Conclusions From the results obtained in the present study the following conclusions may be drawn. It is possible to accept the series of daily anomalous maximum pressures, obtained as the difference between the observed value of daily maximum pressure and that corresponding to mean daily pressure as being representative of the series of daily maximum pressures measured at the weather stations included in the study. The extreme values corresponding to those of the P 05 yp 95 percentiles are established as the thresholds of the highest and lowest AMAGLP values in each working series. These values are very similar in all cases and range between 7.94 and hpa for the lowest values, and between 6.75 and 9.30 for the highest ones. The FAMAGLP series of all the weather stations studied show high variability, but it should be stressed that there is a clear coincidence in the structure of all of them. The years 1963 and 1996 can be considered as the years in which the frequency of days with very low maximum pressures had the highest maxima. Further, 1983 was the year in which the frequency of days with the highest daily maximum pressures showed an absolute maximum. The most extreme year as regards the variability in daily maximum pressure was 1996, since it had an absolute maximum of FAMAGLPs in the case of the highest extreme values and a relative minimum in the lowest extreme values. This anomaly is seen in almost all the series included in this study. The comparative seasonal study shows that on the SCP and each of its two sub-parts (northern subplateau and southern sub-plateau) between 1961

11 TRENDS OF EXTREME VALUES OF PRESSURE IN THE IBERIAN PENINSULA 1237 and 2003 the trends of the annual frequencies of the occurrence of daily extreme maximum pressure values are determined by the behaviour observed in winter. The other seasonal periods (spring, summer and autumn) do not show any appreciable effect since their temporal behaviour does not show any trend, and in some cases such as the summer, for the highest extreme values the trend is opposite to that observed for other times of the year. From the analysis for the period of the total number of extreme events occurring monthly in the study zone, it may be deduced that the months in which the greatest probability of occurrence of such events are December, January and February (the winter period), which are those with the highest incidence of Atlantic storms over the study zone. The behaviour of MAGLP extreme events in the summer months is affected by the existence of a thermal low that affects the study zone, although mainly in the south. This thermal low is a determinant factor in the change of the sign of the trend of the highest FAMAGLP values on the southern submeseta. Although only 44 years of observation were considered, in view of the agreement of the results obtained with those reported in earlier work referring to mean annual temperatures, it may be accepted that the increasing trend of the mean annual frequency of atmospheric pressure previously reported for Castile and Leon (Pinto et al., 2001) is mainly due to an increase in the annual frequency of the extreme cases in MAGLP and to a decrease in the lowest extreme cases. Furthermore, the results obtained are in agreement with the trend seen for the annual value of the NAO index from 1961 to All the foregoing details suggest that over the past 44 years the trend of the pressure field over the SCP has undergone an increase in intensity and/or annual frequency of anticyclonic situations at the expense of cyclonic events. Acknowledgements This work was carried out within the framework of Research Project REN , funded by the Spanish Ministry of Science and Technology. The authors thank the INM National Institute of Meteorology of Spain for providing the data that allowed this study to be conducted. References Allan RJ, Reason CJC, Carroll P, Jones PD A reconstruction of Madras (Chennai) mean sea level pressure using instrumental records from the late 18 th and early 19th centuries. International Journal of Climatology 9: Bärring L, Jönsson P, Achberger C, Ekström M, Alexandersson H The lund instrumental record of meteorological observations: reconstruction of monthly sea-level pressure International Journal of Climatology 19: Brunetti M, Maugeri M, Nanni T Changes in total precipitation, rainy days and extreme events in northeastern Italy. International Journal of Climatology 21: Domonkos P, Kyselý J, Piotrowicz K, Petrovic P, Likso T Variability of extreme temperature events in south-central europe during the 20th century and its relationship with large-scale circulation. International Journal of Climatology 23: Forland EJ, Hanssen-Bauer I Changes in temperature and precipitation in the norwegian arctic during the 20th century. In Detecting and Modelling Regional Climate Change, Brunet India M, López Bonillo D (eds). Springer-Verlag: Berlin Heidelberg; Galán E, Cañada R, Fernández F, Cervera B Annual temperature evolution in the southern plateau of spain from the construction of regional climate time series. In Detecting and Modelling Regional Climate Change, Brunet India M, López Bonillo D (eds). Springer- Verlag: Berlin Heidelberg; García-Herrera R, Gallego D, Hernández E, Gimeno l, Ribera P, Calvo N Precipitation trends in the canary islands. International Journal of Climatology 23: Gouirand I, Moron V Variability of the impact of the El Niñosouthern oscillation on sea level pressure anomalies over the North Atlantic in January to March ( ). International Journal of Climatology 23: Griffiths GM, Salinger MJ, Leleu I Trends in extreme daily rainfall across the South Pacific and relationship to the South Pacific convergence zone. International Journal of Climatology 23: Hellström C Atmospheric conditions during extreme and nonextreme precipitation events in Sweden. International Journal of Climatology 25: IPCC Cambio climático: ciencia, impactos, adaptación y mitigación. In Principales Conclusiones del Tercer Informe de Evaluación. OficinaEspañola de Cambio Climático. Ministerio de Medio Ambiente, Madrid: Spain. IPCC Climate change 2007: the physical science basis. contribution of Working Group I. Fourth Assessment Report of he Intergubernamental Panel on Climate Change, IPCC Secretariat, WMO: Switzerland. Jones PD, Hulme M Calculating regional climate time series for temperature and precipitation: methods and illustrations. International Journal of Climatology 16: Jones PD, Moberg A Hemispheric and large scale surface air temperature variations: an extensive revision and an update 3 to Journal of Climatology 16: Kadioglu M Trends in surface air temperature data over Turkey. International Journal of Climatology 17: Klein Tanh AMG, Können GP Trends in Indices of daily temperature and precipitation extremes in Europe, Journal of Climatology 16: Kutiel H, Paz S Sea level pressure departures in the mediterranean and their relationship with monthly rainfall conditions in Israel. Theoretical and Applied Climatology 60: Lana X, Serra C, Burgueño A Trends affecting pluviometric indices at the Fabra Observatory (Barcelona, NE Spain) from 1917 to International Journal of Climatology 23: Levene H Contributions to probability and statistics. In Essays in Honour of Harold Hotelling, Olkin I (ed.). Stanford University Press: Stanford-California; Manton MJ, Della-Marta PM, Haylock MR, Hennessy KJ, Nicholls N, Chambers LE, Collins DAA, Daw g, Finet A, Gunawan D, Inape K, Isobe H, Kestin TS, Lefale P, Leyu CH, Lwin T, Maitrepierre L, Ouprasitwong N, Page CM, Pahalad J, Plummer N, Salinger MJ, Suppiah R, Tran VL, Trewin b, Tibig I, Yee D Trends in extreme daily rainfall and temperature in Southeast Asia and the South Pacific: International Journal of Climatology 21: Martín-Vide J, Olcina J Climas y tiempos de España, Alianza Editorial: Madrid. España. Maugeri M, Nanni T Air temperature variations in Italy. Recent trends and an update to Theoretical and Applied Climatology 61: Pinto M, Labajo JL, PIORNO A Atmospheric pressure trends between 1945 and 1994 in Castilla and León (Spain). In Detecting and Modelling Regional Climate Change, Brunet India M, López Bonillo D (eds). Springer: Berlin; Polyakov IV, Berkryaev RV, Alekseev GV, Bhatt US, Colony RL, Johnson MA, Maskshtas AP, Walsh D Variability and trends of air temperature and pressure in the maritime artic, Journal of Climatology 16:

12 1238 J. L. LABAJO ET AL. Salinger MJ, Griffiths GM Trends in New Zealand daily temperature and rainfall extremes. International Journal of Climatology 21: Sneyers R Sobre el análisis estadístico de las series de observaciones. OMM, Nota Técnica 143, OMM-N 415, 192. Shouraseni SR, Balling RC, JR Trends in extreme daily precipitation indices in India. International Journal of Climatology 24: Suppiah R, Hennessy KJ Trends in total rainfall, heavy rain events and number of days in Australia, International Journal of Climatology 10: Zangvil A, Karas S, Sasson A Connection between Eastern Mediterranean seasonal mean 500 hpa height and sea-level pressure patterns and the spatial rainfall distribution over Israel. International Journal of Climatology 23: Zveryaev II Decadal and longer changes of the winter sea level pressure fields and related synoptic activity over the North Atlantic. International Journal of Climatology 11: