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Analysis of recent climatic variations in Castile and Leon (Spain)
Atmospheric Research 73 (2005) 69 – 85 www.elsevier.com/locate/atmos
Analysis of recent climatic variations in Castile and Leon (Spain) ´ ngel Penasa, Roberto Fraileb,* Sara del Rı´oa, A a
Departamento de Biologı´a Vegetal (A´rea de Bota´nica), Fac. Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, Campus de Vegazana s/n, E-24071 Leon, Spain b Departamento de Fı´sica, Fac. Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, Campus de Vegazana s/n, E-24071 Leon, Spain Received 1 December 2003; received in revised form 1 March 2004; accepted 10 June 2004
Abstract This paper reports the results of the analysis of annual mean temperature and precipitation series from 171 meteorological stations distributed over Castile and Leon [Castilla y Leo´n in Spanish] in Spain on monthly, seasonal and annual time-scales for a 37-year study period (1961–1997). Various statistical tools were used to detect and characterize significant changes in these series. The magnitude of the trends was derived from the slopes of the regression lines using the least squares method, and the statistical significance was determined by means of nonparametric tests. Positive trends of about 0.33 8C in the annual mean temperature were found for the whole period. Mean temperatures increased in spring and winter, the winter trend being statistically significant. The months of December and March also showed significant trends. Decreases in rainfall were found for three seasons (winter, spring and autumn), with statistically significant trends in March. Summer precipitation showed slight increases over the 37-year period. On this basis, the authors consider that the increase in summer precipitation and the decrease in the range of average temperatures between the warmest and the coldest months of the year (continentality), point towards a trend to a more oceanic climate in Castile and Leon. D 2004 Elsevier B.V. All rights reserved. Keywords: Spain; Climate change; Temperature series; Precipitation series; Trends
* Corresponding author. Tel.: +34 987291543; fax: +34 987291945. E-mail address: [email protected] (R. Fraile). 0169-8095/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2004.06.005
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1. Introduction Climate change over the last century is a subject of great topical interest. This problem worries the scientific community, as it could have a major impact on natural and social systems at local, regional and national scales. Numerous climatologists (Jones et al., 1999; Parker and Horton, 1999; IPCC, 2001; Jones and Moberg, 2003; Vinnikov and Grody, 2003) agree that there has been a large-scale warming of the Earth’s surface over the last hundred years or so. This warming up of the Earth during the 20th century brought with it a decrease in the area of the world affected by exceptionally cool temperatures, and, to a lesser extent, an increase in the area affected by exceptionally warm temperatures (Jones et al., 1999). Some analyses of long time-series of temperatures on a hemispheric and global scale (IPCC, 2001) have indicated a warming rate of 0.3–0.6 8C since the mid-19th century, due to either anthropogenic causes (IPCC, 2001) or astronomic causes (Soon et al., 2000a,b; Landscheidt, 2000). The Third Assessment Report projections for the present century are that average temperature rises by 2100 would be in the range of 1.4–5.8 8C (IPCC, 2001). In Europe, Parry (2000) noted that mean annual temperatures rose by about 0.8 8C during the 20th century, with the last decade (1990–1999) being the warmest on record, both annually and for the winter season. Annual temperatures over Europe are rising at a rate of between 0.1 and 0.4 8C/decade. This warming is greatest in Southern Europe (Spain, Italy, Greece) and North-Eastern Europe (Finland, Western Russia) and least along the Atlantic coastline of the continent. For the Iberian Peninsula, results seem to indicate an increase in the annual mean temperature (Hulme and Sheard, 1999; Parry, 2000) of about 1.6 8C over the last hundred years. The highest increases were found in summer (approximately 2 8C) and the lowest in winter. There is strong evidence that rainfall changes associated to global warming are already taking place on both global and regional scales (Forland et al., 1996; Scho¨nwiese and Rapp, 1997; Hulme et al., 1998; Rodrı´guez-Puebla et al., 1998; Trenberth, 1998; Dohetry et al., 1999; Osborn et al., 2000; IPCC, 2001). The trend was globally positive throughout the 20th century, although large areas were characterized by negative trends (IPCC, 2001). They have demonstrated that during recent decades precipitation has tended to increase in the mid-latitudes, decrease in the Northern Hemisphere subtropical zones, and increase generally throughout the Southern Hemisphere. However, these large-scale occurrences incorporate considerable spatial variability. If the trends observed persist, the general pattern of future changes in annual precipitation over Europe will be for widespread increases in Northern Europe (Forland et al., 1996; Scho¨nwiese and Rapp, 1997) and decreases in Southern Europe (Scho¨nwiese and Rapp, 1997; Buffoni et al., 1999; Piervitali et al., 1997; Brunetti et al., 2000, 2001). In summer, an increasing differential will be seen between Northern Europe (becoming more humid by up to +2%/decade) and Southern Europe (becoming drier by up to 5%/decade) (Parry, 2000; Ventura et al., 2002). For the Iberian Peninsula, the annual precipitation will decrease (Esteban-Parra et al., 1998; Labajo et al., 1998; Hulme and Sheard, 1999; Labajo and Piorno Herna´ndez, 1999; IPCC, 2001; Mossmann, 2002), the greatest decreases coming in summer, while winters will get wetter.
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Few studies have investigated climate on a local or regional scale using the full suite of locations observed. New (1999) points out that the effects of errors and lack of consistency at individual stations are reduced when averaging data to calculate the regional series. Hence, 171 meteorological stations located in Castile and Leon, representative of the whole region, were studied. The region of Castile and Leon [Castilla y Leo´n in Spanish] is located in the NorthWest of Spain and has an area of 94,193 km2, i.e. about one fifth of the whole country. ´ vila, Burgos, Leo´n, Palencia, There are nine different provinces in the region: A Salamanca, Segovia, Soria, Valladolid and Zamora (Fig. 1). Except to the West, Castile and Leon is surrounded by large mountain ranges. These mountain ranges act as a barrier to maritime influences, giving most of the region a continental climate with long cold winters and hot summers. Winter is the rainiest season of the year and most of the region suffers periods of drought in summer (Garcı´a Ferna´ndez, 1986; Font Tullot, 2000).
Fig. 1. Meteorological stations used in this study and Castile and Leon provinces.
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The aims of this study are: !
! !
to determine recent climate variations in mean temperature and annual rainfall on monthly, seasonal and annual time-scales in Castile and Leon, using a range of statistical techniques. Specifically, the direction, magnitude and statistical significance of trends were addressed; to compare the results with other studies on climate change in the Iberian Peninsula; to highlight the fact that seasonal patterns of precipitation and temperature are as important as the absolute figures for these, or even more so. Thus, a knowledge of seasonal patterns is crucial in establishing changes in land uses, ecosystems, crop yields, water resources, and so forth.
2. Methodology and data 2.1. Data set Monthly mean temperature and precipitation series from 171 weather stations covering a period of 37 years (1961–1997) were analyzed for this study. These data were provided by the Spanish National Meteorological Service [Instituto Nacional de Meteorologı´a (INM)]. They were selected according to the length and completeness of records, so as to provide a reasonable spatial coverage over much of Castile and Leon. In order to ensure the reliability of the data, only those weather stations with temperature series that followed a normal distribution were selected. Thus, of the 255 stations considered initially, only 171 were used for this study. This condition was not required in the case of precipitation series, since they do not follow a normal distribution. As all series were complete, no gap filling method was needed. Fig. 1 shows the locations of these weather stations. For each station, monthly values were averaged (for temperature and precipitation, respectively) to obtain seasonal and annual values. Seasons were defined using the standard meteorological definition (i.e. winter=December; January, February; spring=March, April, May; summer=June, July, August and autumn=September, October, November). The next step was to set up new mean temperature and precipitation series on monthly, seasonal and annual time-scales for all 171 stations and for the 37-year period. This is because the analysis of various observations from each station does provide an insight into the climatic trends for those specific locations, but a single station cannot be used to represent the entire climatic region. Hence, monthly, seasonal and annual temperature and precipitation series representing the region of Castile and Leon were used. It was found that seasonal and annual temperature distributions were still normal, and that the precipitation series kept the form of a gamma distribution, but with a different skewness. In order to assess the homogeneity of the regional series, the nonparametric Kruskal– Wallis test (Essenwanger, 1986) was applied. Among the factors affecting homogeneity are changes in the observers, the equipment or the location of a weather station. All of these can have a profound effect on the data, and, therefore, on the results of any study. The homogeneity of the series was guaranteed by applying the Kruskal–Wallis test.
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2.2. Trend analysis Two complementary statistical techniques have been used to detect variations in temperature and precipitation: nonparametric and linear models. As noted by several authors (Yu and Neil, 1993; Suppiah and Hennessy, 1996), more information can be obtained from a combination of such techniques. In the present study, the magnitude of the trends was derived from the slope of the regression line using the least squares method, while the statistical significance was determined by the Spearman and Mann–Kendall tests (Sneyers, 1990). Both measures are based on rank correlation. In precipitation and temperature studies, the moving average or running mean is a conventional procedure used to reduce the inter-annual variability of the series (Sneyers, 1992). A 5-year running average was used in this study. The Spearman coefficient, r s, is the correlation coefficient of the linear regression between the series i and y i and it is obtained from the expression: h X i. ð yi i Þ 2 rs ¼ 1 6 n n2 1 ; where n is the number of data items in the series and i is the order of the elements in the original series. The distribution of r s tends towards a normal distribution (bell curve) with a mean of zero. In order to examine whether the null hypothesis, that there is no trend, can be rejected or not, it is necessary to calculate the probability a ¼ PðjujNðjuðrs ÞjÞ with uðrs Þ ¼ rs ðn 1Þ1=2 : This is calculated using a table of reduced normal distribution. If aba 0, the null hypothesis is rejected for a significance level of a 0. If a trend is detected, it will be an increasing or decreasing trend, depending on whether r sN0 or r sb0. The Mann–Kendall test is used in climate studies because it has major advantages, among which are the following: –
–
no assumptions about distribution are necessary, although it was found that the temperature distribution was normal and that the precipitation distribution followed a gamma distribution, approximately. it is directly applicable to climate data for a given month or season.
For each element x i (i=1, . . ., n), the number of lower elements x j (x j bx i ) preceding it ( jbi) is calculated and the statistical parameter t is given by: X ti ¼ ni : The distribution of the test statistic t under the null hypothesis has an expected value E(t) and variance u(t) such that: Eðt Þ ¼ nðn 1Þ=4 u2 ¼ nðn 1Þð2n 5Þ=72
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The hypothesis is rejected for |u(t)|N1.96 with a statistical significance of 5% and with: 1=2 : uðt Þ ¼ t Eðt Þ= u2 ðt Þ Hence, trend analyses on monthly, seasonal and annual time-scales were performed in order to detect changes in the temperature and precipitation series for Castile and Leon.
3. Results and discussion 3.1. Precipitation trend analysis Fig. 2 shows monthly, seasonal and annual precipitation trends for Castile and Leon over the period 1961–1997 using a linear regression model. The rate of change is described by the slope of the regression line. Table 1 shows the results of applying the Mann–Kendall and Spearman tests, the linear trend for the slope value, and the total trend for all 37 years of the study period on different time-scales. It is striking that the application of the Mann–Kendall and Spearman tests to the various series should yield an agreement between the two tests. Considerable variability was noted between the different years, with a standard deviation of 110 mm, while average annual precipitation was 644 mm. The year 1991 was the driest in the period of study (480 mm). This result concurs with those noted in the Spanish Environment Ministry’s publication Segunda Comunicacio´n Nacional de Espan˜a del Clima (Ministerio de Medio Ambiente, 1997) which indicates that Spain underwent a period of drought beginning in the 1980s and going on to the early 1990s, with 1991 yielding values below the minima for the reference period (1961–1990). In contrast, 1966, with 849 mm, was the rainiest year in the period studied. These extreme values have been fitted to the Gumbel distribution. Return periods of almost 20 years were found for the maximum precipitation, whereas the return period for the driest year was of only 12 years. It has to be noted that these time spans represent approximately one half and one third of the study period, respectively. The annual trend in Castile and Leon is towards a drier climate with an estimated decrease of about 0.91 mm/year (5.23% decline on average over the 37-year period 1961– 1997), although this series was not statistically significant after the application of the Mann–Kendall and Spearman tests. The tendency towards decreasing annual rainfall ties in with the results found by a number of authors that reflect a general trend towards a decrease in rain rates over the Iberian Peninsula (Karas, 1997; Hulme and Sheard, 1999; Parry, 2000; Mossmann, 2002). As numerous authors have pointed out (Zorita et al., 1992; Hurrell, 1995; Corte-Real et al., 1995; Jones et al., 1997; Rodo´ et al., 1997; Rodrı´guezPuebla et al., 1999; Esteban-Parra et al., 1998; Watson et al., 1998; Rodrigo et al., 2000; Goodess and Jones, 2002), this fact seems to be related to the links between rainfall rates and the North Atlantic Oscillation (NAO). The positive phase of the NAO is related to lower precipitation rates over Southern Europe. On a seasonal time-scale, the general trend is towards decreasing precipitation in three out of four seasons in the year (winter, spring and autumn), although these trends are not
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Fig. 2. Five-year moving averages of monthly, seasonal and yearly rainfall and their linear trends for Castile and Leon region. 75
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Fig. 2 (continued).
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Table 1 Monthly, seasonal and annual results of Mann–Kendall and Spearman tests and of least square linear fitting for Castile and Leon rainfall over the period 1961–1997 Slope (mm/year)
u(t)
rs
u(r s)
Standard error
Months December January February March April May June July August September October November
Positive Negative Negative 1.2 Negative Positive Negative Positive Positive Negative Positive Negative
0.68 0.86 1.25 2.35* 0.39 1.83 0.81 0.83 1.62 0.65 0.39 0.57
0.133 0.156 0.2 0.377 0.08 0.294 0.099 0.166 0.269 0.11 0.078 0.069
0.80 0.94 1.20 2.26* 0.48 1.76 0.59 1.00 1.61 0.66 0.47 0.41
– – – 5.4 – – – – – – – –
Seasons Winter Spring Summer Autumn Year
Negative Negative Positive Negative Negative (0.91)
0.78 0.28 0.26 0.07 0.23
0.106 0.079 0.05 0.005 0.075
0.64 0.47 0.30 0.03 0.45
– – – – –
u(t), r s, u(r s) statistics and parameters of Spearman and Mann–Kendall. * Significant trends at a 95% significance level.
statistically significant. This appears to be due to inter-annual variability and differing trends in individual months that make up the various seasons. The greatest decrease in precipitation over the year occurred in winter (0.596 mm/ year). This is in accordance with the results put forward by other authors (Zorita et al., 1992; Hurrell, 1995; Wanner et al., 1997; Va´zquez Lo´pez, 1999; Go´mez Navarro et al., 1999), showing a negative correlation in winter between the NAO and precipitation in Southern Europe. Fraile et al. (2004) have found significant correlations between the NAO index and precipitation in Spain. This implies that a high NAO index causes frequent anticyclones and dry weather over wide areas of Southern Europe. Over the Iberian Peninsula, this is associated to an intensification of the Azores high-pressure system over the western Iberian Peninsula that acts to block Atlantic weather from this territory. An analysis of the summer series showed a nonsignificant positive trend in this season for the period under consideration (0.481 mm/year). It seems likely that this positive trend will have been caused by an increase in storms over the study zone, as convection is the main cause of rain in Castile and Leon during the summer. To detect the month or months that had the greatest influence on seasonal precipitation, a monthly trend analysis was carried out. It was observed that rainfall increased in December, May, July, August and October, with the highest increases in December (0.98 mm/year). Rainfall decreased in the other months of the year, the greatest decreases being those for February (1.14 mm/year) and March (1.24 mm/year). Other studies (Mossmann,
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2002) have also noted an increase in the areas affected by negative trends in February over many areas in the Iberian Peninsula. The first statistically significant negative trend in the year was observed in March because rainfall in this month is mainly caused by Atlantic fronts. Mossmann (2002) has already confirmed that cyclonic situations have recently been decreasing across the Iberian Peninsula in March, implying a trend towards less rainfall in this month. The positive trend observed for July and August should be noted. This result is in accordance with the study of Mossmann et al. (2004) about precipitation in the Iberian Peninsula. In this work, the authors show there is a significant positive trend for these months with increases of about 0.3 mm/year for July and 0.2 mm/year for August in the period 1961–1990. The analysis of the July and August series yielded results for Castile and Leon of a similar order of magnitude: 0.2 mm/year for July and 0.4 mm/year in August. Some of the results relating to the analysis of precipitation trends obtained in this study do not accord with other climate change studies carried out for Europe and the Iberian Peninsula (Hulme et al., 1999), which found increased rainfall in winter and decreases in spring, autumn, and especially in summer. These differences may be explained by taking into account the fact that local or regional factors (topography, orography, geography and land use, among others) are essential in gaining an understanding of variations in local and regional climates. 3.2. Temperature trend analysis Fig. 3 shows monthly, seasonal and annual mean temperature trends for Castile and Leon over the period 1961–1997 using a linear regression. As in the case of precipitation trends, the rate of change is described by the slope of the regression line. Table 2 shows the results of the Mann–Kendall and Spearman tests, the slope value, and the total increase for all 37 years of the study period on different time-scales. The agreement between the results of the application of the Spearman and Mann–Kendall tests should be emphasized. With respect to temperature, the variability between the different years was less than with precipitation data, the standard deviation being 0.54 8C, and the average annual temperature 11.17 8C. The year 1997, with 12.21 8C, was the warmest year in the whole period. This result is in agreement with The Third Assessment of the Intergovernmental Panel on Climate Change (IPCC, 2001), which concluded that the 1990s were the decade with the highest temperatures since 1861. The years 1998 and 1997 were the first and second warmest years in the period. Various authors (Quereda Sala et al., 1999; Jones et al., 1999; Parry, 2000) have found similar results, pointing out that the warmest years on record were during the 1990s and noting that the four hottest years in decreasing order were 1998, 1997, 1995 and 1990. At the other extreme, the data being discussed here indicate that 1972, with 10.07 8C, was the coldest year in the period analyzed. The observed annual trend towards a warmer climate, with an estimated increase of some 0.009 8C/year (0.33 8C over the whole period considered) is not statistically significant. Such a rise in annual mean temperature of about 0.1 8C/decade is in
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Fig. 3. Five-year moving averages of monthly, seasonal and yearly temperature and their linear trends for Castile and Leon region. 79
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Fig. 3 (continued).
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Table 2 Monthly, seasonal and annual results of Mann–Kendall and Spearman tests and of least square linear fitting for Castile and Leon temperature over the period 1961–1997 Slope (8C/year)
u(t)
rs
u(r s)
Standard error
Months December January February March April May June July August September October November
0.07 Positive Positive 0.05 Negative Negative Negative Negative Positive Negative Negative Positive
3.19* 0.91 0.75 2.24* 0.68 0.41 0.39 0.39 0.41 1.04 0.62 1.72
0.522 0.75 0.12 0.374 0.076 0.055 0.068 0.043 0.069 0.179 0.098 0.304
3.13* 0.92 0.72 2.24* 0.46 0.33 0.41 0.26 0.41 1.07 0.59 1.82
0.24 – – 0.24 – – – – – – – –
Seasons Winter Spring Summer Autumn Year
0.03 Positive Negative Negative Positive (0.009)
3.11* 0.86 0.39 0.05 1.49
0.486 0.115 0.045 0.009 0.189
2.92* 0.69 0.27 0.05 1.13
0.13 – – – –
u(t), r s, u(r s) statistics and parameters of Spearman and Mann–Kendall. * Significant trends at a 95% significance level.
accordance with what has been reported by several authors (Hulme and Sheard, 1999; Jones et al., 1999; Parry, 2000), who have noted an increase of between 0.1 and 0.4 8C each decade in Europe. On a seasonal time-scale, temperatures have risen in both winter (0.034 8C/year) and spring (0.009 8C/year). The application of the Mann–Kendall and Spearman tests demonstrated a statistically significant positive trend in winter, because most of the increases in monthly temperature were to be found in this season. As various authors have pointed out (Hurrell, 1995; Easterling et al., 1997; Va´zquez Lo´pez, 1999; Go´mez Navarro et al., 1999; Brunetti et al., 2000; Marshall et al., 2001; Shabbar et al., 2001; Chmielewski and Ro¨tzer, 2001; Watson et al., 1998), this significant increase seems to be related to the influence of the positive NAO phase over the Northern Hemisphere in winter. It seems likely that the positive trend found in winter and early spring may be causing alterations in the length of seasons, with consequential effects on aspects such as production, and the composition and distribution of animal and plant communities, which are crucial in determining the economic impact. On the other hand, slight decreases were noted for autumn (0.001 8C a year) and summer (0.002 8C a year), which do not concur with other studies of climate change in the Iberian Peninsula (Hulme et al., 1999) indicating a considerable increase in summer temperatures. This seems to confirm that the results of climate change studies depend on the time-scale used and on the period taken into consideration.
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As was done for precipitation trends, a monthly analysis was carried out so as to detect the month or months having the greatest influence on the seasonal temperature. On a monthly time-scale, it was observed that average temperatures have risen for December, January, February, March, August and November, with the greatest increases in December. The statistically significant positive trend observed for winter is due to the fact that all the months in this season showed an increase in mean temperature, especially December. The results reported here for winter and early spring confirm findings by other authors (Hurrell, 1995; Rodwell et al., 1999; Chmielewski and Ro¨tzer, 2001) with respect to the influence of the positive NAO phase from January to April. This tends to be associated with prevalently westerly winds and with the advection of warmer and more frequent anticyclones over Southern Europe, causing high mean temperatures in these months. No statistically significant negative trends for the rest of the year were observed. The largest increases have been noted in November. Before closing this section, it is necessary to indicate that the trends found have not been influenced by grouping together and averaging the data from the different weather stations used. We have not carried out a probabilistic study of the influence of the partial grouping of weather stations, although it had been initially decided to do this separately for each of the nine provinces in the region. However, it was observed that the trends remain constant considering the region as a whole. Similarly, it had been decided that the trends should be studied according to the climatic subregions: none of the three climatic subregions in Castile and Leon showed variations with respect to the general trends though. These facts provide enough evidence to claim that, with the exception of very few weather stations, the general trend for temperature and precipitation is in accordance with the patterns described above.
4. Conclusions A number of conclusions may be drawn from the present analysis of precipitation and temperature data from 171 meteorological stations over a 37-year study period: In Castile and Leon, temperature and precipitation show divergent trends on an annual and seasonal time-scale (increasing and decreasing, respectively), with a general trend towards a warmer and drier climate in the latter part of the twentieth century. ! It is likely that the rise in the annual mean temperature has largely been brought about by the significant increases in winter temperatures resulting from the influence of the positive NAO phase. The absence of statistical significance for most seasons is an outcome of the different tendencies of individual months in these seasons. ! A variation in the rainfall regime in Castile and Leon was noted over the study period, with a change from a winterNspringNautumnNsummer model to winterNautumnN springNsummer. This is because the decrease observed in spring rainfall was about twice as great as that recorded in autumn. ! Although there is no significant evidence to allow certainty as to whether the trends detected will vary or remain steady in the future, it does seem that the climate of Castile and Leon has become less continental, due to the increases observed in summer !
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!
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precipitation and the decreases in the range between the average temperatures of the warmest and coldest months of the year over the period analyzed. It should be emphasized how crucial it is to work with local and regional time-scales so as to confirm the real effects of any possible climatic change. This is a consequence of the great impact of local and regional factors on a territory’s climate.
Acknowledgements The authors wish to express their thanks to Dr. Noelia Ramo´n for the translation of this paper into English and to the INM for kindly making data sets available to them. We also thank two anonymous referees for their helpful comments on the original manuscript.
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Analysis of recent climatic variations in Castile and Leon (Spain) ´ ngel Penasa, Roberto Fraileb,* Sara del Rı´oa, A a
Departamento de Biologı´a Vegetal (A´rea de Bota´nica), Fac. Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, Campus de Vegazana s/n, E-24071 Leon, Spain b Departamento de Fı´sica, Fac. Ciencias Biolo´gicas y Ambientales, Universidad de Leo´n, Campus de Vegazana s/n, E-24071 Leon, Spain Received 1 December 2003; received in revised form 1 March 2004; accepted 10 June 2004
Abstract This paper reports the results of the analysis of annual mean temperature and precipitation series from 171 meteorological stations distributed over Castile and Leon [Castilla y Leo´n in Spanish] in Spain on monthly, seasonal and annual time-scales for a 37-year study period (1961–1997). Various statistical tools were used to detect and characterize significant changes in these series. The magnitude of the trends was derived from the slopes of the regression lines using the least squares method, and the statistical significance was determined by means of nonparametric tests. Positive trends of about 0.33 8C in the annual mean temperature were found for the whole period. Mean temperatures increased in spring and winter, the winter trend being statistically significant. The months of December and March also showed significant trends. Decreases in rainfall were found for three seasons (winter, spring and autumn), with statistically significant trends in March. Summer precipitation showed slight increases over the 37-year period. On this basis, the authors consider that the increase in summer precipitation and the decrease in the range of average temperatures between the warmest and the coldest months of the year (continentality), point towards a trend to a more oceanic climate in Castile and Leon. D 2004 Elsevier B.V. All rights reserved. Keywords: Spain; Climate change; Temperature series; Precipitation series; Trends
* Corresponding author. Tel.: +34 987291543; fax: +34 987291945. E-mail address: [email protected] (R. Fraile). 0169-8095/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2004.06.005
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1. Introduction Climate change over the last century is a subject of great topical interest. This problem worries the scientific community, as it could have a major impact on natural and social systems at local, regional and national scales. Numerous climatologists (Jones et al., 1999; Parker and Horton, 1999; IPCC, 2001; Jones and Moberg, 2003; Vinnikov and Grody, 2003) agree that there has been a large-scale warming of the Earth’s surface over the last hundred years or so. This warming up of the Earth during the 20th century brought with it a decrease in the area of the world affected by exceptionally cool temperatures, and, to a lesser extent, an increase in the area affected by exceptionally warm temperatures (Jones et al., 1999). Some analyses of long time-series of temperatures on a hemispheric and global scale (IPCC, 2001) have indicated a warming rate of 0.3–0.6 8C since the mid-19th century, due to either anthropogenic causes (IPCC, 2001) or astronomic causes (Soon et al., 2000a,b; Landscheidt, 2000). The Third Assessment Report projections for the present century are that average temperature rises by 2100 would be in the range of 1.4–5.8 8C (IPCC, 2001). In Europe, Parry (2000) noted that mean annual temperatures rose by about 0.8 8C during the 20th century, with the last decade (1990–1999) being the warmest on record, both annually and for the winter season. Annual temperatures over Europe are rising at a rate of between 0.1 and 0.4 8C/decade. This warming is greatest in Southern Europe (Spain, Italy, Greece) and North-Eastern Europe (Finland, Western Russia) and least along the Atlantic coastline of the continent. For the Iberian Peninsula, results seem to indicate an increase in the annual mean temperature (Hulme and Sheard, 1999; Parry, 2000) of about 1.6 8C over the last hundred years. The highest increases were found in summer (approximately 2 8C) and the lowest in winter. There is strong evidence that rainfall changes associated to global warming are already taking place on both global and regional scales (Forland et al., 1996; Scho¨nwiese and Rapp, 1997; Hulme et al., 1998; Rodrı´guez-Puebla et al., 1998; Trenberth, 1998; Dohetry et al., 1999; Osborn et al., 2000; IPCC, 2001). The trend was globally positive throughout the 20th century, although large areas were characterized by negative trends (IPCC, 2001). They have demonstrated that during recent decades precipitation has tended to increase in the mid-latitudes, decrease in the Northern Hemisphere subtropical zones, and increase generally throughout the Southern Hemisphere. However, these large-scale occurrences incorporate considerable spatial variability. If the trends observed persist, the general pattern of future changes in annual precipitation over Europe will be for widespread increases in Northern Europe (Forland et al., 1996; Scho¨nwiese and Rapp, 1997) and decreases in Southern Europe (Scho¨nwiese and Rapp, 1997; Buffoni et al., 1999; Piervitali et al., 1997; Brunetti et al., 2000, 2001). In summer, an increasing differential will be seen between Northern Europe (becoming more humid by up to +2%/decade) and Southern Europe (becoming drier by up to 5%/decade) (Parry, 2000; Ventura et al., 2002). For the Iberian Peninsula, the annual precipitation will decrease (Esteban-Parra et al., 1998; Labajo et al., 1998; Hulme and Sheard, 1999; Labajo and Piorno Herna´ndez, 1999; IPCC, 2001; Mossmann, 2002), the greatest decreases coming in summer, while winters will get wetter.
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Few studies have investigated climate on a local or regional scale using the full suite of locations observed. New (1999) points out that the effects of errors and lack of consistency at individual stations are reduced when averaging data to calculate the regional series. Hence, 171 meteorological stations located in Castile and Leon, representative of the whole region, were studied. The region of Castile and Leon [Castilla y Leo´n in Spanish] is located in the NorthWest of Spain and has an area of 94,193 km2, i.e. about one fifth of the whole country. ´ vila, Burgos, Leo´n, Palencia, There are nine different provinces in the region: A Salamanca, Segovia, Soria, Valladolid and Zamora (Fig. 1). Except to the West, Castile and Leon is surrounded by large mountain ranges. These mountain ranges act as a barrier to maritime influences, giving most of the region a continental climate with long cold winters and hot summers. Winter is the rainiest season of the year and most of the region suffers periods of drought in summer (Garcı´a Ferna´ndez, 1986; Font Tullot, 2000).
Fig. 1. Meteorological stations used in this study and Castile and Leon provinces.
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The aims of this study are: !
! !
to determine recent climate variations in mean temperature and annual rainfall on monthly, seasonal and annual time-scales in Castile and Leon, using a range of statistical techniques. Specifically, the direction, magnitude and statistical significance of trends were addressed; to compare the results with other studies on climate change in the Iberian Peninsula; to highlight the fact that seasonal patterns of precipitation and temperature are as important as the absolute figures for these, or even more so. Thus, a knowledge of seasonal patterns is crucial in establishing changes in land uses, ecosystems, crop yields, water resources, and so forth.
2. Methodology and data 2.1. Data set Monthly mean temperature and precipitation series from 171 weather stations covering a period of 37 years (1961–1997) were analyzed for this study. These data were provided by the Spanish National Meteorological Service [Instituto Nacional de Meteorologı´a (INM)]. They were selected according to the length and completeness of records, so as to provide a reasonable spatial coverage over much of Castile and Leon. In order to ensure the reliability of the data, only those weather stations with temperature series that followed a normal distribution were selected. Thus, of the 255 stations considered initially, only 171 were used for this study. This condition was not required in the case of precipitation series, since they do not follow a normal distribution. As all series were complete, no gap filling method was needed. Fig. 1 shows the locations of these weather stations. For each station, monthly values were averaged (for temperature and precipitation, respectively) to obtain seasonal and annual values. Seasons were defined using the standard meteorological definition (i.e. winter=December; January, February; spring=March, April, May; summer=June, July, August and autumn=September, October, November). The next step was to set up new mean temperature and precipitation series on monthly, seasonal and annual time-scales for all 171 stations and for the 37-year period. This is because the analysis of various observations from each station does provide an insight into the climatic trends for those specific locations, but a single station cannot be used to represent the entire climatic region. Hence, monthly, seasonal and annual temperature and precipitation series representing the region of Castile and Leon were used. It was found that seasonal and annual temperature distributions were still normal, and that the precipitation series kept the form of a gamma distribution, but with a different skewness. In order to assess the homogeneity of the regional series, the nonparametric Kruskal– Wallis test (Essenwanger, 1986) was applied. Among the factors affecting homogeneity are changes in the observers, the equipment or the location of a weather station. All of these can have a profound effect on the data, and, therefore, on the results of any study. The homogeneity of the series was guaranteed by applying the Kruskal–Wallis test.
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2.2. Trend analysis Two complementary statistical techniques have been used to detect variations in temperature and precipitation: nonparametric and linear models. As noted by several authors (Yu and Neil, 1993; Suppiah and Hennessy, 1996), more information can be obtained from a combination of such techniques. In the present study, the magnitude of the trends was derived from the slope of the regression line using the least squares method, while the statistical significance was determined by the Spearman and Mann–Kendall tests (Sneyers, 1990). Both measures are based on rank correlation. In precipitation and temperature studies, the moving average or running mean is a conventional procedure used to reduce the inter-annual variability of the series (Sneyers, 1992). A 5-year running average was used in this study. The Spearman coefficient, r s, is the correlation coefficient of the linear regression between the series i and y i and it is obtained from the expression: h X i. ð yi i Þ 2 rs ¼ 1 6 n n2 1 ; where n is the number of data items in the series and i is the order of the elements in the original series. The distribution of r s tends towards a normal distribution (bell curve) with a mean of zero. In order to examine whether the null hypothesis, that there is no trend, can be rejected or not, it is necessary to calculate the probability a ¼ PðjujNðjuðrs ÞjÞ with uðrs Þ ¼ rs ðn 1Þ1=2 : This is calculated using a table of reduced normal distribution. If aba 0, the null hypothesis is rejected for a significance level of a 0. If a trend is detected, it will be an increasing or decreasing trend, depending on whether r sN0 or r sb0. The Mann–Kendall test is used in climate studies because it has major advantages, among which are the following: –
–
no assumptions about distribution are necessary, although it was found that the temperature distribution was normal and that the precipitation distribution followed a gamma distribution, approximately. it is directly applicable to climate data for a given month or season.
For each element x i (i=1, . . ., n), the number of lower elements x j (x j bx i ) preceding it ( jbi) is calculated and the statistical parameter t is given by: X ti ¼ ni : The distribution of the test statistic t under the null hypothesis has an expected value E(t) and variance u(t) such that: Eðt Þ ¼ nðn 1Þ=4 u2 ¼ nðn 1Þð2n 5Þ=72
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The hypothesis is rejected for |u(t)|N1.96 with a statistical significance of 5% and with: 1=2 : uðt Þ ¼ t Eðt Þ= u2 ðt Þ Hence, trend analyses on monthly, seasonal and annual time-scales were performed in order to detect changes in the temperature and precipitation series for Castile and Leon.
3. Results and discussion 3.1. Precipitation trend analysis Fig. 2 shows monthly, seasonal and annual precipitation trends for Castile and Leon over the period 1961–1997 using a linear regression model. The rate of change is described by the slope of the regression line. Table 1 shows the results of applying the Mann–Kendall and Spearman tests, the linear trend for the slope value, and the total trend for all 37 years of the study period on different time-scales. It is striking that the application of the Mann–Kendall and Spearman tests to the various series should yield an agreement between the two tests. Considerable variability was noted between the different years, with a standard deviation of 110 mm, while average annual precipitation was 644 mm. The year 1991 was the driest in the period of study (480 mm). This result concurs with those noted in the Spanish Environment Ministry’s publication Segunda Comunicacio´n Nacional de Espan˜a del Clima (Ministerio de Medio Ambiente, 1997) which indicates that Spain underwent a period of drought beginning in the 1980s and going on to the early 1990s, with 1991 yielding values below the minima for the reference period (1961–1990). In contrast, 1966, with 849 mm, was the rainiest year in the period studied. These extreme values have been fitted to the Gumbel distribution. Return periods of almost 20 years were found for the maximum precipitation, whereas the return period for the driest year was of only 12 years. It has to be noted that these time spans represent approximately one half and one third of the study period, respectively. The annual trend in Castile and Leon is towards a drier climate with an estimated decrease of about 0.91 mm/year (5.23% decline on average over the 37-year period 1961– 1997), although this series was not statistically significant after the application of the Mann–Kendall and Spearman tests. The tendency towards decreasing annual rainfall ties in with the results found by a number of authors that reflect a general trend towards a decrease in rain rates over the Iberian Peninsula (Karas, 1997; Hulme and Sheard, 1999; Parry, 2000; Mossmann, 2002). As numerous authors have pointed out (Zorita et al., 1992; Hurrell, 1995; Corte-Real et al., 1995; Jones et al., 1997; Rodo´ et al., 1997; Rodrı´guezPuebla et al., 1999; Esteban-Parra et al., 1998; Watson et al., 1998; Rodrigo et al., 2000; Goodess and Jones, 2002), this fact seems to be related to the links between rainfall rates and the North Atlantic Oscillation (NAO). The positive phase of the NAO is related to lower precipitation rates over Southern Europe. On a seasonal time-scale, the general trend is towards decreasing precipitation in three out of four seasons in the year (winter, spring and autumn), although these trends are not
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Fig. 2. Five-year moving averages of monthly, seasonal and yearly rainfall and their linear trends for Castile and Leon region. 75
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Fig. 2 (continued).
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Table 1 Monthly, seasonal and annual results of Mann–Kendall and Spearman tests and of least square linear fitting for Castile and Leon rainfall over the period 1961–1997 Slope (mm/year)
u(t)
rs
u(r s)
Standard error
Months December January February March April May June July August September October November
Positive Negative Negative 1.2 Negative Positive Negative Positive Positive Negative Positive Negative
0.68 0.86 1.25 2.35* 0.39 1.83 0.81 0.83 1.62 0.65 0.39 0.57
0.133 0.156 0.2 0.377 0.08 0.294 0.099 0.166 0.269 0.11 0.078 0.069
0.80 0.94 1.20 2.26* 0.48 1.76 0.59 1.00 1.61 0.66 0.47 0.41
– – – 5.4 – – – – – – – –
Seasons Winter Spring Summer Autumn Year
Negative Negative Positive Negative Negative (0.91)
0.78 0.28 0.26 0.07 0.23
0.106 0.079 0.05 0.005 0.075
0.64 0.47 0.30 0.03 0.45
– – – – –
u(t), r s, u(r s) statistics and parameters of Spearman and Mann–Kendall. * Significant trends at a 95% significance level.
statistically significant. This appears to be due to inter-annual variability and differing trends in individual months that make up the various seasons. The greatest decrease in precipitation over the year occurred in winter (0.596 mm/ year). This is in accordance with the results put forward by other authors (Zorita et al., 1992; Hurrell, 1995; Wanner et al., 1997; Va´zquez Lo´pez, 1999; Go´mez Navarro et al., 1999), showing a negative correlation in winter between the NAO and precipitation in Southern Europe. Fraile et al. (2004) have found significant correlations between the NAO index and precipitation in Spain. This implies that a high NAO index causes frequent anticyclones and dry weather over wide areas of Southern Europe. Over the Iberian Peninsula, this is associated to an intensification of the Azores high-pressure system over the western Iberian Peninsula that acts to block Atlantic weather from this territory. An analysis of the summer series showed a nonsignificant positive trend in this season for the period under consideration (0.481 mm/year). It seems likely that this positive trend will have been caused by an increase in storms over the study zone, as convection is the main cause of rain in Castile and Leon during the summer. To detect the month or months that had the greatest influence on seasonal precipitation, a monthly trend analysis was carried out. It was observed that rainfall increased in December, May, July, August and October, with the highest increases in December (0.98 mm/year). Rainfall decreased in the other months of the year, the greatest decreases being those for February (1.14 mm/year) and March (1.24 mm/year). Other studies (Mossmann,
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2002) have also noted an increase in the areas affected by negative trends in February over many areas in the Iberian Peninsula. The first statistically significant negative trend in the year was observed in March because rainfall in this month is mainly caused by Atlantic fronts. Mossmann (2002) has already confirmed that cyclonic situations have recently been decreasing across the Iberian Peninsula in March, implying a trend towards less rainfall in this month. The positive trend observed for July and August should be noted. This result is in accordance with the study of Mossmann et al. (2004) about precipitation in the Iberian Peninsula. In this work, the authors show there is a significant positive trend for these months with increases of about 0.3 mm/year for July and 0.2 mm/year for August in the period 1961–1990. The analysis of the July and August series yielded results for Castile and Leon of a similar order of magnitude: 0.2 mm/year for July and 0.4 mm/year in August. Some of the results relating to the analysis of precipitation trends obtained in this study do not accord with other climate change studies carried out for Europe and the Iberian Peninsula (Hulme et al., 1999), which found increased rainfall in winter and decreases in spring, autumn, and especially in summer. These differences may be explained by taking into account the fact that local or regional factors (topography, orography, geography and land use, among others) are essential in gaining an understanding of variations in local and regional climates. 3.2. Temperature trend analysis Fig. 3 shows monthly, seasonal and annual mean temperature trends for Castile and Leon over the period 1961–1997 using a linear regression. As in the case of precipitation trends, the rate of change is described by the slope of the regression line. Table 2 shows the results of the Mann–Kendall and Spearman tests, the slope value, and the total increase for all 37 years of the study period on different time-scales. The agreement between the results of the application of the Spearman and Mann–Kendall tests should be emphasized. With respect to temperature, the variability between the different years was less than with precipitation data, the standard deviation being 0.54 8C, and the average annual temperature 11.17 8C. The year 1997, with 12.21 8C, was the warmest year in the whole period. This result is in agreement with The Third Assessment of the Intergovernmental Panel on Climate Change (IPCC, 2001), which concluded that the 1990s were the decade with the highest temperatures since 1861. The years 1998 and 1997 were the first and second warmest years in the period. Various authors (Quereda Sala et al., 1999; Jones et al., 1999; Parry, 2000) have found similar results, pointing out that the warmest years on record were during the 1990s and noting that the four hottest years in decreasing order were 1998, 1997, 1995 and 1990. At the other extreme, the data being discussed here indicate that 1972, with 10.07 8C, was the coldest year in the period analyzed. The observed annual trend towards a warmer climate, with an estimated increase of some 0.009 8C/year (0.33 8C over the whole period considered) is not statistically significant. Such a rise in annual mean temperature of about 0.1 8C/decade is in
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Fig. 3. Five-year moving averages of monthly, seasonal and yearly temperature and their linear trends for Castile and Leon region. 79
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Fig. 3 (continued).
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Table 2 Monthly, seasonal and annual results of Mann–Kendall and Spearman tests and of least square linear fitting for Castile and Leon temperature over the period 1961–1997 Slope (8C/year)
u(t)
rs
u(r s)
Standard error
Months December January February March April May June July August September October November
0.07 Positive Positive 0.05 Negative Negative Negative Negative Positive Negative Negative Positive
3.19* 0.91 0.75 2.24* 0.68 0.41 0.39 0.39 0.41 1.04 0.62 1.72
0.522 0.75 0.12 0.374 0.076 0.055 0.068 0.043 0.069 0.179 0.098 0.304
3.13* 0.92 0.72 2.24* 0.46 0.33 0.41 0.26 0.41 1.07 0.59 1.82
0.24 – – 0.24 – – – – – – – –
Seasons Winter Spring Summer Autumn Year
0.03 Positive Negative Negative Positive (0.009)
3.11* 0.86 0.39 0.05 1.49
0.486 0.115 0.045 0.009 0.189
2.92* 0.69 0.27 0.05 1.13
0.13 – – – –
u(t), r s, u(r s) statistics and parameters of Spearman and Mann–Kendall. * Significant trends at a 95% significance level.
accordance with what has been reported by several authors (Hulme and Sheard, 1999; Jones et al., 1999; Parry, 2000), who have noted an increase of between 0.1 and 0.4 8C each decade in Europe. On a seasonal time-scale, temperatures have risen in both winter (0.034 8C/year) and spring (0.009 8C/year). The application of the Mann–Kendall and Spearman tests demonstrated a statistically significant positive trend in winter, because most of the increases in monthly temperature were to be found in this season. As various authors have pointed out (Hurrell, 1995; Easterling et al., 1997; Va´zquez Lo´pez, 1999; Go´mez Navarro et al., 1999; Brunetti et al., 2000; Marshall et al., 2001; Shabbar et al., 2001; Chmielewski and Ro¨tzer, 2001; Watson et al., 1998), this significant increase seems to be related to the influence of the positive NAO phase over the Northern Hemisphere in winter. It seems likely that the positive trend found in winter and early spring may be causing alterations in the length of seasons, with consequential effects on aspects such as production, and the composition and distribution of animal and plant communities, which are crucial in determining the economic impact. On the other hand, slight decreases were noted for autumn (0.001 8C a year) and summer (0.002 8C a year), which do not concur with other studies of climate change in the Iberian Peninsula (Hulme et al., 1999) indicating a considerable increase in summer temperatures. This seems to confirm that the results of climate change studies depend on the time-scale used and on the period taken into consideration.
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As was done for precipitation trends, a monthly analysis was carried out so as to detect the month or months having the greatest influence on the seasonal temperature. On a monthly time-scale, it was observed that average temperatures have risen for December, January, February, March, August and November, with the greatest increases in December. The statistically significant positive trend observed for winter is due to the fact that all the months in this season showed an increase in mean temperature, especially December. The results reported here for winter and early spring confirm findings by other authors (Hurrell, 1995; Rodwell et al., 1999; Chmielewski and Ro¨tzer, 2001) with respect to the influence of the positive NAO phase from January to April. This tends to be associated with prevalently westerly winds and with the advection of warmer and more frequent anticyclones over Southern Europe, causing high mean temperatures in these months. No statistically significant negative trends for the rest of the year were observed. The largest increases have been noted in November. Before closing this section, it is necessary to indicate that the trends found have not been influenced by grouping together and averaging the data from the different weather stations used. We have not carried out a probabilistic study of the influence of the partial grouping of weather stations, although it had been initially decided to do this separately for each of the nine provinces in the region. However, it was observed that the trends remain constant considering the region as a whole. Similarly, it had been decided that the trends should be studied according to the climatic subregions: none of the three climatic subregions in Castile and Leon showed variations with respect to the general trends though. These facts provide enough evidence to claim that, with the exception of very few weather stations, the general trend for temperature and precipitation is in accordance with the patterns described above.
4. Conclusions A number of conclusions may be drawn from the present analysis of precipitation and temperature data from 171 meteorological stations over a 37-year study period: In Castile and Leon, temperature and precipitation show divergent trends on an annual and seasonal time-scale (increasing and decreasing, respectively), with a general trend towards a warmer and drier climate in the latter part of the twentieth century. ! It is likely that the rise in the annual mean temperature has largely been brought about by the significant increases in winter temperatures resulting from the influence of the positive NAO phase. The absence of statistical significance for most seasons is an outcome of the different tendencies of individual months in these seasons. ! A variation in the rainfall regime in Castile and Leon was noted over the study period, with a change from a winterNspringNautumnNsummer model to winterNautumnN springNsummer. This is because the decrease observed in spring rainfall was about twice as great as that recorded in autumn. ! Although there is no significant evidence to allow certainty as to whether the trends detected will vary or remain steady in the future, it does seem that the climate of Castile and Leon has become less continental, due to the increases observed in summer !
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precipitation and the decreases in the range between the average temperatures of the warmest and coldest months of the year over the period analyzed. It should be emphasized how crucial it is to work with local and regional time-scales so as to confirm the real effects of any possible climatic change. This is a consequence of the great impact of local and regional factors on a territory’s climate.
Acknowledgements The authors wish to express their thanks to Dr. Noelia Ramo´n for the translation of this paper into English and to the INM for kindly making data sets available to them. We also thank two anonymous referees for their helpful comments on the original manuscript.
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