frost days and tropical nights in the iberian peninsula, 1929-2005 f.s. rodrigo, s....
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FROST DAYS AND TROPICAL NIGHTS IN THE IBERIAN PENINSULA, 1929-2005
F.S. Rodrigo, S. Fernández-Montes Department of Applied Physics, University of Almería La Cañada de San Urbano, s/n, 04120, Almería (Spain) [email protected], [email protected]
1. INTRODUCTION
Seasonal changes of extreme daily temperature indices (frequency of frost days and tropical nights) of25 stations in the Iberian Peninsula from 1929 to 2005 are investigated. Frost days index (FD) is defined as the number of days with Tmin<0ºC, and tropical nights index (TR) as the number of days with Tmin>20ºC.The objective of this work is to analyze the spatio-temporal variability of these indices and to look for possible relationships with the main teleconnection indices affecting the study area.
2. DATA
Daily minimum temperature from 23 Spanish stations (SDATS database) + Lisbon and Perpignan (ECAD) from 1929 to 2005.
3. METHODS
FD and TR indices were calculated on a seasonal basis (winter=DJF, summer=JJA) for each individual station.
Cluster analysis (Ward’s method, euclidean distance) was applied to analyze spatial variability and establish different regions within the Iberian Peninsula.
Non parametric Mann-Kendall test was applied to detect significant trends (95% confidence level)in regional series.
Linear model was used in a first approach to estimate the changing rate (slope of the regression equation)in the cases where the Mann-Kendall test detected a significant trend.
Cummulative anomalies were used to detect different periods in time series. T-test for difference between means, F-test for ratio of variances and Kolmogorov-Smirnov test for comparison of distribution functions were applied to confirm these periods.
Stepwise multiple regression analysis between the regional extreme indices and teleconnection indices since 1951.Teleconnection indices were taken from the Climate Prediction Center (http://www.cpc.noa.gov)
- North Atlantic Oscillation (NAO)- East Atlantic (EA)- East Atlantic/Western Russia (EA/WR)- Scandinavian (SCAN)
4. WINTER FROST DAYS
Comparison between 1929-1965 and 1966-2005 s= standard deviation; t= t-test; F= F-test; KS= Kolmogorov-Smirnov statistic Bold numbers: significant at the 95% confidence level
Trends C1: z(Mann-Kendall) = -0.55 C2: z(Mann-Kendall) = -3.61 (-0.6 days/decade)
5. SUMMER TROPICAL NIGHTS
Comparison between 1929-1980 and 1981-2005 s= standard deviation; t= t-test; F= F-test; KS= Kolmogorov-Smirnov statistic Bold numbers: significant at the 95% confidence level
Trends C1: z(Mann-Kendall) = +6.2 (+3.8 days/decade) C2: z (Mann-Kendall) = +4.3 (+1.4 days/decade) C3: z (Mann-Kendall) = +2.9 (+0.2 days/decade)
6. RELATIONSHIPS WITH TELECONNECTION INDICES Period 1951-2005 Residuals of the models
In summer, positive trends detected must be explained by other causal mechanisms.
SE = standard errorMAE = Mean absolute errorDW = Durbin-Watson statistic
Acknowledgments
This work was supported by the Spanish Ministry of Education and Science (project CGL2007-65546-C03-01).
The authors are in debt to Dr. R. Aguilar from the Climate Research Group of the University Rovira iVirgili (Tarragona, Spain) for providing the original temperature data series (SDATS).
7. Conclusions
Significant decreasing trend from 1965 onwards in the frequency of winter frost days for coastal stations.Significant increasing trend from 1980 onwards in the frequency of summer tropical nights in southern stations.The behaviour of winter frost days is related to the evolution of winter NAO and EA patterns.Although multiple regression models establish a relationship between summer indices and the SCAN pattern, the behaviour of the residuals indicates that summer indices must be explained by other causal mechanisms.