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Actual and future trends of extreme values of temperature for the NW Iberian Peninsula

It is now very well established that yearly averaged temperatures are increasing due to anthropogenic climate change. In the area of Galicia (NW Spain) this trend has also been determined. The main objective of this work is to assess actual and future trends of different extreme indices of temperature, which are of crucial importance for many impact studies.

As direct GCM-output significantly underestimates the variance of daily surface temperature variables in NW Spain, these are obtained by applying a statistical downscaling technique (analog method), using 850hPa temperature and mean sea level pressure as combined predictors. The predictor fields have been extracted from a multi-model multi initial conditions ensemble of 14 runs of three GCMs participating in the IPCC AR4.

For 1960-2006 results show a significant increase of the number of days with maximum temperatures above the 90th percentile. Furthermore, a significant decrease of the days with maximum temperatures below the 10th percentile has been found. The tendencies of minimum temperatures are reverse: less nights with minimum temperatures below the 10th percentile and more with minimum temperatures above 90th percentile. Those tendencies can be observed all over the year, but are more pronounced in summer. Results for relationship between the above mentioned extreme values and different North Atlantic teleconnection patterns show that local tendencies are associated with trends of EA (Eastern Atlantic) and SCA (Scandinavian) patterns. The NAO (North Atlantic Oscillation) only in winter is related to the number of cold days.

The ensemble climate projections generated with the analog method indicate that the observed (linear) trends of maximum and minimum temperatures in NW Spain are expected to continue in the next decades. Furthermore the uncertainty interval of these projections (2021-2050) is dominated by model uncertainties rather than by forcing uncertainties.

We conclude that more frequent hot days as well as an increasing probability of summertime heat waves are to be expected in the next decades. Cold days tend to diminish, decreasing the probability of wintertime cold waves and leaving a greater part of the area under study without frost throughout the year.