Summer temperature extremes in northeastern Spain : spatial 1 regionalization and links to atmospheric circulation ( 1960 - 2006 )

8 A procedure for classifying daily summer temperature extremes in northeastern Spain 9 into homogenous regions has been presented and evaluated. This procedure employed daily 10 temperature series from a dense network of 128 weather stations spanning the period from 11 1960 to 2006. Characteristics of temperature extremes included temperature frequency (e.g. 12 warm days), intensity (e.g. warmest day), and duration (e.g. maximum length of hot spell). 13 Following the results of the principal components analysis (PCA) and the Ward ́s method 14 of clustering, the study area was divided into four homogenous sub-regions in terms of both 15 the geographic and climatic meanings: the Mediterranean region, the mainland and the 16 Cantabrian region, the moderately elevated areas westward and southward, and the 17 mountainous region. Based on an internal cluster validation measure (Silhouette width), the 18 quality of clustering was evaluated and ensured. The temporal evolution of the long-term 19 (1960-2006) temperature extremes clearly showed different behavior amongst these sub20 regions. The Mediterranean and the highly elevated regions revealed the strongest signals 21