Summer night-time temperature trends on the Iberian Peninsula and their connection with large-scale atmospheric circulation patterns

The present paper studies the variability and trends of Tropical Nights (>20 degrees C) (TNs) on the Iberian Peninsula (IP) over the 1961-2007 period during the extended summer (defined as June, July, August, and September, JJAS). The analyses were performed with the mean series calculated for the whole IP and for different subregions. We conducted a regionalisation by means of a S-Mode Principal Component Analysis (PCA), which identified three subregions where the variability of TNs behaves similarly: the Mediterranean (M) region, the centre-north of the Peninsula (CN) region, and the Atlantic (A) region. Throughout the entire period, the mean summer TN series for the whole IP shows a significant increase; there are, however, some regional differences, as the CN and M regions present higher rates of warming than the A region. Using Canonical Correlation Analysis (CCA) we detected the large-scale atmospheric circulation patterns controlling the interannual variability of TNs over the IP. The occurrence of TNs in the M and CN subregions is highly dependent on anticyclonic conditions over Western Europe in the mid-troposphere level. The increase in the positive phase of this canonical mode during the last three decades might explain part of the positive trend of the TNs detected in summer on the IP. Copyright (C) 2011 Royal Meteorological Society