Changes in summer temperatures and rainfall in the northeastern Iberian Peninsula in the late 1960s and the weakening of the Iberian thermal low

Changes in a number of important climatic variables in the northeastern part of the Iberian Peninsula (NE IP) were detected in summers around the end of the 1960s. From 1950 to 1966, we found significant correlations between the Summer North Atlantic Oscillation (SNAO) and frontal precipitation. This could be explained, on the one hand, by the withdrawal of the Azores High to the west which enhanced the frequency of cold advections in the north IP, and, on the other hand, by a high occurrence of an anticyclonic pattern over the west Mediterranean that enhanced the precipitation in NE IP by advecting moist air which, in turn, reinforced the storms linked to the frontal activity coming from the Atlantic. This pattern was also linked in this period to the strengthening of the Iberian Peninsula summer thermal low and to the increase in second-quadrant wind at the Ebro Observatory. Nevertheless, an abrupt rise of the vorticity at sea level in the western Mediterranean was detected around 1967. This rise was related to the steep increase in the SNAO, the expansion of the Azores High to central Europe in summer, and an increase of the anticyclonic regime. This contributes to explaining the simultaneous drop in frontal precipitation in NE IP, the weakening of the IP summer thermal low, the drop of the second-quadrant daily wind run, and the steep rise of the diurnal temperature range (DTR) at the Ebro Observatory.