Circulation types and extreme precipitation days in the Iberian Peninsula in the transition seasons: Spatial links and temporal changes

In the Iberian Peninsula a great amount of precipitation concentrates in relatively few days,primarily conditioned by the atmospheric circulation and the moisture content. This paperinvestigates the relationship between synoptic circulation types (CTs) and the frequency ofprecipitation extremes (N90th percentile) in spring and autumn at 44 stations. From 1950 to2003, in spring, extreme precipitation days diminished in the West and South mainly due to adecreasing frequency of cyclonic Southwest flow. In contrast, in autumn most patternsconducive to extreme precipitation (mainly NW flow) become more frequent, contributing tomore extremes at the central and North-western stations. The observed inter-annualvariability of extreme precipitation days appears well related with changes in the frequencyof the CTs for westernmost Iberia and high altitude stations. In addition, low-frequencychanges within the CTs are analysed throughout the 20th Century: they demonstrate that aremaining part of the variability in the frequency of extreme precipitation must be explainedby other long-term factors, such as changes in air temperature, in the upper tropospherecirculation, and ocean–atmosphere and land–atmosphere processes. In general, the within-type frequency of extreme daily precipitation seems to decrease (increase) in warmer (cooler)periods, except for western and central parts of Iberia under certain CTs. The identification ofchanges in precipitation extremes, both due to CTs frequency and to other factors, takesadvantage of the seasonal basis and the regional responses.© 2013 Elsevier B.V. All rights reserved.