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 paper investigates the relationship between synoptic circulation types (CTs) and the frequency of precipitation extremes (> 90th percentile) in spring and autumn at 44 stations. From 1950 to 2003, in spring, extreme precipitation days diminished in the West and South mainly due to a decreasing frequency of cyclonic Southwest flow. In contrast, in autumn most patterns conducive to extreme precipitation (mainly NW flow) become more frequent, contributing to more extremes at the central and North-western stations. The observed inter-annual variability of extreme precipitation days appears well related with changes in the frequency of the CTs for westernmost Iberia and high altitude stations. In addition, low-frequency changes within the CTs are analysed throughout the 20th Century: they demonstrate that a remaining part of the variability in the frequency of extreme precipitation must be explained by other long-term factors, such as changes in air temperature, in the upper troposphere circulation, 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 of changes in precipitation extremes, both due to CTs frequency and to other factors, takes advantage of the seasonal basis and the regional responses.