Wintertime circulation types over the Iberian Peninsula: long-term variability and relationships with weather extremes

This paper analyses atmospheric surface circulation and climatic extremes in the Iberian Peninsula in winter (December-February). Sea level pressure grids (1850-2003) are classified through a simulated annealing clustering into 7 characteristic circulation types (CTs). Daily series of temperature (29 stations) and precipitation (44 stations) started between 1905 and 1950. We investigate which CTs are conducive to extremes at each station by means of their contribution to extreme days compared to non-extremes days, with significance based on a Monte Carlo resampling. Regional features arise in the relationship between CTs and extremes and, taking them into account, some trends in extreme indices from 1950-2003 (period shared by all stations) agree with trends in the frequency of the CTs. Thus, increases in warm days in northern and central stations are consistent with a positive trend in Anticyclone over North Iberia/France; and negative trends in extreme precipitation in the East Cantabrian coast are consistent with a negative trend in the north-westerly flow. Furthermore, low-frequency temporal analyses reveal large (small) changes in extreme temperature (precipitation) days within the CTs. From the mid-1940s onwards, the extreme cold character of the northerly and north-easterly flows diminished, i.e. the frequency of cold nights within these CTs decreased from similar to 25 to similar to 15%, whereas from the 1920s to mid-1940s an opposite behaviour occurred (towards a higher frequency of cold nights). Throughout the 20th century, the frequency of warm days within the CTs has increased, especially for the south-westerly and westerly flows from the 1920s to 1950s and since the mid-1970s. Westerly flow was less frequent in the 1980s and 1990s but connected with a higher percentage of extreme precipitation days in West Iberia. This changing frequency of extremes within the CTs indicates that, apart from circulation types, other physical forcings have influenced the occurrence of extremes.