Spring and summer extreme temperatures in Iberia during last century in relation to circulation types

Fernández-Montes S., Rodrigo FS., Seuber S., Sousa P.M.
Atmospheric Research. doi: 10.1016/j.atmosres.2012.07.013

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In the Iberian Peninsula the raise of temperatures has been notable from mid-1970s to mid-2000s, especially in spring and summer. This study analyses spatial and temporal relationships between extreme temperatures and atmospheric circulation types (CTs) defined over the Iberian Peninsula (IP) in these seasons. Station series (29) of maximum and minimum temperature are considered, starting from 1905 until 2006. The CTs (9 for spring and 8 for summer) are derived by a cluster method applied to daily mean SLP grids covering the period 1850–2003. Changes in the seasonal frequency of extreme temperatures and of CTs are analysed. Subsequently, the CTs are examined for their effectiveness in leading to moderately extreme temperatures (at each location) using an index that measures the contribution to extreme days with respect to the contribution to non-extreme days. Correlation between regional extreme series and CTs frequency is also tested.
In spring, the decrease in cold nights, which is notable in the 1970s onwards, can be partially attributable to a downtrend in the frequency of Northerly flow. High frequency of Anticyclone in North Iberia in the 1980s and 1990s has contributed to an increase in warm days in West and North stations. To the SE quadrant of the IP, a great part of warm days is related to south-westerly flow, (both) presenting a higher frequency in the 1950s and the 1960s.
In summer warm nights increased remarkably to the SE and SW, and may be in part related to uptrends in Iberian thermal low pattern (1950–2003) and North Atlantic Anticyclone (1850–2003) respectively. Warm days have increased remarkably to the NE especially in the 1990s and 2000s, but this is not found to be related to changes in CTs' frequency.
Furthermore, the existence of within type changes (variations in Tmax, Tmin and extreme indices within the CTs) points to the identification of other physical factors operating on inter-annual to multidecadal time scales. Thus, the consideration of Sea Surface Temperature (SST) of the East Atlantic Ocean and Iberian soil moisture conditions (by means of a drought index) helps to explain the evolution of extreme temperatures.