Recent trends in the frequencies of extreme values of daily maximum atmospheric pressure at ground level in the central zone of the Iberian Peninsula

Using data sets on the daily data of maximum atmospheric pressure at ground level collected at 14 weather stations located in the central zone of the Iberian Peninsula [Spanish central plateau (SCP)], the series of daily maximum pressure anomalies at each station, together with the difference between the daily value and mean daily value for each day of the year for the period between 1961 and 2003, have been established. The regional series of such anomalies were constructed for the whole study zone and for two differentiated parts of the same. As thresholds for the extreme values of the anomaly series, the values corresponding to the P05 and P95 percentiles were used. The series of annual frequencies of days with anomaly values below and above the threshold values were constructed for each of the weather stations, together with the average series for the whole study zone and each of its two parts. The corresponding regional average series of seasonal frequencies were also constructed. From an analysis of the trend of the series of the annual frequency of extreme anomaly values in daily maximum pressure it may be deduced that the lowest values show a decreasing annual trend, while the highest ones show an increasing frequency. This indicates that between 1961 and 2003 the number of days per year on the SCP with the highest extreme atmospheric pressure values at ground level increased along the study period. In contrast the number of days per year with lower extreme values decreased. Additionally, analysis of the seasonal frequency series indicated that it was the winter that dictated such behaviour. Copyright © 2007 Royal Meteorological Society