Definition and temporal evolution of the heat and cold waves over the Spanish Central Plateau from 1961 to 2010

ÁNGEL L. LABAJO, MOISÉS EGIDO, QUINTÍN MARTÍN, JUAN LABAJO, JOSÉ LUIS LABAJO

Abstract

The temporal behavior of the annual frequency of heat and cold waves observed between 1961 and 2010is established for the Spanish Central Plateau and for the two sub-areas in it. The series of daily maximumand minimum temperature anomalies for the working areas were calculated from the daily data concerningtemperature anomalies. The thresholds of these series of anomalies, determined by the P10 and P90 percentilesvalues, were obtained. Heat waves occur when there are two or more consecutive days on which the maximumand minimum temperature anomalies are simultaneously greater than the values of the thresholds obtainedfor the P90 percentile. The heat waves that affected the Spanish Central Plateau and the two sub-areas duringthe period of time considered were identified and their monthly and annual frequencies were obtained. Likewise,a cold wave is considered to have occurred when there are two or more consecutive days on which themaximum and minimum temperature anomalies are simultaneously lower than the thresholds establishedby the corresponding P10 percentiles. The cold waves occurring in the study area were identified and theirmonthly and annual frequencies in the study period were established. According to the results, the monthswith highest number of heat waves between 1961 and 2010 were May (25 waves) and June (23 waves).Trend analysis of the series of annual frequencies indicates that there was an increasing trend towards theoccurrence of heat waves, with a confidence level greater than 99%. The linear model established indicatesthat an increase had occurred in the frequency of heat waves in the Spanish plateau of the order of 0.6 wavesevery 10 years. Regarding cold waves, these were detected in each month of the year and their frequencyranged between eight and 16 events per year. The months with lowest number of cold waves were April(nine), July (eight) and August (nine), and the months with the highest number were March, May, June andOctober, with 16 cold waves. The years with the highest number of cold waves were 1969, 1971 and 1977,with seven cases. In the other years the annual rate was between one and six. Trend analysis of the series ofannual frequencies indicated that there was a decreasing frequency of cold waves, at the confidence levelof 99%. When a linear model was considered for the Spanish Central Plateau a decreasing frequency of thecold waves of the order of 0.54 waves in every 10 years was observed from 1961 to 2010.

Keywords

Anomaly; temperature; heat wave; cold wave; frequency, trend.

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