Climate variability and change in Bulgaria during the 20th century

Climate data used for climate variability and change analyses, must be homogeneous, to be accurate. The data currently used in the Meteo-France homogenization procedure, which does not require computation of regional reference series, was applied to precipitation and average air temperature series in Bulgaria. The Caussinus-Mestre method, with a double-step procedure, was used to detect multiple breaks and outliers in the long-term series of precipitation and average air temperature. A two factor linear model was applied for break correction. The homogenization procedure was run till all or most break risk was gone. Analysis of climate variability and change in Bulgaria during the 20(th) century was done on already homogenized precipitation and average air temperature series. The statistical significance of the trends obtained was evaluated by the coefficient of Spearman rank correlation. The variations of annual precipitation in Bulgaria showed an overall decrease. The country has experienced several drought episodes during the 20(th) century, most notably in the 1940s and 1980s. Seasonal precipitation in spring shows a positive trend at most weather stations across the country. The trend for summer and autumn precipitation is negative. A statistically significant increasing trend of winter precipitation in north Bulgaria was detected. No significant warming trend in the country was found during the last century inspite of the warming observed during the last two decades. Summer in Bulgaria tends to be warmer from the beginning of the 1980s. There is a statistically significant increasing trend of average air temperature during the winter season at the weather stations near the Danube river ( north Bulgaria) during the periods 1901 - 2000 and 1931 - 2000.