Rainfall variability in Bulgaria and its relation with NAO

The variation of climate exerts strong influence on the productivity of agriculture and on the various aspects of human activity. Many research programs at local, regional and global levels show as a main task the action to reduce, avoid, and better understand the risk associated with the climate change. The precipitation determines the availability of drinking water and the level of the soil moisture. The paper is intended to provide information about seasonal and geographical variation of precipitation in Bulgaria and its relation with North Atlantic Oscillation (NAO). A Statistical test is applied to test the homogeneity and distribution of the initial data. In order to characterize the long-term and more recent trends linear regression equations are calculated. Correlation analysis is used to define the relationship between precipitation and global circulation mechanisms. The main question of the research is: whether the observed trend in Bulgaria corresponds to the global climate change? The tasks are: - Investigation of short-term and long-term rainfall variability - Assessment of statistical significance of the linear trend - Establishing the relationship between rainfall variability and NAO - what is the influence of the NAO on rainfall variability in Bulgaria In order to characterize the long-term and more recent trends linear regression equations were calculated individually for each station for two periods 1931-2000 and 1961-2000. The trend is negative or close to 0 for the first period. For the second period it is a negative with the values for January to April between -0.2 and -0.7. Correlation analysis is applied to define the relationship between rainfall variability in Bulgaria and NAO. The relationship is stronger for more recent period 1960-2000. The correlation coefficients are negative. Statistical significant coefficients are found for January, February and March. The paper gives information about climate change in regional scale. The results of the research will have importance for better understanding the relationship between rainfall and NAO. The knowledge for rainfall-variability and physical mechanisms responsible for this variability could be applied in risk management and land use planning in Bulgaria. (Author)