Factors affecting air temperature in Bulgaria

Abstract

The research period is 1984–2007 and includes data for air temperatures at 20 Bulgarian and Serbian stations. Also, data for four radiation fluxes and cloud fraction were used in the study. Atmospheric circulation is represented by six different circulation patterns and indices. Statistical methods such as principal component analysis, trend analysis, and correlation were employed in order to solve different problems. The main factor that affects air temperature in Bulgaria is solar radiation. Among different radiation fluxes, the most important is Surface Longwave Upward Flux followed by Surface Shortwave Downward Flux and radiation balance. Circulation factors also contribute to the values of air temperature in Bulgaria. The most significant is East Atlantic teleconnection pattern. Local physical geographical conditions such as relief and large water bodies are also important, especially by modeling and modifying the temperature field over Bulgaria. Average annual and most of average monthly air temperatures have increased in the period 1984–2007. The only exceptions are months of September and December. The observed increase in air temperature is due mainly to the increase in Surface Shortwave Downward Flux, which in turn is due to the decrease in cloudiness. The decrease in cloudiness leads to redistribution of radiation fluxes, increasing the net balance in shortwave spectrum and decreasing the net balance in longwave spectrum. The latter suggests a decreasing greenhouse effect in the studied period over the territory of Bulgaria.