Frequencies, Variations and Trends in Atmospheric Circulation Types over Bulgaria and their climatological impacts in the last 150 years

Everyday weather conditions and meteorological situations at a given point or area on the earth's surface are primarily a function of the atmospheric circulation conditions above it. On monthly, seasonal or annual basis these daily conditions form atmospheric circulation in climatological aspect. Many climatological studies are dedicated to the baric formations and their positions relative to a given area. Thus, cyclones and anticyclones strongly affect the weather conditions through the values of the variety of the meteorological elements as the wind speed and direction, temperature and precipitation quantities. The multi-year fluctuations of the circulation also shape it from a climatic point of view. The classification of circulation types over Bulgaria in present work is made on daily basis using subjective (manual) approach, covers 150-year period (1871-2020) and is based on the data of two atmospheric levels – mean sea level pressure (SLP) and 500 hPa in the middle troposphere, derived from “20th century reanalysis” visualization maps. The circulation types are composed as the position of the center of the baric formations relative to the territory of Bulgaria. The methods used in composing the atmospheric classification and the circulation types are explained in detail in a previous study. 13 atmospheric circulation types are separated at 500 hPa level (5 anti-cyclonic and 8 cyclonic) and 16 at the surface (6 anti-cyclonic, 8 cyclonic and 2 low gradient). Annual and seasonal frequencies as well as seasonal distribution of all circulation types in the examined period at both levels are presented and analyzed. Some of the types are united in order to emphasize their climatological impact. Trend analyzes are performed on all 29 circulation types and on some type combinations, as for this aim the methods of Man-Kendall and Sen Slope estimator are used. A special attention is paid to the high throws and cyclones at 500 hPa and their major influence on surface cyclogenesis and respectively to the precipitation regime in different parts of the country. Some other suggestions are proposed and conclusions are made, as the increased influence, at least concerning the region of Balkan Peninsula of the Azores anticyclone in the last four decades.