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Rainfall, with its intensity, duration, and seasonal distribution, is among the main factors causing soil erosion, which is a widespread environmental problem in Bulgaria. Rainfall erosivity shows the potential of precipitation to generate erosion processes and is an essential indicator of the climate vulnerability of a region. This paper aims to evaluate rainfall erosivity in a part of the Eastern Rhodopes Mountains, an area that is characterised by high-intensity erosion processes and high erosion risk. Local peculiarities of rainfall erosivity were revealed by the calculation of some precipitation indices based on the monthly precipitation for the period 2000–2021, such as the precipitation concentration index (PCI), Angot precipitation index, Fournier index (FI), and modified Fournier index (MFI). The analysis of the extremely wet and extremely dry months at the annual and seasonal (October–March and April–September) levels was performed to evaluate the susceptibility to erosion. The results from the study show that rainfall erosivity in the studied area varies from low to moderate in the northern part of the study area and from high to very high in the south. According to the MFI, high and very high erosivities have been observed mainly since 2012. The erosivity increases from north to south, to the area with a complex relief, where the combination of orography and atmospheric circulation make favourable conditions for the occurrence of extreme precipitation. The analyses of the calculated indices show that the precipitations in most of the studied area generally have from a low to a moderate erosivity, but this does not exclude the occurrence of cases with high and very high erosivities, which are characteristic of recent years and are related to the increase in annual precipitations and extreme precipitation months. The results of this study can contribute to the development and implementation of measures and preventive activities for the reduction and possible elimination of the negative impacts of extreme precipitation. [ABSTRACT FROM AUTHOR]

The objective of this paper is to determine the influence of circulation factors on precipitation in Bulgaria. The study succeeds investigation on the influence of circulation factors on air temperatures in Bulgaria, as the focus here is directed toward precipitation amounts. Circulation factors are represented through two circulation indices, showing west-east or south-north transport of air masses over Bulgaria and four teleconnection indices (patterns)-North Atlantic Oscillation, East Atlantic, East Atlantic/Western Russia, and Scandinavian. Omega values at 700-hPa level show vertical motions in the atmosphere. Annual precipitation trends are mixed and not statistically significant. A significant decrease of precipitation in Bulgaria is observed in November due to the strengthening of the eastward transport of air masses (strengthening of EA teleconnection pattern) and anticyclonal weather (increase of descending motions in the atmosphere). There is also a precipitation decrease in May and June due to the growing influence of the Azores High. An increase of precipitation happens in September. All this leads to a redistribution of annual precipitation course, but annual precipitation amounts remain the same. However, this redistribution has a negative impact on agriculture and winter ski tourism. Zonal circulation has a larger influence on precipitation in Bulgaria compared to meridional. Eastward transport throughout the year leads to lower than the normal precipitation, and vice versa. With regard to the four teleconnection patterns, winter precipitation in Bulgaria is determined mainly by EA/WR teleconnection pattern, spring and autumn by EA teleconnection pattern, and summer by SCAND teleconnection pattern. [ABSTRACT FROM AUTHOR]

Climate change and the impacts of anthropogenic activity on climate in Europe is documented in many scientific publications but the investigations about climate impact and particularly urban climate are relatively few for Bulgaria. The present paper investigates the variability of seasonal and annual air temperature in Sofia city and surrounding areas (towns Bankya and Bozhurishte) for the period 1961-2015 based on the linear regression method. In order to investigate the urban heat island (UHI) intensity, the air temperature difference between the city and its surroundings is calculated. The study shows increase of air temperature difference between the city and suburban territories during the recent years. The linkages between the observed variability of air temperature difference and atmospheric circulation and urbanization are investigated. [ABSTRACT FROM AUTHOR]

This study reveals the dynamics in the number and intensity of heat waves in Bulgaria in the late 20th and early twenty-first century, as well as their impact on forest fires in the beginning of this century. Statistical methods such as linear regression and Spearman's correlation were the main tools of this study. Multiple linear regression models were used in order to reveal the complex influence of heat wave characteristics on forest fires in Bulgaria. Over the last four decades, there has been a statistically significant trend of increasing number of days belonging to heat waves. There is no significant trend (except in southwestern Bulgaria) in the average maximum temperatures of heat waves in the period 1979–2021. This is due to a change in the atmospheric circulation over Bulgaria in the twenty-first century toward increased transport of air masses from northeast in summer. The heat waves have a significant impact on the forest fires in Bulgaria in summer. In general, more, longer and having higher air temperatures heat waves provide more favorable conditions for the occurrence and development of forest fires. The results of this study can serve to build on the existing methodology for determining the risk of forest fires in the territory of Bulgaria, including factors such as the total duration and intensity of heat waves. [ABSTRACT FROM AUTHOR]

Extreme weather and climate events have received increased attention in the last years due to their impact on different aspects of human activity. The aim of the present paper is to bring to better understanding of the causes of extreme precipitation occurrence in Bulgaria. An extreme precipitation month is defined as a month registering precipitation totals lower or higher than certain thresholds. Extremely wet months are defined as months with total precipitation amounts higher than 90% percentiles of the gamma distribution in the reference period 1961-1990. As extremely dry month, we consider the month with total precipitation lower than 10% of the gamma distribution. Spatial changes in the occurrence of extreme precipitation are investigated by the application of the cluster analysis. The circulation patterns were analyzed with regard to sea level pressure. The relation between the fluctuations of the extreme months and atmospheric circulation mechanism teleconnection is study by the North Atlantic Oscillation Indices (NAOI), the Eastern Atlantic pattern (EA) and the Mediterranean Oscillation Indices (MOI). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The research period is 1950-2012, and includes data for air temperatures in 21 Bulgarian stations. Two circulation indices, covering the same period and showing west-east or south-north directions of transport of air masses, were calculated. Statistical methods were used in the study. The results show a significant positive trend in average annual air temperatures in almost the entire territory of Bulgaria with the exception of its eastern-northeastern part. The warming occurs mainly in March, June and July, with some stations having significant positive values also in January, May and August. The zonal index reveals an insignificant increase of western transport of air masses in the cold half of the year (October-April) and strengthening of the eastern transport in the rest of the year. The meridional index shows an increase of the northern transport of air masses over the entire year and this is particularly visible in March, June, August and September. Correlation coefficients indicate that atmospheric circulation has leading role in determination of air temperatures during the period from November to April. Western transport of air masses leads to higher temperatures in spring, autumn and winter and to lower temperatures in summer. The influence of ENSO on atmospheric circulation over Bulgaria is weak, with a time lag of 2 months. El Niño is associated with increased western and northern transport of air masses, while La Niña is associated with increased eastern and southern transport of air masses over southeastern Europe. [ABSTRACT FROM AUTHOR]

Trends in atmospheric pressure, circulation and some relationships between North Atlantic Oscillation (NAO) indices, sea surface temperatures, and atmospheric circulation over Bulgaria are discussed in this article. Data for measured atmospheric pressure at stations Burgas, Pleven, and Sandanski are used. Information about atmospheric circulation over Bulgaria was obtained using sea level pressure and 700 hPa Omega (vertical velocity) reanalysis daily data for grid cells covering the territory of Bulgaria for the period 1948-2010. Zonal and meridional indices for Bulgaria were also calculated based on the data for sea level pressure. NAO index calculated by NOAA and NCAR is correlated with atmospheric pressure and circulation. A total of 12 areas in three major water basins influencing Bulgarian climate-North Atlantic, Mediterranean, and Black Seas-were studied. Main methods employed in the article are statistical-trend analysis, multiple linear regression, correlation, nonparametric tests, etc. There is no change in the mean values of atmospheric pressure over Bulgaria. Circulation over Bulgaria during the research period increases its anticyclonal patterns mainly due to the decrease of the number of cyclones. Dynamics in zonal and meridional indices for Bulgaria result in an increase of the northwest transport in the winter and an increase of the northeast transport in the summer. Cyclones over Bulgaria determine the values of atmospheric pressure. Influence of the NAO on atmospheric pressure and circulation is stronger in winter. Atmospheric processes, expressed by the number of cyclones and anticyclones, are most active in spring. Current trends are towards increasing of sea surface temperatures (SSTs) at all investigated places. Temporally, the effect of SSTs on the number of cyclones, anticyclones, zonal and meridional indices for Bulgaria during the different seasons comes with a delay of 1 to 3 months. Constructed multiple linear regression (MLR) models with predictors SSTs adequately describe the atmospheric circulation over Bulgaria. There is a clear pattern of SSTs distribution, which leads to a higher number of cyclones over Bulgaria in winter-lower than normal temperatures in the Aegean Sea and higher than normal in the Black Sea. A decrease in the difference of temperatures between the Gulf Stream and western colder parts leads to higher values of winter zonal transport over Bulgaria. Higher than normal temperatures in Black Sea lead to a higher number of cyclones in spring. Higher difference in temperatures of the North Atlantic leads to a stronger cyclogenesis and enhanced zonal transport, which affects autumn circulation over Bulgaria. [ABSTRACT FROM AUTHOR]