The Mountain Observatory Bjelašnica - History, analysis, homogenization and interpretation of a more than 100 years long temperature data set

Bosnia and Herzegovina is situated at the junction of three main climatic zones, namely, the Mediterranean, the Mountain and the Moderate Continental one. The Mountain Observatory Bjelasnica is located at the boundary between these climatic zones. The special location at the sharp transition between different climate regimes creates surnormally variably climate conditions. This - along with the general sparseness of high elevation climate information ― makes a new centennial high elevation data set starting in 1895 particularly interesting for climate and climate change research. Our paper describes the historical background of the observatory - one of the fundamentals of appropriate homogenizing climate time series. The new temperature series is then implemented into the existing data collection HISTALP. Further our paper provides a first comparative analysis of the centennial trend, the decadal variations and the high-frequent variability of the first homogenized centennial summit series at the Balkan Peninsula. Homogenization tests detected four breaks in the 1895-2007 Bjelasnica summit series. The monthly adjustments ranged from -0.9 to +0.9°C. The average length of a homogeneous subinterval was 23 years. Although remote from the other high elevation summit sites of the Alps, the Bjelasnica-series shows astonishingly similar centennial and decadal scale trends and variations. The linear centennial trend 1895-2007 is +0.11°C/10a. Decadal variations are more accentuated in summer than in winter. The former are dominated by a two-step warming from cool 1910s summers to a first maximum near 1950 followed by a significant cooling until 1980 and then the well known recent warming which was strongest from 1980 to 2000. In the cold season decadal scale deviations from the centennial trends are less significant than in the summer half year season. There are only two main divergent subperiods - a cold one at the beginning of the series near 1900 and the main maximum in the 1990s. The Bjelasnica summit series is freely available now in the web on www.zamg.ac.at/histalp.