Multi-annual variability of global solar radiation in the agricultural area of Lower Silesia (SW Poland) and its relationship with the North Atlantic Oscillation

In this paper, the long-term variability of global solar radiation in the agricultural area of Lower Silesia is analyzed based on a 56-year long (1961-2016) measurement series recorded at the Agro- and Hydro-meteorological Wrocław-Swojec Observatory (SW Poland). Yearly and monthly global radiation sums with their extreme and mean values were compared with radiation data from Warsaw (Central Poland) and Potsdam (East Germany). The dynamics of variability between consecutive months, seasons and years was also taken into account. The conducted positive trends show a significant increase in the investigated global radiation sums for Lower Silesia and also for Central Poland and the eastern part of Germany. The trends are strongly related to long-term macro-circulation changes in the North Hemisphere, particularly with the phases and sub-phases of the North Atlantic Oscillation (NAO). The relations between the investigated values of global solar radiation and these macro-circulation patterns are very complicated and they very often have an asynchronous character. The first, juvenile stage of the NAO positive phase (the 1970s and 1980s), when annual sums of global solar radiation in Wrocław-Swojec reached only the average level of about 3700 MJ·m-2 and warm half-year about 2800 MJ·m-2 respectively, was cloudy and rainy. This period was distinctly different than the advanced stage of one (the 1990’s and later years) with bigger sunshine duration and smaller annual precipitation, when the adequate radiation sums amount to 3 900-4000 MJ·m-2 and 3 000-3 100 MJ·m-2 respectively.