Annual sum of precipitation
In the periods of 1951–2013, mean annual precipitation for Poland, based on 50 analyzed stations, amounted to 618 mm and varied from 480 mm in Leszno to more than 1000 mm in Zakopane. In general, the precipitation in the area of central lowland of Poland is the lowest. The precipitation grows to the North and South (Fig. 2a). It has been observed that in present climate, annual sums of precipitation are slightly growing in Poland; however, these changes are not statistically significant in the entire area. More distinct increases are observed in the northern part of Poland (Fig. 2b).
Based on five climate models, as well as on a single MPI-M-REMO climate model, further growth of annual sum of precipitation is predicted in the future years 2061–2090, especially in the northern part of the country. The annual precipitation totals below 600 mm are not projected at all (see Fig. 3a, b). Average projection from five models points that in most areas of Poland, annual precipitation increases will be 200–300 mm and the highest is in the high mountains—over 400 mm. The projection of the MPI-M-REMO model differs significantly from the average projection from five models. This model predicts the highest increases of annual precipitation total on the coast (over 400 mm), while the decreases of over 200 mm in the highest mountains. In most parts of Poland, this projection foresees increases of precipitation in the range of 100–300 mm, slightly higher to the north (Fig. 3c, d).
If we look at the results of analyses for countries neighboring Poland, it turns out that presently, no changes in annual precipitation totals have been observed west of Poland; while east of Poland, there have been noted statistically significant increases at the level of 0.01 for two stations (Baranovici, Pinsk) and of 0.05 for two other stations. Projections for the future point to an increasing trend in the annual precipitation totals for all the stations outside of Poland (both in the east and west). For each station, the average value of the annual precipitation total in the future from five models and from each model separately is higher than the average from the periods of 1951–2013.
Presently, based on the precipitation unevenness index (IPU), intra-year variability of precipitation on the territory of Poland can be described as moderately and medium-unevenly distributed. The mean value of IPU for 50 stations in Poland is 31. The lowest IPU values (marking even distribution) are found for the north-western part of Poland while the highest for the South (Fig. 4a). The distribution of precipitation is uneven during a year, because the summer precipitation is higher than the winter one. The most uneven distribution of precipitation was noted for the 10-year periods of 1951–1960; however, no statistically significant changes in the precipitation unevenness index are observed in the whole periods of 1951–2013 at the 0.05 level. Some weaker tendencies (to more even distribution during a year) were detected in a few stations (Fig. 4b). Based on climate model simulations, the tendency to more even distribution in a year should continue in the future. The expected mean value of IPU for Poland in the future is 16.5 based on five climate models, while about 15 based on MPI-M-REMO model. In the future, there is almost no spatial diversity in the value of IPU across Poland.
Several stations located outside Poland analyzed in this paper are characterized by a medium uneven distribution of precipitation with the values of index IPU varying from 42 to 47% and it does not tend to change during the periods 1951–2013. However, projections for the future predict a clear decline in the value of the unevenness index, estimating it (an average of five models) at a dozen or so percentages in all locations, indicating an even distribution of precipitation over the year.
Summer precipitation total (JJA)
Summer precipitation totals in the periods of 1951–2013 seem to be stable. Neither trends nor even weak tendencies in time series of summer precipitation totals were detected around Poland. There is only one exception from this rule—for the time series of Pułtusk station, an increasing trend was detected of the significance level of 0.05. Of course, the lack of changes in the summer precipitation totals does not indicate their stable share in the total annual sum. However, there are no changes on majority of stations. Some stations showed weak decreasing trends. Such a tendency is most visible in the south of Poland (see Fig. 5). However, the mean share of summer to annual precipitation for Poland in the periods of 1951–2013 (based on 50 stations) reaches 37.7%, and varies from about 31% in coastal areas to more than 43% in the south. There are projected further decreases of share of summer precipitation in the future. The mean share of summer precipitation in the annual total is equal to 24 and 28%, based on five climate models and MPI-M-REMO model, respectively.
As far as the stations outside Poland are concerned, to the west, there have been noted decreases in summer precipitation total and their share in the annual precipitation total for the periods 1951–2013 (including the station of Schwerin, where the decrease is statistically significant at the 0.05 level). In the east, no changes have been recorded. However, projections for the future suggest that by the end of the century, the share of summer precipitation will decrease by about 10% and will amount to 21–26%. These are values close to those estimated for Poland.
Winter precipitation total (DJF)
In this research, winter is interpreted as the sequence of DJF months, where D (December) belongs to the preceding calendar year. This way, the first winter that can be described is the winter of 1952 understood as December 1951, January 1952, and February 1952.
As to the winter precipitation total, Poland is divided into two parts. In the north-west and west areas, there are clear increases in the sum of winter precipitation. At ten stations, these increases are statistically significant at the 0.05 level of and at other stations; some weaker increasing tendencies are detected. For the east and south-east, no changes in winter sum of precipitation are observed. The increases or stability in total winter precipitation lead to increase in their share in annual sum of precipitation, especially visible in the west (Fig. 6). The mean share of winter to annual precipitation for Poland in the analyzed period is equal to 17.4%, and varies from about 12.6% in Kłodzko (the smallest share has been recorded for the south-west) to above 22.4% in Resko (the greatest share of winter precipitation is noted in the north-west). For the future, there are projected further increases of the share of winter precipitation. The mean share of winter precipitation in the annual total is equal to 26.6 and 24.2%, based on five climate models and MPI-M-REMO model, respectively. Comparing the projected changes in the share of winter and summer precipitation to the annual total, one can state that the presently observed changes are likely to continue.
Currently, analyses of winter precipitation in the close vicinity of Poland show a weak increasing tendency, more pronounced to the north. Nevertheless, none of these tendencies are strong enough to be statistically significant at 0.05 level. Projections for the future, however, suggest continuation of such tendencies and increases of winter precipitation total and in the share of winter precipitation in the annual sum in the periods 2061–2090.
Variability in individual months
Analysis of the monthly sums of precipitation shows that the most variable precipitation is observed in July and June, while the most stable is from January to March. The share of June/July precipitation as a rule varies to more than 40% within annual sum of precipitation, while the share of winter months to no more than a dozen or so percent each.
For the periods of 1951–2013, the absolute maximum share of July precipitation within the annual sum was noted in Sandomierz in 2011 and reached the value of 60.0%, while the absolute maximum share of winter month—January—was observed in Słubice in 1976 and amounted to 26.6%. Figure 7 is an example of these characteristics for Toruń station.
Share of march precipitation
As mentioned before, observed annual sums of precipitation have been slightly growing in Poland. It turns out that the most visible increases have been observed during months of colder half-year, especially in March. For the northern and central part of Poland, the statistically significant trend of March precipitation total in the periods 1951–2013 was detected (Fig. 8b). Further grow of March precipitation of about 20 mm in the entire area of Poland is projected by climate models for 2061–2090. It will also cause the increase of share of March precipitation within the annual precipitation total in the future.
To recapitulate, the mean March precipitation total in Poland (for 50 stations) is presently 30.7 mm, while the projection for the future is 63.9 mm—mean value for the five models—and 58.3 mm—based on MPI-M-REMO model. Next, the mean share of March precipitation within annual sum is presently 5.5%. Based on five models, it will grow to 7.7% in the future (or to 7.0% if taking MPI-M-REMO model only), (see Figs. 8 and 9a, b). In general, the smallest increases of share of March precipitation in the annual precipitation total in the future in relation to the present are projected in the west/north-west and will increase in the east/south-east direction (Fig. 9c, d).
Stations outside of Poland perfectly fit in the above-described picture of changes in March precipitation. March increases were recorded only for Potsdam and Schwerin stations (weak trends not statistically significant at the 0.05 level), which corresponds to the greatest changes in north-western Poland. No changes were detected at other stations. However, projections for the future say that in the next time horizon, the changes will cover all analyzed stations and the share of March falls will be similar to that estimated for Poland.
Changes in annual cycle of precipitation
The increase of winter precipitation and decrease of summer precipitation (although not statistically significant) affect the proportion of summer (JJA) to winter (DJF) precipitation, whose mean values for the present and future are 2.2 and 1.2, respectively. The same characteristics for warmer half-year (AMJJAS) and colder half-year (ONDJFM) take the values of 1.8 and 1.1, respectively. Presently, the smallest differences between summer and winter sum of precipitation are noted for the north-western part of Poland, while the highest for the south. This corresponds to the IPU value, presented above in this paper (see Fig. 4). Pearson correlation coefficient between the ratio of JJA to DJF precipitation and the IPU value is about 0.82. No statistically significant changes of the proportion of summer (JJA) to winter (DJF) precipitation totals have been detected for the periods of 1951–2013, but some weak tendencies for a few stations have been noted—Fig. 10a. Similar (slightly visible) changes are noted for the ratio of precipitation in warmer half-year to colder one (Fig. 10b).
These tendencies are projected to continue in the future. As was mentioned before, the unevenness of precipitation will decrease in the future, so the proportion of JJA to DJF precipitation as well as the proportion of AMJJAS to ONDJFM precipitation also is projected to decrease.