The paper identifies relationships between lake water levels and indices of macroscale atmospheric circulations: Arctic Oscillation (AO), North Atlantic Oscillation (NAO), East Atlantic (EA), and Scandinavian pattern (SCAND). Correlation coefficients between synchronous and asynchronous series of monthly water levels and 4 circulation indices were calculated. Based on Ward hierarchical grouping considering 156 correlation coefficients, the groups of lakes were designated due to the strength and term of relation of circulation indices with lake water levels. It was found that these links are not strong but noticeable. The strength of relationships varies in space and time, and the designated groups of lakes refer not only to the climatic diversity of the studied area, but also to some extent to the types of water levels regime. The observed relationships are the most important in the case of AO and NAO (particularly in winter period), and slightly weaker for EA and SCAND. The analysis used mean monthly water levels of 15 lakes in northern Poland from 1976–2015.

The hydrology of coastal lakes differs significantly from that of inland water bodies due to the influence of the neighboring sea. Observed climatic changes are expected to enhance the effect of the sea on coastal lake ecosystems, which makes research on sea–lake interactions even more significant. In this study, on the basis of maximum annual and monthly values of water level, dependencies among the water levels of six lakes located along the southern coast of the Baltic Sea in Poland, and the Baltic Sea water levels were analyzed. First, the Spearman rank correlation and the linear regression method were applied. Then, selected copulas were used to find joint distributions of the studied time series. In the next stage, the degrees of synchronous and asynchronous occurrences of maximum water levels in lakes and the sea were calculated. The study revealed that correlations between the maximum annual water levels in coastal lakes and in the Baltic Sea in the selected gauge stations were very strong and statistically significant. These results were confirmed by a synchronicity analysis carried out with the help of a copula function. The highest relationship was detected in the case of Lake Resko Przymorskie (correlation coefficient 0.86, synchronicity 75.18%), while the lowest were observed in Lakes Jamno (0.62 and 58.20%, respectively) and Bukowo (0.60 and 56.82%, respectively). The relation strength between maximum water levels of the sea and coastal lakes may increase in the future due to sea level rises caused by climate warming.

The paper presents the characteristics of hydrological periods in an average annual cycle in Polish lakes. The types of period and their sequence determine the regime of water stages in lakes. The article applies the unsupervised approach to analysis of water level fluctuation patterns, where the regime is identified by grouping analytical parameters. Hydrological periods were designated by grouping elementary time units of the hydrological year (pentads) based on the similarity of their parameters, namely water level frequency distributions. The analysis covered daily water stages in 33 lakes in Poland from the period from 1984 to 2012. Five types of hydrological period were designated. The studied lakes differ in the number, type and sequence of hydrological periods in an average annual cycle. Most of the lakes (19 lakes) have a 4-period temporal structure of water stages with the course of water stages in a year characteristic of this geographical zone. No spatial patterns occurred in the location of lakes from particular groups. This suggests the dominant role of local factors in determining the seasonality of water stages.

Water level fluctuations of coastal lakes are more dynamic than in case of other lakes located in the medium latitude zone. In this study, mean annual values of water level of 6 coastal lakes (Resko Przymorskie, Jamno, Bukowo, Gardno, Łebsko, Druzno) of southern coast of the Baltic Sea and sea water level were analyzed, using correlation coefficient, it was also pointed at possibility of modelling dependencies between chosen variables using Copula theory. The highest correlation coefficient was noted for the pair Łebsko Lake–Baltic Sea, gauge station in Łeba (0.91), and the lowest one for the pair Jamno Lake–Baltic Sea, gauge station in Kołobrzeg (0.67). The results of correlation analysis were proved by analysis using Archimedean Copulas. The highest synchronicity in relation to sea water levels was observed for Łebsko Lake (77.54%), the lowest one for Jamno Lake (59.98%). The method on the basis of Copula functions applied in the study shows that not only traditional correlation methods allow to verify dependencies and their strength between data series. There was noted strong dependency between correlation coefficient values and synchronicity. The obtained results allow to conclude that there is diversified strength of dependencies between mean annual water levels of coastal lakes in Poland and mean annual water levels of the Baltic Sea.