3.1. Description of Ice Conditions, and Probability of Ice Occurrence on Coastal Lagoons
Coastal lagoons, in comparison to the open southern Baltic Sea, are much shallower, less saline, and more sheltered, which is reflected in the considerably more frequent (common, in nearly every year) occurrence of ice phenomena on the lagoons cf. [
21,
22,
23]. Ice conditions on the coastal lagoons are also more severe than on the Odra River, and slightly milder than on the coastal lakes cf. [
13,
24,
25]. Fast ice cover is the most frequent ice form on the coastal lagoons (especially on Vistula Lagoon), and fine ice forms such as grease ice, shuga, and pancake ice, as well as ice floes, are dominant on lakes and rivers.
Coastal lagoons are characterized by relatively weak water dynamics. The strongest motion of water masses is due to wind-induced wave action. Water currents are very weak, and no tides occur [
22,
23]. Mean depths of the study basins range from 2.6 m for Vistula Lagoon to 3.8 m for Szczecin Lagoon. For each study basin, the surface area to average depth ratio is high, as reflected by the very high exposure index values (
Table 1).
Coastal lagoons freeze considerably earlier than the unsheltered, coastal marine waters of the southern Baltic. Ice occurs on the coastal lagoons nearly every winter. The first ice occurs on the lagoons rather early, even as early as November. On Vistula Lagoon, ice has been observed as early as late October (22 October 1979, Ušakovo). Ice occurs over a relatively long period, in extreme cases even up to 166 days (Krasnoflotskoye), and its maximum thickness reaches up to 70 cm (Tolkmicko) [
21]. In the sheltered basins of the southern Baltic, higher ice thicknesses are noted only on the Curonian Lagoon (up to 90 cm) [
27,
28].
On Szczecin Lagoon, fast ice disintegration and drift occur earlier than on the remaining two lagoons. Fast ice is divided on Szczecin Lagoon by a furrow which is kept ice-free, thus sustaining shipping via a shipping lane that enables seagoing vessels to enter Szczecin Lagoon from Pomeranian Bay (Baltic Sea), en route to the seaport in Szczecin. The furrow accelerates the disintegration of fast ice cover. Further, an extensive current polynya occurs where the waters and ice ouflow to the sea, i.e., at the southern end of Piastowski Channel (an artificial waterway connecting Szczecin Lagoon to the Pomeranian Bay). The inflow of waters from the Odra River also accelerates the disintegration of fast ice cover in the southern part of the lagoon. Fast ice survives the longest in the northeastern part of Szczecin Lagoon. This is facilitated not only by bathymetric conditions (shoals) but also by a clear dominance of winds blowing from the SW in winter.
On Puck Lagoon, the relatively early fast ice cover disintegration is facilitated by an influx of marine waters from the southeast. Winds blowing from the W/NW push ice floes toward the Gulf of Gdańsk. On Puck Lagoon, fast ice survives the longest in the NW part of the basin. This is facilitated by the presence of land sheltering this part of the lagoon, especially Hel Spit.
Ice drift usually occurs on the study basins during the period of fast ice cover disintegration (late winter/early spring). During periods of strong winds, ice floes drift in an eastward direction, inducing rafted ice and piled ice formation. Ice hummocks form along eastern coasts of the lagoons, and on shoals, often reaching a height of several meters, with a maximum height of 10 m. Mathematical models concerning ice rafting and piling and the height of ice piles on southern Baltic coastal lagoons are presented in the paper by Girjatowicz [
29]. As shown by Kolerski et al. [
30], ice conditions will locally undergo modification due to the ongoing construction of the channel across Vistula Spit, which separates Vistula Lagoon from the sea, and the ongoing construction of artificial islands within Vistula and Szczecin Lagoons.
The earliest ice phenomena observed, mostly during wave action, are coastal grease ice, shuga, and pancake ice, and when conditions are still—coastal ice rind. Such ice conditions occur the earliest in small, shallow bays that are sheltered from wind and waves.
Characteristic values of ice parameters on southern Baltic coastal lagoons are tabulated in
Table 2 and presented in
Figure 2,
Figure 3 and
Figure 4. The first ice phenomena occur the earliest in the eastern part of the coast. The earliest observed first occurrences are in November (11–13), and the latest observed first occurrences arein February (6–27). On average, however, ice occurs in December, from 11 December on Vistula Lagoon to 25 December on Szczecin and Puck lagoons. Last ice disappears the latest in the eastern part of the coast. On Vistula Lagoon, ice disappears on average on 15 March, and the latest on 19 April. On Szczecin Lagoon, the respective dates are 5 March and 10 April.
The last ice phenomena are fine floes and brash ice. The location of the last ice disappearance on the lagoons is determined by wind direction. In spring, the prevailing wind direction is from the SW, which causes the last ice to melt in NE parts of the respective lagoons. The period between F and L (i.e, the ice season) is progressively longer toward the east. On average, S equals 64 days for Szczecin Lagoon to 69 days for Puck Lagoon to 94 days for Vistula Lagoon (
Table 2,
Figure 2). The longest ice season took place on Vistula Lagoon during the very severe winter of 1962/63 and lasted for 151 days (18 November–17 April). Similarly, N also increases toward the east, on average from 51 days for Szczecin Lagoon to 80 days for Vistula Lagoon. The maximum observed N values are 123 to 138 days, respectively (
Table 2,
Figure 3). Additionally, H increases toward the east, on average from 17 cm for Szczecin Lagoon to 28 cm for Vistula Lagoon (
Table 2,
Figure 4).
An analysis of variability coefficients for ice parameters (
Table 2) indicates that these coefficients are higher for Szczecin and Puck Lagoons, and lower for Vistula Lagoon. Vistula Lagoon is located in the eastern part of the southern Baltic coast. In winter, it is characterized by more severe, and more stable climatic conditions. The variability in ice parameters on Puck Lagoon is strongly influenced by the inflows of marine waters. In this case, contact with open marine waters is considerably larger than in the case of Szczecin and Vistula lagoons. This results in a stronger motion and exchange of waters, and consequently, in relatively high ice parameter variability coefficients (
Table 2). In the case of Szczecin Lagoon, high variability in ice parameters is influenced by the relatively high traffic on the shipping lane.
Ice occurs the most frequently on the lagoons located in the eastern part of the southern Baltic coast. On Vistula Lagoon, the easternmost basin, no ice occurred only once, during the winter of 2019/20. No ice occurred on Puck Lagoon four times, during the winters of 1974/75, 2007/08, 2014/15, and 2019/20. On the westernmost Szczecin Lagoon, no ice occurred seven times, during the winters of 1974/75, 1987/88, 1988/89, 1989/90, 2006/07, 2014/15, and 2019/20. It is clear from this overview that ice-free conditions tend to occur in the latter half of the study period. Ice occurrence probability (p) increases toward the east. For Szczecin Lagoon, p equals 0.900, for Puck Lagoon p = 0.943, and for Vistula Lagoon p = 0.986 (
Table 3). The standard deviation of ice occurrence probability (SD) is higher in the western part of the study area (Szczecin Lagoon, 0.036) than in the east (Vistula Lagoon, 0.014). The eastward decrease in the number of ice-free days, increase in p (along with a concomitant decrease in SD) are influenced not only by winters being more severe in the east but also by the higher stability of climatic conditions in that direction. This is related to the stronger continental influence in the eastern part of the southern Baltic coast.
Notably, relatively strong relationships occur between individual ice parameters on the coastal lagoons, with correlation coefficients usually exceeding 0.80 (
Table 4). Only F correlates less strongly with the studied ice parameters (especially with L). The strongest relationship concerns N and S. On Puck Lagoon, the correlation coefficient for both variables equals 0.90. All relationships between the studied ice parameters are statistically significant at α < 0.001 level. Only some relationships with F are statistically significant at a slightly lower level (
Table 4). This section may be divided into subheadings. It should provide a succinct, precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.
The southern Baltic coast is located in a temperate climate zone and is characterized by seasonal changes in temperature and insolation. Two main and two transient seasons occur, determined by the quantity of solar energy influx, and are characterized, especially in the cooler half of the year, by a high intensity of atmospheric circulation. This causes high variability in weather conditions, and diverse, and temporal variable air temperatures. Air temperature amplitudes between summer (Jun-Aug) and winter (Dec-Feb) are very pronounced and exceed 17 °C [
31]. Absolute amplitudes, however, equal 61 °C for Świnoujście (from 37.4 to −23.6 °C), and 66.6 °C for Elbląg (from 36.5 to −30.1 °C) [
32]. Mean monthly air temperatures are the lowest in January and range from 0 °C (Świnoujście) to −1.5 °C (Elbląg) [
31]. A higher air temperature in the western than the eastern part of the coast influences the diversity of ice conditions on the coastal lagoons. Ice conditions are milder in the western part of the coast, which is manifested in a later ice occurrence and earlier disappearance, shorter ice season duration, a lower number of days with ice, and a lower maximum ice thickness cf. [
1,
33].
3.2. Analysis of Relationships between Coastal Lagoon Ice Parameters and Winter Temperature Conditions
Southern Baltic coastal lagoons are located at a similar latitude and within the same climatic zone. They are also characterized by similar physiographic (surface area, depth) and hydrologic conditions. This influences the high similarity among the strength of relationships between ice parameters and AT, but some diversity is evident. The relationships between ice parameters and winter AT are usually highly statistically significant (α < 0.001,
Table 5). They have high correlation coefficients (R), indicating a very strong inverse correlation (except for correlation to F), ranging from −0.81 to −0.93. The strongest relationships, with average correlation coefficients concern N (−0.92). Slightly weaker relationships were observed for L (−0.87), H (−0.86), and S (−0.83). The relationships for F, however, are distinctly weaker. This is because F is influenced not only by AT, but also by water temperature, wind, snowfall, and marine water incursions. The relationships of F and L with AT
Nov-Dec and AT
Feb-Mar, respectively, were stronger than the relationships obtained when AT was averaged for the whole standard winter period (Dec-Mar). This was because F most frequently occurred in the Nov-Dec period, and L most frequently occurred in the Feb-Mar period.
On the study basins, AT makes the strongest impact on N. Linear regression determination coefficients in these cases range from 0.80 for Vistula Lagoon to 0.86 for Puck Lagoon (
Figure 5). This means that the variability in winter (December–March) AT explains the variability in N on the studied basins as 80–86%. As indicated by the y-intercept of the regression line, a 1 °C increase in AT from December to March will cause N to become reduced by as many as 5 and 19 days on these basins, respectively (
Figure 5).
The AT influences N more strongly than S because N includes only those days on which ice actually occurred. S is defined by the dates of F and L. Ice-free days may occur in between. Thus, this parameter may not correspond to temperature conditions as closely as N. N, which correlates the strongest with temperature conditions of a given winter, will describe ice conditions on a given basin more accurately (the most accurate among the studied parameters).
Very strong correlations (negative) concern also the relationships of L with AT
Feb-Mar. Determination coefficients range from 0.75 for Szczecin Lagoon to 0.77 for Vistula Lagoon (
Figure 6). This means that air temperature variability explains the variability in L on the lagoons as 75–77%. As indicated by the regression equation, a 1 °C increase in AT in the Feb-Mar period will cause L on the lagoon to occur on average 9–11 days earlier (
Figure 6).
No spatial diversity was observed in the strength of relationships between ice parameters and winter AT on the studied basins. The values of correlation coefficients of the respective ice parameters among individual lagoons are minor, of the order of several 0.01. AT was observed to make the strongest influence on N (R = −0.93), S (R = −0.84), and F (R = 0.70), in the eastern part of the coast, mainly on Puck Lagoon (
Table 5). Variability in N, S, and F is explained by AT variability as 86, 71, and 49%, respectively (
Figure 7). As indicated by the regression equations, a 1 °C air temperature increase on Puck Lagoon will reduce N (and S) by 19 days, and delay F by 12 days (
Figure 7). On Vistula Lagoon, L (R = −0.88) and H (R = −0.087;
Table 5) are the most strongly dependent on AT changes. Szczecin Lagoon is characterized by slightly weaker relationships. Such differences in relationship strength for individual basins are influenced by the degree of winter severity and the stability of winter temperature conditions. The western part of the coast is under a strong influence of the oceanic climate that is milder and more variable than the climate of the eastern part of the southern Baltic coast.
The overall conclusion is that the relationships between ice parameters and AT are very strong. This is indicated by high correlation and determination coefficient values and their high statistical significance. This can be explained by the very shallow depth (high exposure index), and the isolation of the coastal lagoons from marine waters, which together enable rapid reaction of the studied ice parameters to changes in AT.
Of all the ice parameters studied for the coastal lagoons, N displays the strongest relationships with AT. This parameter is the best descriptor of the coastal lagoon ice conditions and is closely linked to the severity of winter. In turn, of all the ice parameters studied for the coastal lagoons, F displays the weakest relationships with AT. This is because the formation of first ice is influenced not only by AT, but also by other factors, such as water temperature, snow cover, wind, or marine water incursions (salinity).
In addition to air temperature, future studies should also focus on the influence of water temperature on the first ice occurrence date, and the influence of solar factors on the last ice disappearance date.
3.3. Analysis of Trends in Coastal Lagoon Ice Parameters
Climate warming has influenced the mildening of ice conditions on the southern Baltic coastal lagoons. AT
Dec-Mar values measured at Świnoujście, Gdynia, and Elbląg weather stations display a positive trend, statistically significant at α < 0.01 level. Correlation coefficients for these trends range from 0.35 to 0.39, and determination coefficients, respectively, from 0.12 to 0.16 (
Figure 8 and
Figure 9). This means that AT increases on the southern Baltic coast are explained as 12–16% over the passage of time. In earlier periods, up to the winter of 1986/87, low AT and high ice parameter values (L, S, N, H) occurred relatively frequently. From the winter of 1987/88, mild winters definitely prevail, which is manifested in a clear mildening of ice conditions. As indicated by the regression equations of linear trends in AT at the studied weather stations, winter air temperature rises along the southern Baltic coast by 0.03–0.04 °C/year, or 3–4 °C per 100 years (
Figure 8 and
Figure 9).
The statistical significance of trends in ice parameters is high for all the studied lagoons, and mostly equals α < 0.001 (
Table 6). The average values of correlation coefficients range from −0.32 (H) to −0.50 (S). The largest changes, i.e., the strongest decreasing trends, concern S. The slope of the regression line (a) values for regression lines range from −0.89 for Vistula Lagoon to −1.08 for Puck Lagoon. These are very high values, indicating a reduction in S by about a day per year (
Figure 8). Determination coefficients for the discussed trends equal 0.28 and 0.26, respectively. For Vistula Lagoon, the reduction in S is thus explained as 28% over the passage of time. A similar trend was obtained for N on Vistula Lagoon (
Figure 9). The regression equations indicate that N and S are both being reduced on average by 0.9 days/year. The first ice occurs on average 0.3 days/year later, and the last ice disappears on average 0.5 days/year earlier. H is diminishing on average by 0.3 cm/year (
Figure 9).
Nearly all trends in ice parameters, except for F, display an increase in intensity toward the east (
Table 6). Correlation coefficients for the relations between individual ice parameters and passage of time increase from the west (Szczecin Lagoon) to the east (Vistula Lagoon) by 0.07 for S; 0.09 for L, 0.10 for N, and 0.19 for H, respectively. However, absolute values of average correlation coefficients increase by 0.08, that is, from 0.36 for Szczecin Lagoon to 0.44 on Vistula Lagoon (
Table 6). An increase in correlation coefficients for trends in ice phenomena toward the east indicates the impact of climate warming is stronger in the eastern than in the western part of the southern Baltic coast. Negative values of correlation coefficients for trends in ice parameters indicate that L will occur 0.36–0.52 days/year earlier, S will become 0.89–1.08 days/year shorter, N will be diminishing by 0.72–0.91 days/year, and H will become 0.14–0.32 cm/year thinner. It is a clear manifestation of climate warming in Europe, like in the whole of the Northern Hemisphere.
In general, both the determined trends in winter AT and in coastal lagoon ice parameters indicate a pronounced mildening of climate conditions. An acceleration in both temperature and ice condition mildening occurred in the late 1980s, and especially in the two last decades of the study period (2000–2020). Trends in AT and individual ice parameter changes are relatively strong and highly statistically significant. The reason for mildening in winter ice conditions may be explained by progressive climate warming. The strongest trends observed for the coastal lagoons concern S and N. H and F are undergoing changes to a considerably lesser extent.