Relationship Between Air Temperature Change and Southern Baltic Coastal Lagoons Ice Conditions

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Article Relationship between Air Temperature Change and Southern Baltic Coastal Lagoons Ice Conditions

Józef Piotr Girjatowicz and Małgorzata Swi´ ˛atek*

Institute of Marine and Environmental Sciences, University of Szczecin, ul. Mickiewicza 16, 70-383 Szczecin, Poland; [email protected] * Correspondence: [email protected]

Abstract: The relationship between air temperature (mainly winter, December-March) in Swinouj´scie,´ Gdynia, and Elbl ˛agand ice parameters (dates of the first ice and disappearance of the last ice, the length of the ice season, number of days with ice, maximum ice height) of southern Baltic coastal lagoons (Szczecin, Puck, and Vistula) was investigated. Trends in these parameters were determined, too. The observation material comes from the archives of the Institute of Meteorology and Water Management and spanned the winters from 1950/51 through to 2019/20. Relationships between the selected ice parameters for the study basins and the values of air temperature were examined using correlation and regression methods. The regression equations and trends, as well as their correlation and determination coefficients, were determined. The statistical significance of these relationships was examined using the Fisher-Snedecor test. Strong correlations between ice parameters values and air temperature were obtained, characterized by high values of both correlation coefficients and statistical significance. All trends of ice parameters indicate mitigation of ice conditions. An acceleration in both temperature and ice condition mildening occurred in the late 1980s, and especially

in the last years of the study period. These trends, except the first ice date, are statistically significant, some even at α < 0.001. The length of the ice season becomes significantly shorter, the number of days

Citation: Girjatowicz, J.P.; Swi´ ˛atek, with ice and the maximum thickness is smaller, and the last ice is disappearing early. An increase M. Relationship between Air in the correlation and determination coefﬁcients and a characterized trend of ice parameters values Temperature Change and Southern towards the East was found. It shows the increased impact of a warming climate in this direction on Baltic Coastal Lagoons Ice Conditions. the southern Baltic coast. Strong correlations and trends may be of prognostic signiﬁcance. Atmosphere 2021, 12, 931. https:// doi.org/10.3390/atmos12080931 Keywords: coastal lagoons; sea ice parameters; Baltic Sea; temperature trends

Academic Editor: Vladimir Ivanov

Received: 30 June 2021 1. Introduction Accepted: 18 July 2021 Published: 21 July 2021 Southern Baltic coastal lagoons are important basins for the maritime economy. These basins are crossed by shipping lanes leading to large ports: to Police, Stepnica and Szczecin

Publisher’s Note: MDPI stays neutral via Zalew Szczeciński;to Puck, Ku´znicaand Jastarnia via Puck Lagoon, and to Baltiysk, with regard to jurisdictional claims in Kaliningrad, and Elbl ˛agvia Vistula Lagoon. In addition to the ports mentioned above, published maps and institutional affil- situated along the coasts of the lagoons are numerous smaller harbors, mostly fishing ports iations. and marinas. The lagoons are sheltered basins, separated from the sea by islands (Szczecin Lagoon) or spits (Puck Lagoon and Vistula Lagoon). Only Puck Lagoon is in direct contact with marine waters via the Gulf of Gdańsk.The remaining lagoons are linked to the sea via channels. In the present paper, the inner part of Puck Bay (which is the western part of the Gulf of Gdańsk)is termed Puck Lagoon due to the considerable shallowing of Rewa Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Mewia (in some papers referred to as Seagull Shallow or Mewia Shallow), protruding This article is an open access article toward the SW from Hel Peninsula, and Rewa Cape, protruding from the mainland coast distributed under the terms and toward the NE opposite the Seagull Shallow. Rewa Mewia is an elongated shoal, only conditions of the Creative Commons about 1 m deep along its whole length, and in some places (especially close to the shore), it Attribution (CC BY) license (https:// is entirely emerged. creativecommons.org/licenses/by/ Both these features distinctly separate the NW part of the bay from the remainder, 4.0/). effectively making it a lagoon with respect to water circulation, temperature and ice

Atmosphere 2021, 12, 931. https://doi.org/10.3390/atmos12080931 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 931 2 of 20

conditions, and biota. Between the shallow and the cape, there is a dredged furrow which enables a crossing for the vessels traveling from Gdynia to Puck. The common occurrence of ice phenomena on coastal lagoons, and their rapid development, requires, nearly every winter, a continuous, comprehensive study, that would enable, for instance, forecasting ice conditions. Assessing the time of ice occurrences and intensity has considerable significance for shipping companies and fishing fleets, and for other enterprises, including tourism. Ice phenomena also have a direct impact on shore erosion. Ice cover, especially its duration, impacts biota in a given basin [1–3]. It changes the living conditions of biota, enables or disables photosynthesis, and regulates both eutrophication and biodiversity to a high degree. Ice cover is also an important factor influencing water circulation and bottom sediment accumulation [4]. Previous research largely focused on ice phenomena on the southern Baltic coastal lagoons from a physical perspective [1,5–9]. Rukšéniené et al. [10] and Jakimiviˇciuset al. [11] investigated ice phenomena on the Curonian Lagoon, located a short distance NE from the basins studied in the present work. However, there are no studies concerning forecasting analyses of the basic ice parameters, such as: dates of the first ice occurrence and last ice disappearance, ice season duration, number of days with ice, and maximum ice thickness, which precisely determine the ice conditions in a winter season. Although first attempts at analyzing the relationships between ice parameters and air temperature at the southern Baltic coast were undertaken in the 1990s [12], these early works did not investigate all ice parameters. For instance, first ice occurrence and last ice disappearance dates, and maximum ice thickness were not investigated. Having a seasonal forecast (for winter) of the basic ice parameters would enable the enterprises to use the studied basins to make rational plans for their respective economic activities. The scientific and applied value of such forecasts increases with the length of time series for the studied parameters included in the observational data set. It is also essential to apply objective methods of quantitative analysis. Because of this, long-term (seasonal) forecasts are based on statistical methods, including correlation and regression analysis. Given the above, the aim of the present paper is to determine and investigate empirical relationships between winter air temperature and individual ice parameters. In order to determine the impact of trends in air temperature changes on ice conditions on the sheltered southern Baltic basins, we determined linear trends in multi-year changes in ice parameter values for Szczecin, Puck, and Vistula Lagoons.

2. Materials and Methods This study is based on data characterizing the ice conditions on Szczecin, Puck, and Vistula Lagoons, situated along the southern Baltic coast. Ice monitoring stations for these lagoons are located as follows: Trzebiez˙ (Szczecin Lagoon), Puck (Puck Lagoon), and Tolkmicko (Vistula Lagoon; Figure1). Air temperature (AT) data come from weather stations located in close proximity to the studied basins: Swinouj´scie,Gdynia,´ and Elbl ˛agrecording air temperatures for Szczecin Lagoon, Puck Lagoon, and Vistula Lagoon, respectively. All source materials were retrieved from the database of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB), which serves as the state hydrological and meteorological survey in Poland. The data series spans the winters 1950/51 through to 2019/20. Data from the period 1950/51–1999/2000 were published as part of the “Catalogue of ice conditions ... ”[13]. Data from the period 2000/01–2019/20 were retrieved directly from the online IMGW-PIB database. Ice conditions were described by five ice parameters, including first ice (F), last ice (L), ice season duration (S), number of days with ice (N), and maximum ice thickness (H) in a given winter season. There are standard parameters, commonly accepted as the best descriptors of ice conditions during a given winter in a study region cf. [14–16]. The first ice is the date when any ice phenomenon occurred in a given season. Last ice is the last day of ice occurrence in a given winter season. Ice season duration is the number of days Atmosphere 2021, 12, 931 3 of 20

Atmosphere 2021, 12, x FOR PEER REVIEWfrom ﬁrst ice to last ice, inclusively, and the number of days with ice is the sum of3 days,of 21

during which ice occurred on a given basin in a given winter season.

(a)

(b) (c)

Figure 1. Cont.

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(d)

Figure 1. ((a)) LocationLocation ofof coastalcoastal lagoons lagoons on on the the southern southern Baltic Baltic Sea Sea coast; coast; and and maps maps of (ofb) ( Szczecinb) Szczecin Lagoon; Lagoon; (c) Puck (c) Puck Lagoon; La- andgoon; (d and) Vistula (d) Vistula Lagoon. Lagoon. In figure In figure (a) the (a dotted) the dotted line marks line marks the border the border of the of Puck the Puck Lagoon. Lagoon. In figures In figures (b–d ()b the–d) black the black line underlineline underline weather weather stations stations and and the redthe red lines lines show show ice monitoringice monitoring stations, stations, the the pink pink line line marks marks the the state state border. border. Source Source of of maps (b–d): https://en.mapy.cz/, accessed on 14 July 2021, maps were slightly modified by authors. maps (b–d): https://en.mapy.cz/, accessed on 14 July 2021, maps were slightly modified by authors.

IceIn orderconditions to assess were the described diversity ofby distributionfive ice parameters, among these including ice parameters, first ice (F), we last used ice a (L),coefficient ice season of variability duration (i.e.,(S), number the standard of days deviation with ice to (N), the arithmeticand maximum mean ice ratio thickness for a given (H) invariable). a given Forwinter regression season. equationsThere are concerningstandard parameters, calendar dates, commonly the actual accepted dates as of the F and best L descriptorswere converted of ice to conditions numerical during values. a 1 given November winter was in a assigned study region number cf. [14–16].1, 2 November The first— icenumber is the 2, date and when so on, any until ice the phenomenon latest date (19 occurred April), in which a given was season. assigned Last the ice consecutive is the last daynumber of ice 170. occurrence In order in toa given examine winter the season. relationship Ice season between duration AT and is the ice number parameters of days in frommore first detail, ice weto last used ice, mean inclusively, AT values and not th onlye number for the of standarddays with winter ice is periodthe sum spanning of days, duringDecember which through ice occurred March (Dec–Mar) on a given but basin also in for a given the periods winter Nov–Dec, season. Dec–Jan, Feb–Mar, and Mar–Apr.In order to Thisassess was the prompted diversity of by distri the observationbution among that these the firstice parameters, ice tended we to occurused awithin coefficient the former of variability two intervals, (i.e., the while standard the lastdeviation ice tended to the to arithmetic disappear mean from ratio the study for a givenbasins variable). in the latter For two regression intervals. equations concerning calendar dates, the actual dates of F and LEmpirical were converted relationships to numerical between values. ice parameters 1 November (F, L,was S, N,assigned H) and number average 1, winter2 No- vember—numberAT were investigated 2, and using so on, correlation until the andlatest regression date (19 April), methods. which Ice was parameters assigned were the consecutivethe assumed number dependent 170. In variables order to (predicted examine the variables, relationship y), and between AT was AT regarded and ice param- as the etersindependent in more variabledetail, we (predictor used mean variable, AT values x). We not applied only for linear the regressionstandard winter expressed period by spanningthe equation December y = ax +through b, where March a is the (Dec–Mar slope of) thebut regressionalso for the line periods and b Nov–Dec, is the intercept. Dec– Jan,The obtainedFeb–Mar, relationships and Mar–Apr. were This evaluated was prompted statistically. by Correlationthe observation (R) and that determination the first ice tended(R2) coefficients to occur werewithin computed. the former Statistical two interv significanceals, while of the the last relationships ice tended was to disappear examined fromusing the the study Fisher-Snedecor basins in the test latter [17]. two intervals. EmpiricalIce occurrence relationships probability between was calculated ice parameters according (F, toL, theS, N, rules H) and set out average in the winter papers AT by wereJevrejeva investigated et al. [18 ],using Leppäranta correlation [19], and and regres Karetnikovsion methods. et al. [20 ].Ice Ice parameters occurrence were probability the assumed dependent variables (predicted variables, y), and AT was regarded as the inde- pendent variable (predictor variable, x). We applied linear regression expressed by the equation y = ax + b, where a is the slope of the regression line and b is the intercept. The

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on a given basin in consecutive years (p), and standard deviation of the estimator (SD) were computed using the following formulas:

1 N p = I(n), (1) N ∑n=1 r p(1 − p) SD = (2) N where: N represents the total number of seasons (years), n—the season number, I(n)— binary variable, I(n) = 0—season n is ice-free, I(n) = 1—ice occurs during season n. Ice condition value variability coefﬁcients (i.e., the standard deviation to the arithmetic mean ratio) were also computed in the present work.

3. Results 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. [21–23]. 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 (Table1).

Table 1. Morphometric and bathymetric data for the coastal lagoons of the southern Baltic Sea after: [22,23,26].

Surface Average Maximum Shoreline Lake Exposure Lagoon Volume (km3) Area (km2) Depth (m) Depth (m) Length (km) (km2m−1) Szczecin 686.9 2.582 3.8 8.6 243 181 Puck 102.7 0.320 3.1 9.7 52 33 Vistula 838.0 2.300 2.6 5.1 270 322

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. Atmosphere 2021, 12, 931 6 of 20

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 Table2 and presented in Figures2–4. 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.

Table 2. Means and extreme values and coefﬁcients of variability (v) of ice parameters: date of ﬁrst ice occurence (F), date of last ice disappearance (L), number of days with ice (N), duration of ice season (S), maximum ice thickness (H) in southern Baltic coastal lagoons (1950/51–2019/20).

Lagoons Ice Parameters Values Szczecin Puck Vistula earliest 13 November 11 November 11 November mean 25 December 25 December 11 December F (date) latest 27 February 22 February 6 February v 0.40 0.42 0.48 earliest 13 January 8 December 16 December mean 5 March 7 March 15 March L (date) latest 10 April 12 April 19 April v 0.18 0.22 0.18 shortest 0 0 0 mean 64 69 94 S (days) longest 135 139 151 v 0.62 0.62 0.36 minimum 0 0 0 mean 51 56 80 N (days) maximum 123 128 138 v 0.69 0.68 0.45 Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 21

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Table 2. Cont. minimum 0 0 0 mean 51 56 80 N (days) Lagoons Ice Parameters maximumValues 123 128 138 v Szczecin0.69 Puck0.68 Vistula0.45 lowestlowest 0 0 0 meanmean 17 20 28 HH (cm) (cm) highesthighest 50 70 70 v 0.72 0.72 0.71 0.55

180

160

140

120

100

20 Median 25%-75%

Consecutive number of the day starting from 1 Nov 1 from starting day the of number Consecutive 0 Min - Max Sz-F Sz-L P-F P-L V-F V-L Mean

Figure 2. BoxBox and and whisker whisker plots, plots, presenting presenting descriptive descriptive me measuresasures for for first ﬁrst ice ice (F) (F) and and last last ice ice(L) (L)on onSzczecin Szczecin Lagoon Lagoon (Sz–F (Sz–F and and Sz–L, Sz–L, respectively), respectively), Puck Puck Lagoon Lagoon (P–F (P–F and andP–L, P–L, respectively), respectively), and andVis- Vistulatula Lagoon Lagoon (V–F (V–F and and V–L, V–L, respectively) respectively) thro throughugh the the period period 1950/51–2019/20. 1950/51–2019/20. Boxes Boxes indicate indicate the thepositions positions of the of lower the lower and andupper upper quartile, quartile, and andwhiskers whiskers represent represent the maximum the maximum and andminimum minimum val- valuesues of ice of iceparameters. parameters. Consecutive Consecutive day day numbers numbers (wit (withh 1 Nov 1 Nov as asday day 1) 1)are are given given instead instead of of dates. dates.

160The 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 wind140 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 120 the east. On average, S equals 64 days for Szczecin Lagoon to 69 days for Puck Lagoon to 94 days100 for Vistula Lagoon (Table2, Figure2). 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).80 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, respectively60 (Table2, Figure3). Additionally, H increases toward the east, on average from

17Number cmof days for Szczecin Lagoon to 28 cm for Vistula Lagoon (Table2, Figure4). 40 An analysis of variability coefficients for ice parameters (Table2) indicates that these coefficients20 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 characterized0 by more severe, and more stable climatic conditions. Median The variability in ice parameters on Puck Lagoon is strongly influenced by the 25%-75% inflows of marine waters. In this-20 case, contact with open marine waters is considerably Min larger - Max than in the case of Sz-S Sz-N P-S P-N V-S V-N Mean Szczecin and Vistula lagoons. This results in a stronger motion and exchange of waters, and consequently, in relatively high ice parameter variability coefficients (Table2). In the

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minimum 0 0 0 mean 51 56 80 N (days) maximum 123 128 138 v 0.69 0.68 0.45 lowest 0 0 0 mean 17 20 28 H (cm) highest 50 70 70 v 0.72 0.71 0.55

180

160

140

120

100

20 Median 25%-75%

Consecutive number of the day starting from 1 Nov 1 from starting day the of number Consecutive 0 Min - Max Sz-F Sz-L P-F P-L V-F V-L Mean

Atmosphere 2021, 12, 931 Figure 2. Box and whisker plots, presenting descriptive measures for first ice (F) and last ice (L)8 of on 20 Szczecin Lagoon (Sz–F and Sz–L, respectively), Puck Lagoon (P–F and P–L, respectively), and Vis- tula Lagoon (V–F and V–L, respectively) through the period 1950/51–2019/20. Boxes indicate the positionscase of Szczecin of the lower Lagoon, and upper high quartile, variability and inwhiskers ice parameters represent the is inﬂuenced maximum and by theminimum relatively val- ueshigh of trafﬁcice parameters. on the shipping Consecutive lane. day numbers (with 1 Nov as day 1) are given instead of dates.

160

140

120

100

60 Number of days 40

0 Median 25%-75% Atmosphere 2021, 12, x FOR PEER REVIEW 8 of 21 -20 Min - Max Sz-S Sz-N P-S P-N V-S V-N Mean

Figure 3. Box and whisker plots, presenting descriptive measures for ice season duration (S) and the Figure 3. Box and whisker plots, presenting descriptive measures for ice season duration (S) and the number of days with ice (N) on Szczecin Lagoon (Sz–S and Sz–N, respectively), Puck Lagoon (P–S number of days with ice (N) on Szczecin Lagoon (Sz–S and Sz–N, respectively), Puck Lagoon (P–S and P–N, respectively), and Vistula Lagoon (V–S and V–N, respectively) through the period 1950/51– and P–N, respectively), and Vistula Lagoon (V–S and V–N, respectively) through the period 1950/51–2019/20.2019/20. Boxes indicate Boxes indicate the positions the positions of the lower of the and lower upper andquartile, upper quartile, and whiskers and whiskers represent repre- the sentmaximum the maximum and minimum and minimum values of values ice parameters. of ice parameters.

20 Thickness of ice (cm) ice of Thickness

0 Median 25%-75% -10 Min - Max Sz-H P-H V-H Mean

Figure 4. BoxBox and and whisker whisker plot, plot, presenting presenting descriptive descriptive measures measures for formaximum maximum ice thickness ice thickness in sea- in sonseason (H) (H) on onSzczecin Szczecin Lagoon Lagoon (Sz–H), (Sz–H), Puck Puck Lagoon Lagoon (P–H), (P–H), and and Vistula Vistula Lagoon Lagoon (V–H) (V–H) through thethe period 1950/51–2019/20. Boxes Boxes indicate indicate the the positions positions of of the the lower lower and and upper upper quartile, and whiskers represent the maximum and minimum valuesvalues ofof iceice parameters.parameters.

TheIce occurs last ice the phenomena most frequently are fine on floes the lagoons and bras locatedh ice. inThe the location eastern of part the of last the ice southern disap- pearanceBaltic coast. on Onthe Vistulalagoons Lagoon, is determined the easternmost by wind direction. basin, no iceIn spring, occurred the only prevailing once, during wind directionthe winter is of from 2019/20. the SW, No which ice occurred causes on the Puck last Lagoonice to melt four in times, NE parts during of the wintersrespective of 1974/75,lagoons. The 2007/08, period 2014/15, between and F and 2019/20. L (i.e, the On ice the season) westernmost is progressively Szczecin Lagoon,longer toward no ice theoccurred east. On seven average, times, S during equals the 64 wintersdays forof Szcz 1974/75,ecin Lagoon 1987/88, to 69 1988/89, days for 1989/90, Puck Lagoon 2006/07, 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. Vis- tula 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,

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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 (Table3). 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.

Table 3. Probability of ice (p) and its standard deviation (SD) in southern Baltic coastal lagoons (1950/51–2019/20).

Lagoons Winters without Ice p SD Szczecin 7 0.900 0.036 Puck 4 0.943 0.028 Vistula 1 0.986 0.014

Notably, relatively strong relationships occur between individual ice parameters on the coastal lagoons, with correlation coefficients usually exceeding 0.80 (Table4). 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 (Table4). 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.

Table 4. Correlation coefﬁcients between the individual ice parameters (designations as in Table2) in southern Baltic coastal lagoons (1950/51–2019/20).

FLSNH Szczecin Lagoon F −0.41 *** −0.86 *** −0.68 *** −0.53 *** L −0.41 *** 0.82 *** 0.82 *** 0.71 *** S −0.87 *** 0.82 *** 0.89 *** 0.73 *** N −0.68 *** 0.82 *** 0.89 *** 0.85 *** H −0.53 *** 0.71 *** 0.73 *** 0.85 *** Puck Lagoon F −0.31 * −0.79 *** −0.61 *** −0.50 *** L −0.31 * 0.83 *** 0.84 *** 0.71 *** S −0.79 *** 0.83 *** 0.90 *** 0.75 *** N −0.61 *** 0.84 *** 0.90 *** 0.84 *** H −0.50 *** 0.71 *** 0.75 *** 0.84 *** Vistula Lagoon F −0.14 −0.70 *** −0.49 *** −0.37 ** L −0.14 0.81 *** 0.83 *** 0.68 *** S −0.70 *** 0.81 *** 0.89 *** 0.71 *** N −0.49 *** 0.83 *** 0.89 *** 0.82 *** H −0.37 ** 0.68 *** 0.71 *** 0.82 *** *—value significant at α < 0.05; **—value significant at α < 0.01; ***—value significant at α < 0.001. Atmosphere 2021, 12, 931 10 of 20

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 Swinou-´ j´scie(from 37.4 to −23.6 ◦C), and 66.6 ◦C for Elbl ˛ag(from 36.5 to −30.1 ◦C) [32]. Mean monthly air temperatures are the lowest in January and range from 0 ◦C(Swinouj´scie)to´ −1.5 ◦C (Elbl ˛ag)[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, Table5). 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 ATNov-Dec and ATFeb-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.

Table 5. Correlation coefﬁcients between the air temperature in individual periods and ice parameters (designations as in Table2) in southern Baltic coastal lagoons (1950/51–2019/20).

Ice Parameters Lagoons Mean and Periods Szczecin Puck Vistula F, Dec-Mar 0.45 *** 0.44 *** 0.30 * 0.40 L, Dec-Mar −0.83 *** −0.83 *** −0.83 *** −0.83 S, Dec-Mar −0.84 *** −0.84 *** −0.81 *** −0.83 N, Dec-Mar −0.92 *** −0.93 *** −0.90 *** −0.92 H, Dec-Mar −0.85 *** −0.86 *** −0.87 *** −0.86 F, Nov-Dec 0.65 *** 0.70 *** 0.64 *** 0.66 F, Dec-Jan 0.46 *** 0.51 *** 0.35 ** 0.44 L, Feb-Mar −0.87 *** −0.87 *** −0.88 *** −0.87 L, Mar-Apr −0.69 *** −0.76 *** −0.77 *** −0.74 *—value significant at α < 0.05; **—value significant at α < 0.01; ***—value significant at α < 0.001.

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 (Figure5). This means that the variability in winter (December–March) AT explains the Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 21

Atmosphere 2021, 12, 931 On the study basins, AT makes the strongest impact on N. Linear regression deter-11 of 20 mination 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 ex- plainsvariability the variability in N on the in N studied on the basinsstudied as basi 80–86%.ns as 80–86%. As indicated As indicated by the by y-intercept the y-intercept of the regression line, a 1 ◦C increase in AT from December to March will cause N to become of the regression line, a 1 °C increase in AT from December to March will cause N to be- reduced by as many as 5 and 19 days on these basins, respectively (Figure5). come reduced by as many as 5 and 19 days on these basins, respectively (Figure 5).

(a) (b)

(c)

FigureFigure 5. Correlations betweenbetween the the number number of of days days with with ice (N)ice (N) on ( aon) Szczecin (a) Szczecin Lagoon; Lagoon; (b) Puck (b) Lagoon;Puck Lagoon; (c) Vistula (c) Lagoon,Vistula Lagoon,with winter with (Dec–Mar) winter (Dec–Mar) air temperatures air temperatures (ATs) through (ATs) the thro periodugh the 1950/51–2019/20, period 1950/51–2019/20, along with along the with regression the regression line equation line equation and determination coefficient (R2). and determination coefﬁcient (R2).

TheThe AT AT influences influences N N more more strongly strongly than than S S because N N includes only only those those days days on on whichwhich ice actually occurred. SS isis defineddefined byby thethe dates dates of of F F and and L. L. Ice-free Ice-free days days may may occur occur in inbetween. between. Thus, Thus, this this parameter parameter may may not not correspond correspond to to temperature temperature conditions conditions as as closely closely as asN. N. N, N, which which correlates correlates the the strongest strongest with with temperature temperature conditionsconditions ofof a given winter, will describedescribe ice ice conditions conditions on on a a given given basin basin more more accurately accurately (the (the most most accurate among among the the studiedstudied parameters). parameters). VeryVery strong correlations (negative) concern alsoalso thethe relationshipsrelationships ofof LL withwith ATATFeb-MarFeb-Mar. . DeterminationDetermination coefficients coefficients range range from from 0.75 0.75 for for Szczecin Szczecin Lagoon Lagoon to to 0.77 0.77 for for Vistula Vistula Lagoon Lagoon (Figure(Figure 66).). This means that air temperature variabilityvariability explainsexplains thethe variabilityvariability inin LL onon thethe lagoonslagoons as as 75–77%. 75–77%. As As indicated indicated by by the the regression regression equation, equation, a a 1 1 °C◦C increase increase in in AT AT in in the the Feb-MarFeb-Mar period period will cause LL onon thethe lagoonlagoon toto occur occur on on average average 9–11 9–11 days days earlier earlier (Figure (Figure6). 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 (Table5). Variability in N, S, and F is explained by AT variability as 86, 71, and 49%, respectively (Figure7). 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 (Figure7). On Vistula Lagoon, L (R = −0.88) and H (R = −0.087; Table5) are the most strongly dependent on AT changes. Szczecin Lagoon is characterized by slightly weaker relationships. Such differences in

Atmosphere 2021, 12, 931 12 of 20

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 Atmosphere 2021, 12, x FOR PEER REVIEWstrong inﬂuence of the oceanic climate that is milder and more variable than the climate12 of 21 of

the eastern part of the southern Baltic coast.

(a) (b)

(c)

Figure 6. CorrelationsCorrelations betweenbetween the the date date of of the the last last ice ice disappearance disappearance (L) on(L) ( aon) Szczecin (a) Szczecin Lagoon; Lagoon; (b) Puck (b) Lagoon;Puck Lagoon; (c) Vistula (c) VistulaLagoon, Lagoon, with Feb–Mar with Feb–Mar air temperatures air temperatures (ATs) through (ATs) thethro periodugh the 1950/51–2019/20, period 1950/51–2019/20, along with along the with regression the regression line equation line equation and determination coefficient (R2). and determination coefﬁcient (R2).

NoThe spatial overall diversity conclusion was isobserved that the in relationships the strength betweenof relationships ice parameters between andice param- AT are etersvery and strong. winter This AT is indicatedon the studied by high basins. correlation The values and determinationof correlation coefficients coefficient valuesof the respectiveand their highice parameters statistical significance.among individual This can lagoons be explained are minor, by theof the very order shallow of several depth 0.01.(high AT exposure was observed index), to and make the isolationthe strongest of the influence coastal lagoons on N (R from = −0.93), marine S (R waters, = −0.84), which and Ftogether (R = 0.70), enable in the rapid eastern reaction part ofof thethe studiedcoast, mainly ice parameters on Puck Lagoon to changes (Table in AT. 5). Variability in N, OfS, and all theF is iceexplained parameters by AT studied variability forthe as 86, coastal 71, and lagoons, 49%, respectively N displays (Figure the strongest 7). As indicatedrelationships by the with regression AT. This equations, parameter a is1 the°C air best temperature descriptor increase of the coastal on Puck lagoon Lagoon ice willconditions reduce andN (and is closely S) by 19 linked days, to and the delay severity F by of 12 winter. days (Figure In turn, 7). of On all theVistula ice parameters Lagoon, L (Rstudied = −0.88) for and the H coastal (R = −0.087; lagoons, Table F displays 5) are the the most weakest strongly relationships dependent withon AT AT. changes. This is Szczecinbecause theLagoon formation is characterized of first ice by is influencedslightly weaker not onlyrelationships. by AT, but Such also differences by other factors, in re- lationshipsuch as water strength temperature, for individual snow cover,basins wind, are influenced or marine by water the incursionsdegree of winter (salinity). severity and theIn additionstability of to winter air temperature, temperature future condit studiesions. The should western also focuspart of on the the coast influence is under of awater strong temperature influence of on the the oceanic first ice climate occurrence that is date, milder and and the influencemore variable of solar than factors the climate on the oflast the ice eastern disappearance part of th date.e southern Baltic coast.

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. ATDec-Mar values measured at Swinouj´scie,Gdynia,´ and Elbl ˛agweather 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,

Atmosphere 2021, 12, 931 13 of 20

from 0.12 to 0.16 (Figures8 and9). 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 Atmosphere 2021, 12, x FOR PEER REVIEWat the studied weather stations, winter air temperature rises along the southern Baltic 13 coast of 21

by 0.03–0.04 ◦C/year, or 3–4 ◦C per 100 years (Figures8 and9).

(a) (b)

(c)

Figure 7.7. Correlations between: (a) date of ﬁrstfirst iceice occurrenceoccurrence (F);(F); ((bb)) iceice seasonseason durationduration (S);(S); ((cc)) thethe numbernumber ofof daysdays withwith iceice (N)(N) onon PuckPuck Lagoon,Lagoon, andand airair temperaturetemperature (AT)(AT) fromfrom selectedselected periodsperiods (Nov–Dec,(Nov–Dec, Dec–Mar),Dec–Mar), alongalong withwith thethe regressionregression 2 lineline andand determinationdetermination coefﬁcientcoefficient RR2.. AT datadata spanspan 1950/51–2019/20.1950/51–2019/20.

The statisticaloverall conclusion significance is ofthat trends the relationships in ice parameters between is high ice for parameters all the studied and lagoons, AT are andvery mostlystrong. equals This isα indicated< 0.001 (Table by high6). correlation The average and values determination of correlation coefficient coefficients values range and fromtheir high−0.32 statistical (H) to − significance.0.50 (S). The This largest can changes,be explained i.e., theby the strongest very shallow decreasing depth trends, (high concernexposure S. index), The slope and the of theisolation regression of the line coastal (a) valueslagoons for from regression marine waters, lines range which from to- −gether0.89 forenable Vistula rapid Lagoon reaction to − of1.08 the for studied Puck Lagoon.ice parameters These areto changes very high in values,AT. indicating a reductionOf all the in Sice by parameters about a day studied per year for (Figurethe coastal8). Determination lagoons, N displays coefficients the strongest for the discussedrelationships trends with equal AT. This 0.28 andparameter 0.26, respectively. is the best descriptor For Vistula of Lagoon,the coastal the lagoon reduction ice con- in S isditions thus explainedand is closely as 28% linked over to the the passage severity of time.of winter. A similar In turn, trend of wasall the obtained ice parameters for N on Vistulastudied Lagoonfor the coastal (Figure lagoons,9). The regression F displays equations the weakest indicate relationships that N and with S areAT. both This being is be- reducedcause the on formation average of by first 0.9 days/year.ice is influenced The firstnot only ice occurs by AT, on but average also by 0.3 other days/year factors, later,such andas water the last temperature, ice disappears snow on cover, average wind, 0.5 or days/year marine water earlier. incursions H is diminishing (salinity). on average by 0.3In cm/year addition (Figure to air 9temperature,). 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 Bal- tic coastal lagoons. ATDec-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 (Figures 8 and 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

Atmosphere 2021, 12, x FOR PEER REVIEW 14 of 21

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 Atmosphere 2021, 12, 931 14 of 20 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 (Figures 8 and 9).

(a)

(b)

(c)

FigureFigure 8. Variations8. Variations and and trend trend lines lines for for ice iceseason season durationduration (S) on on ( (aa)) Szczecin Szczecin Lagoon; Lagoon; (b ()b Puck) Puck Lagoon; Lagoon; (c) (Vistulac) Vistula Lagoon, Lagoon, and winter (Dec–Mar) air temperature (AT), along with the regression line equation and determination coefficient R2 (AT and winter (Dec–Mar) air temperature (AT), along with the regression line equation and determination coefﬁcient R2 (AT data span 1950/51–2019/20). data span 1950/51–2019/20).

Nearly all trends in ice parameters, except for F, display an increase in intensity toward the east (Table6). 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 (Table6). 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. AtmosphereAtmosphere2021, 122021, 931, 12, x FOR PEER REVIEW 15 of 2115 of 20

(a)

(b)

(c)

(d)

Figure 9. Variations and trend lines for: (a) ﬁrst ice occurrence (F); (b) last ice disappearance (L); (c) the number of days with ice (N); (d) maximum ice thickness (H) on Vistula Lagoon, and air temperature (AT) from selected months (Nov–Dec, Feb–Mar, and DecMar), along with the regression line equations and determination coefﬁcient R2 through the period 1950/51–2019/20. Atmosphere 2021, 12, 931 16 of 20

Table 6. Correlation coefﬁcients of linear trends of ice parameters (designations as in Table2) in southern Baltic coastal lagoons (1950/51–2019/20).

Lagoons Ice Parameters Mean Szczecin Puck Vistula F 0.38 ** 0.41 *** 0.30 * 0.36 L −0.33 ** −0.36 ** −0.42 *** −0.37 S −0.46 *** −0.51 *** −0.53 *** −0.50 N −0.42 *** −0.47 *** −0.52 *** −0.47 H −0.23 −0.31 * −0.42 *** −0.32 Averages of 0.36 0.41 0.44 0.40 obsolute values *—value significant at α < 0.05; **—value significant at α < 0.01; ***—value significant at α < 0.001.

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.

4. Discussion The analyses performed as part of this study have shown a very strong influence of AT changes on ice conditions on the studied basins, although the investigated relationships between AT and ice parameters were spatially diverse. Various parameters were dependent on temperature increase to a variable degree, and not always were the relationships very strong. Although AT, dependent on insolation and air mass circulation, is the main factor influencing ice phenomena formation, there are also other factors playing significant parts. These include water temperature, which obviously depends mostly on AT. Most of all, water temperature influences the occurrence of the first ice phenomena in a given season. Water temperature has no significant influence on the remaining ice parameters, because in the period from the occurrence of the first ice all the way until the disappearance of the last ice, water temperature is nearly constant and close to 0 ◦C. L is to some degree influenced by the insolation and intensity of solar radiation. L is also influenced by H in a given season [19], and the occurrence of rainfall (liquid precipitation) and strong wind [34]. H is in turn influenced to a high degree by the occurrence of snow cover during a given winter [19]. Ice conditions are influenced also by local geographic conditions such as bathymetry, distance from the open sea, or the degree to which a basin is sheltered from the open sea. Shallow and more exposed basins cool more rapidly, which results in a more intense development of ice phenomena cf. [25,35]. Notably, on unsheltered, marine, deeper basins characterized by more intense water dynamics, the influence of factors other than the temperature on ice phenomena is stronger than on lagoons. Such factors do weaken the relationships with air temperature on lagoons, but the degree of weakening is higher on marine basins. For instance, along the southern Baltic coast (in Hel), the correlation coefficient for the relationship between N and AT was lower than those for the studied lagoons and equaled −0.53 [12]. Such relationships may also be weakened by anthropogenic factors, such as icebreaking services, or discharge of heated and saline waters that disrupt the natural development of ice phenomena. These and other factors (wind, marine water incursions, waves, currents) will obviously weaken the relationship between ice conditions and winter AT. Including these factors in the analysis is impossible, for instance, due to Atmosphere 2021, 12, 931 17 of 20

the lack of observational data. For this reason, the current work is restricted to the most important factor, i.e., AT variations. Differences in salinity do not impact the differences in ice conditions among the studied lagoons. Of all the basins, Puck Lagoon is characterized by the highest salinity, on average 5.97 PSU (Puck station) [26]. This value is only slightly lower than average water salinity in the southern part of the open Baltic Sea, equal to as little as 7.5 PSU [36]. The salinity of Vistula Lagoon in Tolkmicko is 2.34 PSU, and in Szczecin Lagoon in Podgrodzie the salinity is only 0.81 PSU [37]. The latter lagoon may therefore be considered a freshwater basin. Even though Szczecin Lagoon has the lowest salinity, it is characterized by the mildest ice conditions. In this case, the water chemistry is a much less significant factor than climatic conditions, specifically an increase in the severity of winters toward the east. Scientific papers on the contemporary climatic changes emphasize progressive global warming, especially in central and northern Europe. In higher latitudes, it is anticipated that permafrost will recede, glaciers will melt, ice phenomena will develop less intensely, and progressively larger areas of seas and oceans will remain free of ice cf. [38–40]. The influence of the greenhouse effect on the mildening of ice conditions was evident on the Baltic Sea as early as the end of the 18th century [41,42]. Since then, ice conditions on the Baltic Sea have been progressively milder cf. [43–45]. It is estimated that during the 21st century, the maximum sea ice extent on the Baltic Sea will become reduced by 50 to 80% [46]. Through the 20th century (specifically, from 1896 to 1993), on the basins located along the southern Baltic Sea coast, S was becoming 1–3 days shorter per decade [47]. The largest changes in the Baltic Sea ice cover concern the western part of the basin, and areas east of Bornholm [48]. These changes were not as pronounced in the whole of the Baltic. For instance, in the Gulf of Finland, through the latter half of the 20th century, no statistically significant trend in S was observed [49]. Progressively more intense mildening of ice conditions on the Baltic Sea is manifested by the decrease in values of the basic ice parameters. N is decreasing [18,49], as is H [18,50]. The ice season is becoming shorter, depending on locality, by 14 to 44 days per century [18,50,51]. In addition, ice disintegrates by 8 to 12 days per century earlier [18]. Ice disappearance is also progressively earlier in individual winters [49,52]. Similar to the studied lagoons, changes in S (0.8 days per year) were observed also on the Curonian Lagoon [11], located several tens of km NE from the northern coast of Vistula Lagoon. Further, from the latter half of the 19th century, there has been a clear decreasing trend in the annual maximum sea ice extent on the Baltic Sea [50,53]. The results of the present work show that already in the late 1980s there was a clear acceleration of mildening in temperature conditions and ice conditions on the southern Baltic coastal lagoons. Additionally, in distant Japan (Saroma-ko Lagoon, Hokkaido Island), changes in ice parameter values displayed similar dynamics. A sharp decrease in the number of days with ice has been observed there since 1988. Since then, winters without complete ice coverage occur there frequently. Before 1988, such winters were rare on Saroma-ko Lagoon [54].

5. Conclusions Southern Baltic coastal lagoons are shallow, brackish basins characterized by high exposure indices, which facilitate rapid cooling, and intense development of ice phenomena. Their waters are susceptible to changes in temperature and solar conditions, especially to AT variations. In the autumn-winter period, these basins quickly release the accumulated warmth and become covered in ice, predominantly fast ice (ice rind, ice cover). The lagoons are linked to the sea via straits, which may serve as a conduit for marine water incursions in the autumn-winter period. Such marine waters are slightly warmer and more saline than those in the lagoons. Only Puck Lagoon, from the SE, via Puck Bay, is open to marine waters. The close proximity of the Baltic Sea inﬂuences the waters of the lagoons by warming them in winter. - On coastal lagoons, winters are more severe and ice phenomena are more intense toward the east. In the eastern part of the southern Baltic coast, ice occurs earlier and Atmosphere 2021, 12, 931 18 of 20

disappears later, the ice season is longer, the number of days with ice is higher, and the maximum ice thickness is larger. - Additionally, ice cover stability (N/S ratio), p, correlation, and determination coefficients for relationships between ice parameters and AT all increase in an eastward direction. However, coefficients of ice parameter variability, except for F, decrease toward the east. - Physiographic conditions distinguishing the studied basins from the open sea significantly influence the relations between coastal lagoon ice parameters and AT. The relationships are considerably more significant for the coastal lagoons, in comparison to the marine waters of the southern Baltic. - The relationships between ice parameters and AT are weakened by inflows of warmer and more saline marine waters into the lagoons in autumn and winter. Other factors, like strong wind causing water and ice movement, or human activity (discharge of heated and saline waters, shipping traffic, icebreaking services, especially on Szczecin Lagoon), are also significant in this respect. - The strength of trends in ice parameters increases in an eastward direction, which indicates a stronger climate warming toward the east. This pattern is displayed the most clearly by H and N. - Winter AT is clearly increasing along the southern Baltic coast. At the same time, first ice phenomena occur later on the coastal lagoons, and last ice phenomena disappear earlier, S is distinctly shortening, and both N and H are diminishing. - The correlations between ice parameters and AT, and trends in ice parameters and AT, characterized by high correlation coefficients and high statistical significance, may be utilized for forecasting.

Author Contributions: Conceptualization, J.P.G.; methodology, J.P.G. and M.S.;´ software, M.S.;´ validation, J.P.G. and M.S.;´ formal analysis, J.P.G. and M.S.;´ investigation, J.P.G. and M.S.;´ resources: J.P.G. and M.S.;´ data curation, J.P.G. and M.S.;´ writing—original draft preparation, J.P.G. and M.S.;´ writing—review and editing, J.P.G. and M.S.;´ visualization, M.S.;´ supervision, J.P.G. Both authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable (study not involving humans or animals). Informed Consent Statement: Not applicable (study not involving humans). Data Availability Statement: The data used in the work are available on the website: https:// danepubliczne.imgw.pl/data/dane_pomiarowo_obserwacyjne/, accessed on 10 March 2021 and in the catalogs provided in the references list. Acknowledgments: The authors of the article would like to thank the employees of IMGW-PIB for preparing and providing access to source data and two anonymous reviews for their help in improving the quality of the manuscript. Józef Girjatowicz praises the God for the care and protection he experienced during 50 years of his oceanographic studies particularly during ice studies on the southern Baltic coast. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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