Hydrometeorological Conditions for the Occurrence of Aeolian Processes on the Southern Baltic Coast in Poland

This article presents the temporal and spatial variability of hydrometeorological conditions conducive aeolian processes on the Southern Baltic coastal zone in Poland. The analysis made use of daily meteorological (wind, temperature, and rainfall) and hydrological (sea level) data from 1961 to 2010. Data for four stations (Świnoujście, Kołobrzeg, Ustka, Hel) were provided by the Institute of Meteorology and Water Management in Warsaw. A time decomposition of hydrometeorological conditions conducive to the initiation and intensification of aeolian processes in the coastal zone was also performed. In addition to their scientific significance, the temporal and spatial conditions for aeolian processes on the Baltic coast of Poland have an essential utilitarian significance. Modern aeolian processes on the Baltic coast limit the development potential of the coastal zone. Aeolian processes have a positive and negative impact on geomorphological transformation of the sea coast. They take part in the reconstruction of the beach and foredunes after storms. In periods between storms, coastal wind is seen to decrease the balance of beach sediments and lowers the beach area. On the other hand, onshore wind favors, among other things, filling of tourist infrastructure and development located at the hinterland of the beach and dunes. Hydrometeorological conditions especially favorable to the intensification of aeolian processes are the main determinants of geomorphological changes in the coastal zone (some of which can be extreme). Temporal and spatial analysis of hydrometeorological conditions conducive to aeolian processes is important for many areas of human activity, especially those concerning protection, management, and development of the coast.


Introduction
To determine how geo-ecosystems of the coastal zone function, it is necessary to identify in detail the hydrometeorological conditions that most influence the trend, cyclicality, and seasonality of geomorphological processes. An example of such a process is aeolian transport. It constitutes an important factor for shaping beaches, coastal dunes and sandy cliff slopes. Research on aeolian processes was carried out irregularly and in various parts of the Polish Baltic coast, including: Swina Gate Sandbar [1], Wolin island [2,3], Kołobrzeg [4], Mierzeja Łebska [4][5][6][7][8][9][10][11], Lubiatowo [12], and Hel [13]. The dynamics of aeolian transport depend on hydrometeorological factors and the type of substrate surface, especially its roughness and moisture [4,9,10,14,15]. Meteorological conditions (including the direction and speed of wind, atmospheric precipitation, and air and ground

Aim and Methods
The main aim of this work was to conduct a temporal and spatial analysis of hydrometeorological conditions potentially favorable (initiating) and particularly conducive (intensifying) to aeolian processes in the Southern Baltic's Polish coastal zone. The occurrence of these conditions was accompanied by fulfillment of all threshold values. The conditions were classified into two groups (threshold values): -Potentially favorable to initiation of aeolian processes (maximum wind speed ≥4 m·s −1 , no precipitation in the last two days, average daily air temperature >0 • C, maximum sea level (storm level) <570 cm). -Particularly conducive to intensification of aeolian processes (maximum wind speed ≥10 m·s −1 , precipitation <5 mm/2 days, minimum air temperature >0 • C, maximum sea level (average level over many years) <~502 cm).
The above meteorological threshold values were derived from direct field studies of aeolian transport dynamics on the Polish Baltic coast. To determine the impact of weather conditions on the occurrence of aeolian processes in the South Baltic coastal zone, field research was conducted in both the dune coastal zone [1,3,4,17,18,34,35] and the cliff coastal zone [2,36,37]. It is assumed that the mobilization of sandy sediments on the beach begins at wind speeds of 4-5 m·s −1 . This threshold estimated to be 4.4 m·s −1 for the finest dry sands, and 10 m·s −1 for moist material [38]. Marked intensification of aeolian transport occurs during winds blowing from the sea, as well as parallel to the coast, as they buffet the entire beach zone and the slopes of dunes and cliffs. Aeolian processes are limited by atmospheric precipitation, which increases the moisture of surface sediments and limits their ability to move. Field studies [2] and regression analysis (atmospheric precipitation-aeolian transport) revealed that aeolian transport occurs when the sum of atmospheric precipitation from the last two days is <5 mm. In the case of pluviometric conditions, the mobilization of sand as a result of splashing was not accounted for. Apart from wind speed higher than 8 m·s −1 , movement of sand grains due to splashing is responsible for many factors related to, among others, kinetic energy of rain, morphology and forms of land cover during rainfall [10]. Many factors are responsible for the movement of sand as a result of splashing during rainfall. These include the kinetic energy of the rain, and the morphology and types of surface cover. Splashing of sand on the beach takes place even in situations when wind shear speeds are too low to initiate aeolian transport [39]. Aeolian transport is also determined by thermal conditions [7,10]. When surface-level sediments are frozen, the movement of sand is impossible. The initiation of aeolian transport therefore begins when the average daily air temperature is >0 • C. Yet, there are no limitations on the movement of sand when the temperature is above freezing. The dynamics of aeolian processes are also related to the availability of beach sediments. During storm surges (≥570 cm in the Polish Baltic coastal zone), the sea level is high, making the availability of beach sediments for aeolian processes low (beach sediment may be available in the upper beach area), and sometimes cancelling it altogether (e.g., a narrow beach in the cliff coastal zone). For this reason, it was assumed that the conditions for initiation of aeolian processes occurred on days when sea level was below 570 cm. Particularly conducive to intensification of aeolian processes were wide beaches and a maximum sea level lower than the average sea level over the analyzed period, i.e.,~502 cm. This temporal and spatial analysis of how sea conditions affect aeolian processes was based on daily sea level data from mareographic stations inŚwinoujście, Kołobrzeg, Ustka, and Hel. Only the Baltic High System (BHS) (based on the Kronstadt sea-gauge) was used. The estimated difference between the Normal-Null (NN)-and BHS-based systems is about 15 cm (the Kronstadt system is higher). Although Poland uses a high system based on the Kronstadt sea gauge, the registration and recording of sea levels is based on the Normal-Null reference system [29].

Hydrometeorological Data
This study is based on daily hydrometeorological data from 1961 to 2010 concerning: Average, maximum, and minimum air temperature; average and maximum wind speed; the sum of atmospheric precipitation; and average, maximum, and minimum sea level. The data was provided by the Institute of Meteorology and Water Management in Warsaw (https://danepubliczne.imgw.pl). Data was collected from four coastal stations inŚwinoujście, Kołobrzeg, Ustka, and Hel ( Figure 1). One drawback of these stations is that they are located behind the coastal dunes, which means that recorded wind speeds may be lower than those actually occurring on the beach.

Study Area
The Polish coast has a length of 500 km [40]. It represents two basic types of coastline: Dune and cliff ( Figure 1) [41]. Out of this, 80% is constituted by dune, developing in the Holocene. These are usually sandy barriers with a height of 2 to 35 m. When it comes to dune shores, about 15% are constituted by accumulative stretches and around 35% by erosional stretches [1]. Cliffs occupy about 15% of the sea shore [40]. They were created during the Holocene and contemporary erosion of frontal moraines. Moraines are built of glacial clays, usually 10-95 m high, as well as fluvioglacial sand and clays accumulated in the form of ground moraines, most often 6-15 m high. A small part (a few %) in estuarine sections of rivers is constituted by a low coast of flood plains or organic accumulation. The height of this type of coastline is 0-5 m above sea level. Aeolian processes occur on all coastal types, especially in the beach zone. Aeolian processes are also observed on the slopes of dunes and cliffs.
Four areas were selected for the analysis: Świnoujście, Kołobrzeg, Ustka, Hel. Distances between these areas are around 90-130 km. The Świnoujście area covers the section located east of Świnoujście. It is a dune section of the coast, which currently comprises an accumulative section. The Kołobrzeg zone west of the Parsęta River is a dune section. On the other hand, there is a clay shore east of the Parsęta River, with a height of 2-6 m above sea level. The western part of the Ustka area is a dune section. There are sand and clay cliffs up to 41 m above sea level in the eastern part of this area near Dębina. The Hel area, on the other hand, constitutes a spit section of the coast, which is 36 km long. There are also inland dunes in the final part of the Hel Peninsula.

Hydrometeorological Conditions
Analysis of meteorological data revealed that from 1961 to 2010, the maximum daily wind speed was lower than 10 m·s −1 only in Kołobrzeg in 1989Kołobrzeg in , 1991Kołobrzeg in , 2005Kołobrzeg in , 2009, and 2010. In these years, the proper anemometric conditions for aeolian processes in beach sediments wet from precipitation did not occur. The highest maximum wind speed occurred on the eastern coast (Ustka 26 m·s −1 in 2004, Hel 23 m·s −1 in 1999). The average annual wind speed was highest in Hel 4.4 m·s −1 and Ustka 4.3 m·s −1 (exceeded the threshold value of 4 m·s −1 for initiation of aeolian processes in dry sandy sediments), and significantly lower in Świnoujście 3.7 m·s −1 and Kołobrzeg 3.2 m·s −1 . The anemometric conditions in the eastern zone of the Polish Baltic coast were more favorable to aeolian processes ( Figure 2). Wind directions were not taken into account in the study. It was assumed that aeolian processes operating on the beach occur with all directions of wind: Onshore, offshore, alongshore.

Study Area
The Polish coast has a length of 500 km [40]. It represents two basic types of coastline: Dune and cliff ( Figure 1) [41]. Out of this, 80% is constituted by dune, developing in the Holocene. These are usually sandy barriers with a height of 2 to 35 m. When it comes to dune shores, about 15% are constituted by accumulative stretches and around 35% by erosional stretches [1]. Cliffs occupy about 15% of the sea shore [40]. They were created during the Holocene and contemporary erosion of frontal moraines. Moraines are built of glacial clays, usually 10-95 m high, as well as fluvioglacial sand and clays accumulated in the form of ground moraines, most often 6-15 m high. A small part (a few %) in estuarine sections of rivers is constituted by a low coast of flood plains or organic accumulation. The height of this type of coastline is 0-5 m above sea level. Aeolian processes occur on all coastal types, especially in the beach zone. Aeolian processes are also observed on the slopes of dunes and cliffs.
Four areas were selected for the analysis:Świnoujście, Kołobrzeg, Ustka, Hel. Distances between these areas are around 90-130 km. TheŚwinoujście area covers the section located east ofŚwinoujście. It is a dune section of the coast, which currently comprises an accumulative section. The Kołobrzeg zone west of the Parsęta River is a dune section. On the other hand, there is a clay shore east of the Parsęta River, with a height of 2-6 m above sea level. The western part of the Ustka area is a dune section. There are sand and clay cliffs up to 41 m above sea level in the eastern part of this area near Dębina. The Hel area, on the other hand, constitutes a spit section of the coast, which is 36 km long. There are also inland dunes in the final part of the Hel Peninsula.

Hydrometeorological Conditions
Analysis of meteorological data revealed that from 1961 to 2010, the maximum daily wind speed was lower than 10 m·s −1 only in Kołobrzeg in 1989Kołobrzeg in , 1991Kołobrzeg in , 2005Kołobrzeg in , 2009, and 2010. In these years, the proper anemometric conditions for aeolian processes in beach sediments wet from precipitation did not occur. The highest maximum wind speed occurred on the eastern coast (Ustka 26 m·s   Each year, the average annual air temperature was similar across the entire Baltic coast zone. This confirms the rule that thermal conditions are more spatially homogenous than pluviometric and anemometric conditions. The average annual air temperature for the analyzed period decreased from  Świnoujście P annual total Kołobrzeg P annual total Ustka P annual total Hel P annual total Świnoujście P max daily Kołobrzeg P max daily Ustka P max daily Hel P max daily Świnoujście P number of days Kołobrzeg P number of days Ustka P number of days Hel P number of days Pluviometric conditions in the open coastal zone of the Baltic (Ustka and Kołobrzeg) were less favorable to aeolian processes than those in bay areas (Świnoujście and Hel). In Ustka and Kołobrzeg, the following were confirmed to be significantly higher: Average annual sum of precipitation (702.2 mm and 636.6 mm); average annual number of days with precipitation (181 and 180 days); and maximum daily sum of precipitation (94.2 and 85.2 mm). InŚwinoujście and Hel (near the Pomeranian and Gdańsk Bay), the following were confirmed to be significantly lower: Average annual sum of precipitation (561.5 and 586.1 mm); average annual number of days with precipitation (169 and 177 days); and maximum daily sum of precipitation (58.7 and 77.1 mm). Pluviometric conditions in the central zone of the Baltic coast (Kołobrzeg and Ustka) were thus less favorable to aeolian processes than those in the western (Świnoujście) and eastern (Hel) zones ( Figure 3).  Each year, the average annual air temperature was similar across the entire Baltic coast zone. This confirms the rule that thermal conditions are more spatially homogenous than pluviometric and anemometric conditions. The average annual air temperature for the analyzed period decreased from  Świnoujście P annual total Kołobrzeg P annual total Ustka P annual total Hel P annual total Świnoujście P max daily Kołobrzeg P max daily Ustka P max daily Hel P max daily Świnoujście P number of days Kołobrzeg P number of days Ustka P number of days Hel P number of days Each year, the average annual air temperature was similar across the entire Baltic coast zone. This confirms the rule that thermal conditions are more spatially homogenous than pluviometric and anemometric conditions. The average annual air temperature for the analyzed period decreased from 8.5 • C inŚwinoujście, to 8.2 • C in Kołobrzeg, to 8.1 • C in Ustka and Hel. The occurrence of aeolian processes is limited by the freezing of surface settlements (t max ≤ 0 • C). The most days with freezing temperatures were recorded in the eastern coastal zone in Hel, with an annual average of 28 days. Slightly fewer days with freezing temperatures were recorded in the remaining coastal zone, with an annual average of 23 to 25 days. The conditions for aeolian processes were particularly unfavorable in 1963, 1969, 1997, and 2010, when there were over 50 days with freezing temperatures recorded on the entire coast ( Figure 4).
8.5 °C in Świnoujście, to 8.2 °C in Kołobrzeg, to 8.1 °C in Ustka and Hel. The occurrence of aeolian processes is limited by the freezing of surface settlements (tmax ≤0 °C). The most days with freezing temperatures were recorded in the eastern coastal zone in Hel, with an annual average of 28 days. Slightly fewer days with freezing temperatures were recorded in the remaining coastal zone, with an annual average of 23 to 25 days. The conditions for aeolian processes were particularly unfavorable in 1963, 1969, 1997, and 2010, when there were over 50 days with freezing temperatures recorded on the entire coast ( Figure 4).       Particularly favorable conditions for intensification of aeolian processes occurred in the entire coastal zone when the maximum sea level was lower than the average sea level from the analyzed period (~502 cm), and especially when the sea level was exceptionally low (H min < 430 cm). It was then that surface-level beach sediments were most available for aeolian processes. Such events (H min < 430 cm) occurred in the entire coastal zone in 1972, 1979, and 2010. The greatest differences in sea level were recorded on the west coast (Świnoujście and Kołobrzeg), where storm surges and post-storm level drops were particularly significant. Relatively small differences in maximum and minimum sea level occurred on the east coast in Hel.

Potentially and Particularly Favorable Hydrometeorological Conditions for the Occurrence of Aeolian Processes
Temporal and spatial analysis of hydrometeorological conditions for initiation of aeolian processes revealed a greater annual average of events on the coast of the Pomeranian Bay (Świnoujście, 92 days) and the Gdańsk Bay (Hel, 96 days) than in the open coastal zone (Kołobrzeg, 64 days; Ustka, 87 days) ( Table 1)  This is why the occurrence of all these conditions at once in the entire Baltic coastal zone is significantly rarer (annual average of 85 days) than their occurrence individually (sea level, average 360 days; air temperature, average 319 days; wind speed, average 296 days; and precipitation, average 128 days). During the analyzed period, there was a statistically significant (p < 0.05) increase in the number of days per year with an average air temperature of >0 • C (r~+0.3), and a decrease in the number of days per year with a maximum wind speed of ≥4 m·s −1 (r~−0.5). Only in Ustka was there an increase in the number of days per year with a wind speed of ≥4 m·s −1 (r~+0.9). The remaining hydrometeorological conditions, i.e., precipitation of 0 mm/2 days and sea level <570 cm, exhibited a statistically insignificant (p > 0.05) downward trend in the number of days per year, with a low correlation (r < −0.3).
Analysis of the number of days per year with favorable conditions for initiation of aeolian processes revealed a statistically insignificant downward trend forŚwinoujście and Hel (r~−0.2), and a statistically significant downward trend for Kołobrzeg (r~−0.7). Only for the central coast in the region of Ustka was a statistically significant upward trend recorded (r~+0.4) ( Figure 6). Analysis of the number of days per year with favorable conditions for initiation of aeolian processes revealed a statistically insignificant downward trend for Świnoujście and Hel (r ~ −0.2), and a statistically significant downward trend for Kołobrzeg (r ~ −0.7). Only for the central coast in the region of Ustka was a statistically significant upward trend recorded (r ~ +0.4) ( Figure 6). Temporal and spatial analysis of hydrometeorological conditions particularly conducive to intensification of aeolian processes revealed a greater average annual number of events in Świnoujście and Hel (4 days) than in Ustka (3 days) and Kołobrzeg (1 day) ( Table 2) Temporal and spatial analysis of hydrometeorological conditions particularly conducive to intensification of aeolian processes revealed a greater average annual number of events inŚwinoujście and Hel (4 days) than in Ustka (3 days) and Kołobrzeg (1 day) ( Table 2). Conditions particularly conducive to intensification of aeolian processes occurred on the most days in the decade of 1971-1980 (annual average of 5 days), and on the least days in the decade of 1991-2000 (annual average of only 1 day). This happened on the most days in 1976 (average of 9 days) and the least in 1990, 1991, and 1997 (no days). Intensification of aeolian processes in the coastal zone may only occur when all hydrometeorological criteria are fulfilled simultaneously. Such events in the entire Baltic coastal zone are thus very rare (annual average of 3 days). The individual hydrometeorological conditions of these events occur more frequently (air temperature-average of 290 days, precipitation-average of 281 days, sea level-average of 140 days, and wind speed-average of just 10 days). During the analyzed period, there was a statistically significant (p < 0.05) increase in the number of days per year with a minimum daily air temperature of >0 • C (r~+0.3), and a decrease in the number of days per year with a maximum wind speed of ≥10 m·s −1 (r~−0.6). Only in Ustka was there an increase in the number of days per year with a wind speed of ≥10 m s −1 (r~+0.5). The remaining hydrometeorological conditions, i.e., precipitation of <5 mm/2 days and sea level <502 cm, exhibited a statistically insignificant (p > 0.05) downward trend in the number of days per year, with a low correlation (r < −0.3). Analysis of the number of days per year with conditions conducive to intensification of aeolian processes revealed a statistically significant downward trend for Kołobrzeg and Hel (r~−0.6), and a statistically significant upward trend for Ustka (r~+0.5) (Figure 7). ForŚwinoujście, this trend was statistically insignificant.  Analysis of the number of days per year with conditions conducive to intensification of aeolian processes revealed a statistically significant downward trend for Kołobrzeg and Hel (r ~ −0.6), and a statistically significant upward trend for Ustka (r ~ +0.5) (Figure 7). For Świnoujście, this trend was statistically insignificant. The occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone clearly depended on the seasons (Figure 8). During the cold half-year, especially from November to March, the number of days with conditions for the occurrence of aeolian processes was lower than from April to October. The cold half-year is the storm period, during which, despite higher wind speeds, storm surges are not conducive to aeolian processes, as they limit the availability of sandy beach sediments. Furthermore, during this period, there is a higher frequency of days with precipitation, as well as low temperatures causing ground frost. In the warm half-year, there are significantly fewer storm surges  The occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes in the Polish Baltic coastal zone clearly depended on the seasons (Figure 8). During the cold half-year, especially from November to March, the number of days with conditions for the occurrence of aeolian processes was lower than from April to October. The cold half-year is the storm period, during which, despite higher wind speeds, storm surges are not conducive to aeolian processes, as they limit the availability of sandy beach sediments. Furthermore, during this period, there is a higher frequency of days with precipitation, as well as low temperatures causing ground frost. In the warm half-year, there are significantly fewer storm surges and days with precipitation, and no days with freezing temperatures. Conditions particularly favorable to the initiation and intensification of aeolian processes occurred especially in the spring, from April to June. At the beginning of spring, hydrometeorological conditions for the occurrence of aeolian processes were exceptionally favorable, as vegetation was not fully developed at the beginning of the season, and therefore did not limit the availability of sandy sediments for aeolian processes. Additionally, the inflow of dry masses of continental air from the Northeast were frequently recorded during this time. and days with precipitation, and no days with freezing temperatures. Conditions particularly favorable to the initiation and intensification of aeolian processes occurred especially in the spring, from April to June. At the beginning of spring, hydrometeorological conditions for the occurrence of aeolian processes were exceptionally favorable, as vegetation was not fully developed at the beginning of the season, and therefore did not limit the availability of sandy sediments for aeolian processes. Additionally, the inflow of dry masses of continental air from the Northeast were frequently recorded during this time. For the Polish Baltic coastal zone, analysis of the general trend in occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes (based on the maximum number of days per year with conditions for aeolian processes recorded by all four measurement stations) revealed statistically significant patterns (Figure 9). For hydrometeorological conditions potentially favorable to initiation of aeolian processes, an upward trend was observed. However, for hydrometeorological conditions particularly conducive to initiation of aeolian processes, a downward trend was observed. The hydrometeorological conditions from 1961 to 2010 were thus characterized by a certain increase in the number of events (days) with conditions for the occurrence of aeolian processes, albeit of relatively low intensity. potential initiation of aeolian processes potential intensification of aeolian processes For the Polish Baltic coastal zone, analysis of the general trend in occurrence of hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes (based on the maximum number of days per year with conditions for aeolian processes recorded by all four measurement stations) revealed statistically significant patterns (Figure 9). For hydrometeorological conditions potentially favorable to initiation of aeolian processes, an upward trend was observed. However, for hydrometeorological conditions particularly conducive to initiation of aeolian processes, a downward trend was observed. The hydrometeorological conditions from 1961 to 2010 were thus characterized by a certain increase in the number of events (days) with conditions for the occurrence of aeolian processes, albeit of relatively low intensity.

Discussion
Research on the mechanism of aeolian processes in the coastal zone (especially quantitative values and the laws governing the movement of particles by wind) conducted in natural conditions [11,42,43], as field experiments [5,10,15,34,44,45], and especially as laboratory experiments [46,47] has often yielded divergent results. The number of variables determining the mechanism of aeolian processes in the coastal zone is much higher than in desert areas, especially in laboratory conditions (wind tunnels).
Field observations have shown that aeolian processes have positive and negative effects in geomorphological transformations of the coastal zone. The first group contains reconstruction of the beach after storms and during interstorm periods [9]. At this time, sand can be transported by winds from the beach to the dunes [1,9,10,34,45,48]. Ripple marks form on the surface of the beach, as well as aeolian shadows behind various obstacles, transverse sand patches, or even small barchans [10]. The second group contains lowering of the beach surface by seaward and alongcoast winds [4,9,49]. The visible effect of lowering of the beach is aeolian pavement and microshadows behind small obstacles [10].
During a storm build-up, the transverse profile of the beach (width and height) is of great importance; the beach can be flooded completely. During weakening of the storm, the aeolian processes act the most quickly on the upper beach fragment at the base of the dunes or cliff. During large storms, at sea level of 570 cm, sea waves cause erosion of sand dunes [1,41]. Such a situation occurs in many places on the Polish coast, even on beaches with a width of up to 70 m, which have low altitudes.
The threshold hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes determined in the study are a kind of generalization. The threshold values were determined based on a review of literature concerning field research on the dune coastal zone [5,9,10,38], and original research on the cliff coastal zone [2,3]. We can assume that the hydrometeorological criteria used in this study are very general, but appropriate for the South Baltic coastal zone.
Analysis of the variability of hydrometeorological conditions over time on the Polish coast revealed patterns similar to those found in other studies on the South Baltic coast [50][51][52]. Current trends and forecasts of climatic conditions in the South Baltic coastal zone indicate that for every 10-

Discussion
Research on the mechanism of aeolian processes in the coastal zone (especially quantitative values and the laws governing the movement of particles by wind) conducted in natural conditions [11,42,43], as field experiments [5,10,15,34,44,45], and especially as laboratory experiments [46,47] has often yielded divergent results. The number of variables determining the mechanism of aeolian processes in the coastal zone is much higher than in desert areas, especially in laboratory conditions (wind tunnels).
Field observations have shown that aeolian processes have positive and negative effects in geomorphological transformations of the coastal zone. The first group contains reconstruction of the beach after storms and during interstorm periods [9]. At this time, sand can be transported by winds from the beach to the dunes [1,9,10,34,45,48]. Ripple marks form on the surface of the beach, as well as aeolian shadows behind various obstacles, transverse sand patches, or even small barchans [10]. The second group contains lowering of the beach surface by seaward and alongcoast winds [4,9,49]. The visible effect of lowering of the beach is aeolian pavement and microshadows behind small obstacles [10].
During a storm build-up, the transverse profile of the beach (width and height) is of great importance; the beach can be flooded completely. During weakening of the storm, the aeolian processes act the most quickly on the upper beach fragment at the base of the dunes or cliff. During large storms, at sea level of 570 cm, sea waves cause erosion of sand dunes [1,41]. Such a situation occurs in many places on the Polish coast, even on beaches with a width of up to 70 m, which have low altitudes.
The threshold hydrometeorological conditions potentially favorable to initiation and particularly conducive to intensification of aeolian processes determined in the study are a kind of generalization. The threshold values were determined based on a review of literature concerning field research on the dune coastal zone [5,9,10,38], and original research on the cliff coastal zone [2,3]. We can assume that the hydrometeorological criteria used in this study are very general, but appropriate for the South Baltic coastal zone.
Analysis of the variability of hydrometeorological conditions over time on the Polish coast revealed patterns similar to those found in other studies on the South Baltic coast [50][51][52]. Current trends and forecasts of climatic conditions in the South Baltic coastal zone indicate that for every 10-year interval, the average annual air temperature will increase by 1.9 • C, and the annual sum of precipitation will decrease by 4% (meaning around 30 mm) [53]. The trend of increasing temperatures, especially during winter (1.7-2.3 • C), will limit the number of days on which freezing temperatures freeze the ground, and as a consequence, improvement of conditions unfavorable to the functioning of aeolian processes. Furthermore, the drop in precipitation will most likely reduce the number of days with precipitation, and thus increase the occurrence of conditions favorable to aeolian processes. Additionally, the observed increase in sea level by 3-4 cm every 10 years [41] may reduce the width of beaches and the availability of sandy sediments for aeolian processes. The increase in the level of the South Baltic Sea is insignificantly higher than the forecasted rise in global sea level, which is estimated at 1-2 mm·a −1 [54].
The high number of days with conditions potentially favorable to aeolian processes from 1971 to 1980 was accompanied by high annual intensity of potential aeolian transport. In this decade, the value of aeolian transport inŚwinoujście was estimated at around 1075 t·m −1 -from~80 t·m −1 in 1972 to~150 t·m −1 in 1977 [4]. During the same period in Kołobrzeg, there were significantly fewer hydrometeorological events favorable to initiation of aeolian processes. The average annual value of aeolian transport was around 395 t·m −1 -from~30 t·m −1 in 1980 to~60 t·m −1 in 1975 [4]. According to Reference [4], 80% of the total aeolian transport takes place during 8% of the year (for about 1 month). The period of potential increase in aeolian transport (1 month) therefore constituted 30% to 50% of the average annual number of days with conditions favorable to aeolian processes (92 days inŚwinoujście and 64 days in Kołobrzeg).

Conclusions
The relationship between hydrometeorological conditions and aeolian processes in the coastal zone is not linear. Extremely high wind speed does not always generate extreme aeolian erosion, transport, and accumulation on beaches and the slopes of dunes and cliffs, especially when strong winds are accompanied by storm surges and significant precipitation. The geomorphological effects of wind in the coastal zone are determined by many other factors that disturb the relationship between hydrometeorological conditions and aeolian processes. Among the most important of these factors are: Morpholytic conditions on beaches and the slopes of dunes and cliffs; surface exposure to wind; land cover from vegetation in different seasons; the dynamics and frequency of previous extreme hydrometeorological events; and human activity (e.g., hydraulic engineering).
Temporal and spatial analysis of hydrometeorological conditions determining initiation and intensification of aeolian processes in the South Baltic coastal zone in Poland yielded the following conclusions: - The hydrometeorological and morpholithodynamic conditions of the coastal zone are conducive to aeolian processes. The average annual number of days with conditions favorable to initiation of aeolian processes is particularly high in terms of sea level (360 days) and air temperature (319 days). This number is slightly lower for wind speed (296 days), and lowest for precipitation (128 days). All of these factors must occur simultaneously for the initiation of aeolian processes. For this reason, the average annual number of potentially favorable events was 85. The number of days with conditions particularly conducive to intensification of aeolian processes was significantly lower (only three). While the average annual number of days with conducive conditions in terms of air temperature and precipitation was high (290 and 281 days), this number was significantly lower for sea level (140 days), and negligible for wind speed (only 10 days). Therefore, the average annual number of days with conditions potentially favorable to initiation of aeolian processes is nearly 30 times greater than the number of days with conditions particularly conducive to their intensification. - The Pomeranian Bay (Świnoujście) and Gdańsk Bay (Hel) are particularly predisposed to the occurrence of hydrometeorological conditions potentially favorable to aeolian processes. In these areas, aeolian processes can occur for over 3 months per year on average. In the open coastal zone, aeolian processes can occur for around 2 months per year on average, e.g., in Kołobrzeg.

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The upwards trend of conditions particularly favorable to the intensification of aeolian processes for Ustka is probably related to the coastline's exposure to seaward winds. An additional effect may be the increase in share of winds associated with storms. -Clustering of high frequencies of potential aeolian processes occurred in the 1970s. The lowest frequency of hydrometeorological conditions for the occurrence of aeolian processes was recorded in the first decade of the 21st century. Aeolian processes may occur on as few as 32 days per year (Kołobrzeg, 2010), and as many as 143 days per year (Świnoujście and Hel, 1989). In seasonal terms, the hydrometeorological conditions most conducive to initiation and intensification of aeolian processes occurred in the spring, especially from April to June. -Conditions conducive to intensification of aeolian processes indicate that there is positive and negative wind activity within the coastal zone. Seaward wind directions cause formation of forms on the beach and build up foredunes. In addition, they cause flooding of promenades, pavements and streets in seaside resorts. Inland wind directions cause dissipation of dunes and clear off sandy material to the sea. Seaward and alongshore directions contribute to lowering of the beach area and transporting sand to other sections of the beach.
Regional climate models (RCM) predict climate change in the upcoming few decades involving the increase of temperature and precipitation [55]. The efficiency of daily rainfall is also to increase [56], and their frequency will be lower [57]. More frequent and longer periods of heat waves are predicted [16] as well as the possibility of long periods of drought [58], which will be conducive to the functioning of aeolian processes. The number of days potentially beneficial and particularly favorable to aeolian processes is likely to increase. An increase in their intensity is also expected.
Considering the current trend of rising sea levels, climatic changes, and the increasing frequency of extreme hydrometeorological events in the South Baltic coastal zone, it is not possible to reliably forecast the frequency of aeolian processes. However, the results of this study may be useful for determining how the coast functions, especially its beaches, coastal dunes, and moraine cliffs, whose low resistance to aeolian erosion is characteristic for the Baltic coast from Germany to Estonia.