**1. 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 [4–11], 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

temperature) determine the initiation, intensity, and duration of aeolian processes. Sea level, on the other hand, determines the availability of beach sediments, which may be subject to deflation, transport, and aeolian accumulation.

Since the internal climate of the Earth varies constantly, climate models are devoted to analyze the non-linear interactions and different time responses of the components in the climate system. With regard to the atmosphere, changes in temperature, rainfall, wind speed and direction are mainly subject to particular analysis [16–18]. Atmospheric parameters may vary over the year. Changes in wind direction affect regional changes in atmospheric factors, such as rainfall and cloudiness [19]. What is more, changes in wind direction and other meteorological factors affect the morphological changes of the coast [20]. Statistical modeling is also used; it takes into account temperature and atmospheric precipitation in the Baltic Sea basin [21]. Research has shown that there are changes in wind directions and speeds in the Estonia region [22]. Sandy beaches, separated from the land by foredunes, favor transport of sand along the coast. Prolonged, above-average winds have a greater impact on sand transport than relatively short-lived strong winds during storms [23]. The analysis of available meteorological data allows to observe changes on the coast occurring as a result of intensifying aeolian processes.

The following take part in development of the beach: Coastal currents in the littoral zone, waves in the inflow zone causing beach erosion during the development of storms and accumulation of sand on the beach during the weakening of storms and aeolian processes in interstorm periods [9]. Material provided by marine alongshore currents comes from eroded cliffs [4,24].

Wind speed, air temperature, as well as air and substrate humidity affect the fraction of exhausted mineral material [10]. Granulation of dunes and sands of the Southern Baltic coast is mainly made up of sands. Research on sediments of sandy beaches in the Baltic Sea coastal zone in Lithuania [25] and in Poland [12] has shown that beaches in these countries are made of fine-grained sands. Other studies on beach sediments on the Curonian Spit, Vistula and Hel have shown that the beaches are dominated by medium- and coarse-grained sand, whereas foredunes are built of fine-grained sands [13,26]. This is in line with the results of a study by Reference [12], which indicated that the graining of beach sediments in Swinouj´ ´ scie, Kołobrzeg, Ustka and Hel is dominated by medium- and fine-grained sands. Coarse sands are found on beaches along which currents and sea waves blur clay thresholds [12].

So far, research carried out in the Baltic Sea basin has been related to various hydrometeorological factors. The rise of sea level [27,28], storm frequencies [29–32] and climatic conditions (also those in other parts of the world) [33] may affect the functioning of aeolian processes in the geosystem of the South Baltic coasts. This work aims to analyze meteorological data, which will allow to establish to answer to the following question: Do hydrometeorological conditions changing in time and space translate into variability of conditions for potential initiation and intensity of aeolian processes in the Polish coastal zone of the Southern Baltic? For this purpose, hydrometeorological threshold values for the potential favorable conditions of aeolian processes were defined, time dynamics and spatial variability of hydrometeorological conditions were indicated, and frequency of intensification of aeolian processes in relation to particularly conducive hydrometeorological conditions was presented. In the Polish coastal zone of the Baltic Sea, this type of research has not yet been done.

#### **2. Materials and Methods**

#### *2.1. 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):


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−<sup>1</sup> for the finest dry sands, and 10 m·s−<sup>1</sup> 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 in Swinouj´ ´ scie, 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].

#### *2.2. 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 in Swinouj´ ´ scie, 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.

**Figure 1.** Area of research—location of measurement stations in the Polish Baltic coastal zone.

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.

#### *2.3. 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: Swinouj´ ´ scie, Kołobrzeg, Ustka, Hel. Distances between these areas are around 90–130 km. The Swinouj´ ´ scie area covers the section located east of Swinouj´ ´ scie. It is a dune section of the coast, which currently comprises an accumulative section. The Kołobrzeg zone west of the Pars ˛eta River is a dune section. On the other hand, there is a clay shore east of the Pars ˛eta 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 D ˛ebina. 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.

#### **3. Results**

#### *3.1. Hydrometeorological Conditions*

Analysis of meteorological data revealed that from 1961 to 2010, the maximum daily wind speed was lower than 10 m·s−<sup>1</sup> only in Kołobrzeg in 1989, 1991, 2005, 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−<sup>1</sup> in 2004, Hel 23 m·s−<sup>1</sup> in 1999). The average annual wind speed was highest in Hel 4.4 m·s−<sup>1</sup> and Ustka 4.3 m·s−<sup>1</sup> (exceeded the threshold value of 4 m·s−<sup>1</sup> for initiation of aeolian processes in dry sandy sediments), and significantly lower in Swinouj´ ´ scie 3.7 m·s−<sup>1</sup> 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).

**Figure 2.** Annual dynamics of wind speed in the Polish Baltic coastal zone.

Pluviometric conditions in the open coastal zone of the Baltic (Ustka and Kołobrzeg) were less favorable to aeolian processes than those in bay areas (Swinouj´ ´ scie 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). In Swinouj´ ´ scie and Hel (near the Pomeranian and Gda ´nsk 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 (Swinouj´ ´ scie) and eastern (Hel) zones (Figure 3).

**Figure 3.** Annual dynamics of precipitation in the Polish Baltic coastal zone.

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 in Swinouj´ ´ scie, 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).

**Figure 4.** Annual dynamics of air temperature in the Polish Baltic coastal zone.

The annual dynamics of the maximum, average, and minimum sea level from 1961 to 2010 are very temporally and spatially varied (Figure 5). The average sea level ranged from 500 cm in Swinouj´ ´ scie to 504 cm in Hel. The absolute maximum sea level was 661 cm (Swinouj´ ´ scie—4 November 1995), and the absolute minimum sea level was 375 cm (Swinouj´ ´ scie—4 November 1979). The amplitude of sea-level fluctuations in the Polish Baltic coastal zone was, thus, 286 cm. It was only in 1966 that no excess of storm level Hmax > 570 cm was recorded at any of the stations. There were no limitations on the availability of beach sediments for aeolian processes on any day of that year.

**Figure 5.** Annual dynamics of sea level in the Polish Baltic coastal zone.

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 (Hmin < 430 cm). It was then that surface-level beach sediments were most available for aeolian processes. Such events (Hmin < 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 (Swinouj´ ´ scie 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.
