Czasowa i przestrzenna charakterystyka ekstremalnych poziomów wód Morza Bałtyckiego

Date of release of electronic version under CC-BY-SA license: April 2025

Spatial and temporal characteristics of the extreme sea levels of the Baltic Sea

The purpose of this monograph was to show spatial and temporal characteristics of the extreme sea levels of the Baltic Sea by means of analysis of the long-term tendencies in extreme sea levels and by identification of their geographical pattern. The purpose along with the scientific tasks, which were mentioned in the introduction, have been implemented in the subsequent chapters of this monograph by studying the extreme sea levels in three timeframes: long-term changes within the entire 50-year period 1960–2010, seasonal changes which means month-to-month changes in a year and short-term changes which concern storm surge and fall events lasting for a couple of days. All the analyses were possible to perform thanks to collecting detailed, 1-hour sea level data from several tens of tide gauges located along the Baltic Sea coast. Such detailed approach played a key role, since it allowed to depict temporary state of the Baltic surface topography in any moment of the multiyear period and it made available the performance of precise quantitative statistics for the entire investigated timeframe. It is worth noticing that some countries are not willing to share the high resolution hydrological data due to commercial (e.g. Latvia) or security reasons (e.g. Russia). Other Baltic countries provided reliable hourly observations of the sea level derived from relevant hydrological and meteorological institutions.

The second basic factor, which allowed for the implementation of the scientific tasks of this monograph, was an acceptance of the one reference datum – Normaal Amsterdams Peil (NAP). European Vertical Reference System is based on this datum. In this situation it was possible to convert the observed sea level data derived from particular Baltic countries to the NAP datum. Picture of the Baltic surface obtained in this manner and demonstrated in relation to a uniform reference datum allowed for spatial display of the geographical pattern of the extreme sea level occurrence in the Baltic Sea. This research issue is unique and has not been investigated in the oceanological literature in relation to the Baltic Sea.

The main feature of this monograph is a fact that all analyses of extreme sea levels are based on the observational data and not prognostic ones derived from a model. A comparison of measurement data and prognostic data obtained from a hydrodynamic model HIROMB, which is the most advanced model applied to the Baltic Sea, revealed significant differences during storm simulations. The HIROMB model most frequently underestimated the falls in sea level in the Western Baltic and overestimated the height of surges in the Gulfs of Finland, Gulf of Riga and Gulf of Bothnia. The differences for tide gauges located within the gulfs were the greatest among the entire Baltic (above 60 cm), while the smallest (under 30 cm) referred to the open waters of the Baltic Proper (tab. 10.2).

True measurement data applied to the work allowed to carry out precise analyses of the extreme sea levels and then to show reliable results.

Tendencies in extreme sea levels in the long-term: 1960–2010

Analyses of temporal changes in extreme sea levels for the period 1960–2010 proved for most of the tide gauges that there exist an evident process consisting in the increment of the amount of hours with high sea levels (≥70 cm above NAP), the increment of frequency of occurrence of storm events and the increment of maximum annual sea level heights (tables 6.2–6.6, fig. 6.2–6.4). The amount of hours with high sea levels of the Baltic doubled, on average, in 1960–2010, whereas the increment of the maximum annual Baltic sea level height, averaged for 34 of 37 tide gauges, amounted to 3 mm∙year–1. Such a large scale of this phenomenon indicates the reliability of the process of constant increment of extreme sea levels.

On the other hand, a decline in the amount of hours with low sea levels (≤–70 cm) was noticed along with unambiguous tendencies of changes in minimum annual sea level heights (tables 6.5– 6.6). The main reason for such situation is an intensification of inflow of western air masses and increase in the cyclonic activity, i.e. intensification of western circulation, which is observed in the second half of the 20th century, in particular (subchapter 2.4 and 2.5 and fig. 2.6). This interpretation is justified in many scientific papers regarding the Baltic written by Heyen et al. (1996), Suursaar i Söör (2007), Johansson et al. (2001), Miętus et al. (2004), Jakusik et al. (2010). Some of scientists claim that mean Baltic Sea level rise is responsible for increments of extreme sea levels apart from changes in atmospheric circulation (Rotnicki and Borzyszkowa 1999). Similar tendencies – increment of storm activity and extreme sea levels are observed across the world as well. The latest 5th IPCC report (IPCC 2013) and latest scientific works regard most often the increment of the mean sea level (MSL) and dominant indicators of climate changes, especially ENSO and NAO, as the reasons for the aforementioned changes (Bengtsson et al. 2006, Bindoff et al., 2007, Lowe et al. 2010, Walsh 2011, Pugh and Woodworth 2014).

Geographical pattern of the extreme sea levels of the Baltic Sea

From presented the geographical distribution and numerical characteristics of extreme water levels and the number of storm surges, and as a result of research carried out in the framework of the project NCN (Wolski 2011–2014) determined a clear regularities on the occurrence of extreme sea levels of the Baltic Sea. These patterns also confirmed in the publication Wolski et al. (2014, 2016) as well as Wolski and Wiśniewski (2016). These are the following regularities:

1. Eastern and northeastern coasts of the Baltic Sea, which are exposed to the inflow of western air masses related to a western atmospheric circulation, including the dominant tracks of pressure systems, are especially at risk of extreme hydrological events. In particular, it refers to Gulf of Riga along with Pärnu Bay, Gulf of Finland, and Gulf of Bothnia. These water basins experience the largest amounts of storm surges, the longest duration of high sea levels (≥70 cm) and the highest water levels in general (fig. 6.6–6.8, tables 9.1–9.3). On the contrary, Swedish coasts of Central and Northern Baltic are the least endangered by extreme sea levels within the Baltic Sea basin (fig. 6.7–6.8, table 9.5). It is explained mostly by their eastern exposition, which constitutes an opposite direction to the inflow of western air masses and to the direction of low pressure systems propagation. In the conditions of western circulation, the filling up of the Baltic Sea increases and the inclination of water surface towards eastern coasts of the Baltic Sea increases as well. This characteristic regularity is in line with the results of works of Averkiev and Klevanny (2007, 2010), Suursaar et al. (2003, 2006a, 2006b, 2007, 2009), Johansson et al. (2001), Wolski et al. (2014, 2016), according to which, the eastern and northeastern coasts of the Baltic (Gulf of Riga and Gulf of Finland and a part of Gulf Bothnia) are exposed to dangerous storm events and extreme sea levels induced by deep low pressure systems that cross the investigated area and by occurrence of strong landward winds.

2. South-western coasts of the Baltic Sea: Bay of Mecklenburg and Bay of Kiel are water basins of the most frequent and the deepest falls as well as of the extremely low sea levels (≤–70 cm) (fig. 6.6–6.8). Eastern exposition of these bays favours the water outflow from their basins by fast-moving mesoscale low crossing the Baltic Sea from SW to NE. At the same time, Bay of Mecklenburg and Bay of Kiel give way only to big north-eastern gulfs only in terms of frequency of occurrence of high sea levels, maximum heights during high sea level periods and number of storm events (table 9.4), what is a peculiar phenomenon among the basins of the Baltic Sea.

3. Southern part of the Gulf of Bothnia (Bothnian Sea), north-eastern part of the Northern Baltic, Southern Baltic and Danish Straits are the water basins of moderate risk of extreme hydrological events occurrence. It follows mainly from the geographical location of these basins and transitional characteristics of extreme sea level parameters (fig. 6.6–6.8, tables 9.6–9.8). The amount of hours with high and low sea levels a year, average annual number of storm events and 100-year water level of these basins fall within parameters of basins of the highest extremes (Gulf of Finland, Gulf of Riga, Bothnian Bay) and basins of small sea level oscillations which are typical for Swedish coast of Central and Northern Baltic.

4. Extreme phenomena, related to water dynamics, increase from the open sea waters of the Baltic Sea (Baltic Proper) to the innermost parts of its bays (Gulf of Bothnia, Gulf of Finland, Gulf of Riga, Bay of Mecklenburg and Bay of Kiel). The responsibility for this situation is taken by the so-called bay effect, i.e. the impact of geomorphological and bathymetrical configuration of the coastal zone on water dynamics. This effect cause an increase in extreme sea levels and in time of their occurrence at bay stations of the Baltic Sea from the seaside boundary of the single bay towards its farthest, innermost, cut-into-the-land point (the end of the bay) (fig. 6.6– 6.9). Narrowing of the bays is the one of the main reasons explaining this phenomenon. The defined volume of water removed or added to a narrowing and shoaling part of the bay will incur more extreme water level compared to its widened seaside part. This interpretation is compliant with results obtained by Sztobryn et al. (2005 and 2009), Ekman (1996) and Johansson et al. (2001), who claim that the highest values of sea level oscillations should be expected in the innermost part of these bays.

Probability of occurrence (return periods) of extreme sea levels

Probabilistic forecasts allow to identify the phenomena of extreme sea levels and storm events (Wróblewski 1992, 2001, Suursaar et al. 2009, Wolski and Wiśniewski 2012) or to identify the ongoing climatic changes (Lowe and Gregory 2005, Meier 2006, Gräwe and Burchard 2012). Such forecasts are also widely applied to the coastal zone engineering, hydraulic engineering, management of flood plains and to flood protection (Pugh 1996, 2004, Hay and Mimura 2005, Pirazzoli et al. 2006, Hallegatte et al. 2011).

The monograph provides with the theoretical sea levels together with the probability of occurrence which were set on the basis of annual minimal and maximal sea levels recorded at tide gauge stations of the Baltic Sea in 1960–2010 (tables 6.8–6.9, annex 1). The results obtained show that levels of theoretical water at particular tide gauge station depend on their location and distribution of theoretical water, for example hundred-year water is compliant with the geographical pattern of distribution of extreme sea levels (fig. 6.11). The most extreme values of theoretical hundred- year water maximum levels (>220 cm NAP) and theoretical minimum water levels (levels <–100 cm) would occur in the innermost parts of Gulf of Bothnia, Gulf of Riga, Gulf of Finland and Bay of Mecklenburg (fig. 6.12). This is a consequence of the bay effect, which was described in chapter 6.2. On the other hand, the Swedish coasts of the central Baltic have the lowest theoretical hundred-year water levels (< 140 cm NAP for the maximum theoretical levels and >−100 cm for the minimum theoretical levels). Owing to their transitory location between the North Sea and central Baltic, the Danish Straits (Skagerrak, Kattegat, Sund, the Belts) are regions with intermediate theoretical hundred-year levels, since the Danish Straits hydraulically balance the water levels between the North Sea and the Baltic Sea.

Theoretical maximum sea levels for two time periods: 1910–1960 and 1960–2010 were compared in this monograph. The determined differences in the levels indicate that theoretical sea levels were higher for the period 1960–2010 for all investigated tide gauges within the full scope of quantiles (from 0.1% to 99%). However, the return period has been reduced for these tide gauges at the same time. For instance, the hundred-year water level for Kungsholmsfort moved to the level of fifty-year water after 50-year period, whereas for Helsinki – to approximately fifteen-year water (fig. 6.13). This is the evidence of constant increment of theoretical, and thereby, observed maximal annual sea levels in the last half century. The exhibits hereto are the integral parts of this monograph and they provide with results of setting the theoretical water for maximal and minimal levels within the full scope of quantiles (from 0.1% to 99%) at 37 investigated tide gauge stations of the Baltic Sea.

Seasonal extreme sea levels of the Baltic Sea

Seasonal variations in the occurrence of extreme sea levels of the Baltic has been investigated herein apart from the multiyear analyses. Changes in the sea level occurring during a year, which are the most frequently associated with filling up of a given water basin, are considered as seasonal variations. The variations in the mean sea level, the amplitude of which reaches several cm, are only the background for short-term variations that are responsible for extreme sea levels. The course of the seasonal changes in extreme sea levels (higher high water HHW and lower low water LLW) for all analysed tide gauges is alike – the highest values occur in autumn-winter season while the lowest ones occur in summer. The occurrence of significantly greater monthly amplitudes between the extreme levels (HHW and LLW) at gauges located in bays is another regularity. Amplitudes there exceed 3 m comparing to the gauges of the open water coasts (Baltic Proper), where they exceed only 2 m (fig. 7.1, table 7.1).

As a part of analysis of the seasonal variations, geographical distribution of occurrence time (number of hours) of high (≥70 cm) and low (≤70 cm) sea levels of the Baltic Sea in particular months during a year have been also taken into consideration. Both, number of hours with high and low sea levels decrease from the highest value in January to the lowest one in summer (June– July) and then they increase until January next year (fig. 7.2–7.7). Total time of high sea levels occurrence for January amounts from 10 to over 50 hours for most of the Baltic Sea basins, with the greatest values at the northern and eastern coasts. On the other hand, total time of low sea levels occurrence for the same month ranges from 10 and 20 hours for the Western Baltic and from 3 to 5 hours at the ends of Bay of Bothnia, Bay of Finland, Bay of Riga and Kattegat. It is typical and compliant with the geographical pattern of extreme sea levels distribution for the long-term characteristics, that the occurrence time of high and low levels increases from the open water basins of the Baltic (Baltic Proper) to the innermost parts: Gulf of Bothnia, Gulf of Riga, Gulf of Finland, Bay of Mecklenburg and Bay of Kiel. It is visible throughout the year, except for summer time, when low and high sea levels appear occasionally or do not appear at all.

The number of storm events corresponds well to the above-mentioned annual time of occurrence of extreme sea levels. Storm events predominate in autumn-winter season with January as the month of the highest number of storm surges for most of Baltic basins. Total amount of storm events in January for the long-term period 1960–2010 ranged from 1 for the Central Baltic (Visby) to 97 for Gulf of Finland (Hamina) (fig. 7.8). By contrast, spring and summer time (from May to August) are periods, when storm events do not occur or they appear occasionally within the bays. The annual course of storm events, which are responsible for low and high sea levels, is compliant with the annual variability of atmospheric circulation not only on local and regional scale but also globally. The reason for the highest numbers of autumn-winter surges is the frequent occurrence of low pressure systems coming from the Northern Atlantic during the period of the most intense western circulation, North Atlantic Oscillation (positive NAO index) in particular (Girjatowicz 2009, Sztobryn and Stigge 2005, Sztobryn et al. 2009, Suursaar et al. 2002, 2006, 2007, Jaagus and Suursaar 2013, Ekman 2003, 2007, 2009).

The confirmation of the influence of circulation variability on extreme sea levels was received, when the research on the extent of dependencies between analysed mean, maximum and minimum levels of the Baltic Sea and indicators of NAO and AO zonal circulations together with SCAND pattern were performed for all months of the long-term period 1960–2010. The results obtained indicate the occurrence of the strongest relationships between NAO and AO and sea levels to appear in winter months, what is then followed by autumn months. The greatest dependencies in a year occur in January. The relations between sea levels and NAO range from 0.29 in Wismar to 0.82 in Spikarna, whereas between sea levels and AO – from 0.31 in Wismar to 0.80 in Kemi.

Next regularity, that can be observed in the analyses, is a spatial diversification of correlation and the increment in its values along the main axes of the Baltic: West–East and North–South, which is especially evident for autumn-winter season (tables 7.2–7.3 and 7.6–7.7 and annex 2). Such diversification can result from the fact that at positive NAO and AO (high values), western air masses distribute the waters from the Danish Straits into eastern and northern edges of the Baltic Sea causing an inclination of the Baltic surface from NE to SW. It magnifies the impact of NAO index on sea levels in the north-eastern part and attenuates the effect in the south-western part of the Baltic, what is in line with the conclusions of the work of Joansson et al. 2003, Jevrejev et al. (2005), Ekman (2007, 2009) or Suursaar and Sooäär (2007) Jaagus and Suursaar (2013).

On the basis of the determined dependencies (tables 7.2–7.3 and 7.6–7.7 and annex 2) a good correlation between both indices (NAO and AO), in terms of phase coverage, has been revealed, what might be explained by the fact that both indices refer to the western circulation in circumpolar and mid-latitudes. This conclusion is in accordance with scientific research which demonstrate AO and NAO spatial patterns as very similar to each other within the Atlantic sector (Delworth and Dixon 2000, Wallace 2000).

The analyses concerning the relations between SCAND index and sea levels proved that these correlations were negative throughout the year and statistically significant in most cases. It means that cyclonic conditions and not anticyclonic were predominant over the Baltic Sea in the period 1960–2010.

It shall be kept in mind that zonal circulations (NAO and AO) cause an intensification of extreme sea level events on weekly and monthly scale by an increased inflow of air masses from northern and western directions. Therefore, these air masses are responsible for a slow process of filling up the Batic Sea with North Sea waters. The filling up effect is an important but not the only one component contributing to extreme sea levels during storm event in the Baltic Sea.

Extreme sea levels of the Baltic Sea during storm events

The occurrence of extreme sea levels, which are the result of storm surges on the Baltic coasts, depends on three components (Wiśniewski i Wolski 2009 a, 2011a, Wolski et al. 2014):

a) filling up of the Baltic Sea (the initial sea level prior to the occurrence of an extreme event),

b) the action of tangential wind stresses within the given area (wind directions: whether they are shore- or seaward; wind velocities; and duration of wind action),

c) deformation of the sea surface by the mesoscale, deep low pressure systems crossing rapidly the Baltic, which then generate the so-called baric wave (ground effect below the pressure system) and seichelike variations of the sea level in the Baltic. Features of the low pressure system – its value, track and velocity – are the most important for the deformation to appear. As a result of the interactions between these three factors, the extremely high sea levels can occur at the positive phase of the storm surge (sea level rise) or the extremely low levels at the negative phase (sea level fall) (fig. 3.4).

The participation of these factors in the storm event has been applied to determine three main types of storm surges:

I. First type of a surge with a stable wind field, in terms of direction and velocity of the wind, affects the sea surface for several days and causes evident drift currents (the velocity of these current might reach 50 cm∙s–1 in the open water and even 100 cm∙s–1 in straits). Such wind field can be generated in the conditions of shallow and slow going low pressure system (>980 hPa). Tangential wind stress impact not only induce the wave action, but also transfers the energy for creation of a water surface inclination, the so-called wind set-up. This surge is a wind-induced type.

II. Second type of a surge – a vacuum-induced one – occur when a mesoscale, concentric low pressure system crosses nearby or over the Baltic Sea with a relatively high velocity (≥16 m∙s–1). In this situation, the vacuum triggers setting up of waters under concentric low pressure which might be named as „ground effect of the low” or „baric wave” (fig. 3.4). If the baric wave moves at the same or similar velocity as the low pressure system, then a considerable flood hazard arises along the particular section of the coast – the significant seichelike variations of the sea level (resonance).

III. Third type of a surge is a mixed one: pressure-wind. In fact, wind (tangential wind stress impact) and pressure field (mesoscale low pressure system crossing the Baltic quickly) act simultaneously during each surge. It is important to distinguish the participation of which factor (wind or vacuum) was clearly predominant. If it cannot be determined, both factors are considered as equal and the mixed type of storm surge is accepted.

Baltic coasts are exposed variously, either to the wind field or the track of the low pressure system and therefore identifying the real impact of given low pressure or wind field on a magnitude of storm surge or storm fall and on time of their occurrences might be complicated. Sometimes wind fields and vacuum acting against each other along the given section (e.g. positive phase of the baric wave at simultaneous seaward wind impact causing sea level fall). However, if these factors act harmoniously at the same time, then an extensive storm surge is expected.

The monograph contains 8 storm situations which are characterized by different extent of the filling up of the Baltic Sea, height of a storm surge or fall and various features of the low pressure system (advancing velocity, pressure in the system centre and track as well as wind velocity, direction and impact time). On the basis of the aforementioned characteristics, 3 situations were assigned to the wind-induced type, next 3 to the vacuum dominant type and 2 situations, where the dominant factor causing the storm surge could not be unambiguously determined, were classified to the mixed one (subchapter 8.2). Studies on storm situations provided with typical features of the surge with dominant participation of vacuum and of the wind-induced surge.

Basic features of the storm surges induced by the vacuum of the dynamic low pressure system:

− A mechanism of dynamic tilting from the equilibrium by a mesoscale low pressure system (cyclone) is acting. Its impact triggers deformation of the Baltic Sea surface by the so-called „ground effect of the low” and wavy nature of the deformation movement along the low pressure system track and surrounding water basins;

− Dynamic increase in the sea level at mesoscale, deep and fast-moving (≥16 m∙s–1) low pressure systems decides whether the extreme of +100 cm in relation to the NAP is exceeded;

− Winds have no significant influence on the vacuum-type surge, but they may sustain high sea level at best;

− Vacuum-induced surges commence with a deep sea level decrease at the stations located in the Western Baltic;

− Maximum rate of changes in the sea level caused by the vacuum-induced surge is 10–40 cm∙h–1 on average. The most rapid rises and falls of the sea level occur at the tide gauges located in bays and straits. Maximum rise and fall durations are short and amount to 2–3 hours.

− An alternating effect of the sea level variations occurring between south-western basins: Western Baltic (Bay of Mecklenburg, Bay of Kiel), Sund and north-western basins of the Northern Baltic (Gulf Riga along with Bay of Pärnu, Gulf of Finland, Gulf of Bothnia) is the characteristic phenomenon. The mesoscale low pressure system decreases the sea level in the southern part and increases in the northern one causing sea surface to incline in a few or several hours. These movements of the water masses can be named as seichelike variations of the Baltic Sea (fig. 8.17–8.20, fig. 8.22–8.24, fig. 8.26–8.28);

− The lowest sea levels of the Baltic Sea occur during storm falls in the Western Baltic if the western circulation appears, the generation of which is associated with deep low pressure systems.

− The highest sea levels and most dangerous storm surges occur when the filling up of the Baltic Sea is initially high (≥50 cm).

Main features of the storm surges induced by the wind:

− Wind-induced storm surges last for a relatively long period of time (2–3 days). A transfer of water masses performed by drift currents during such period causes the so-called wind set-up occurring at the coasts;

− Maximum rate of changes in the sea level is generally lower comparing to vacuum-induced surges and it amounts to 5–25 cm∙h–1 on average. It is worth noticing that the rate of sea level fall is mostly slightly slower than the rate of rise (gravitational falling).

− In case of eastern atmospheric circulation and well-developed anticyclonic system over the Scandinavia or western Russia, the storm surges occur exclusively in south-western basins of the Baltic Sea. In Western and Southern Baltic and Danish Straits, the duration of high sea levels ≥70 cm for such surges ranges from 15 to 50 hours, while duration of very high sea levels ≥100 cm – from 5 to 25 hours.

− Wind direction is of greater percentage in reaching the maximum sea level during the storm surge comparing to wind velocity, when considering the wind-induced surge. Weaker wind blowing more perpendicular to the shore will cause higher wind set-up, the maximum of which will be reached faster, than a stronger wind blowing to the shore at smaller angle (example of the tide gauge in Wismar for the situation of January 1987 and November 1995, fig. 8.8 and 8.12).

General regularities in occurrence of the extreme sea levels in the Baltic Sea

Extreme sea levels, both falls and surges, occur during the storm events. Significant minimum ≤–70 cm in relation to the NAP and high maximum ≥70 cm of the surge are frequently reached in the same storm. The water basin, where an evident minimum and maximum (negative and positive phase) of the storm surge occur, is the Western Baltic.

However, the low sea levels appear not only during the storm events. The genesis of these sea levels results also from an action of totally different process. Low sea levels occur in the eastern circulation conditions during permanent and longstanding eastern or northern winds associated with a well-developed high pressure system over the Scandinavia or north-western Russia. Then, the duration of relatively low sea levels might amount to even a couple of weeks (Majewski and Dziadziuszko 1985, Suursaar et al. 2003, Sztobryn et al. 2009).

As it has been mentioned, extreme sea levels during storm surges in the Baltic Sea can be reached as a result of action of several factors. Initial high sea level, which might be higher by 50 to 70 cm than mean sea level (the so-called filling up of the water basin), is the first factor. Such hydrological situation is possible during longstanding western winds blowing through even a couple of weeks which cause the inflow of water masses of volume from 100 to 150 m3 from the North Sea to the Baltic Sea via Danish Straits. Such filling up favours the development of sudden storm surges, which then occur as a result of deep low baric depression (<980 hPa) incoming from the Northern Atlantic and crossing the Baltic Sea (II, III and IV track of the low pressure system according to Schinze or the modifications of these tracks).

When the active, concentric low pressure system of high velocity (≥16 m∙s–1) enters the Baltic Sea, the sea level in the south-western basins falls, whereas in the central and north-eastern ones – it rises at the same time. This is a visible impact of the vacuum, which „collects water” from shallow and non-capacious bays of the Western Baltic (the negative phase causing storm fall mainly at Bay of Kiel and Bay of Mecklenburg) and leaves it (set it up) in the northern and eastern basins, in order to create the ground effect. This phenomenon is associated with a transfer of the baric wave shape and not the water mass. Once the baric depression enters the land in the area of Finland, Estonia, Latvia or Lithuania, the culmination of the surge is most likely to occur creating the greatest deformations of the Baltic Sea surface between south-western and north-eastern basins, which can exceed 4 m in the extreme events (minimum of the negative phase in the Western Baltic and maximum of the positive phase of the surge in the north-eastern coasts of the Baltic). Central Baltic area located in the middle of the deformation (tilt) will experience the smallest range of sea level fluctuations among the Baltic basins (the so-called node point of the deformation). Great depth and vast volume of the Central Baltic comparing to the other basins has an additional influence for this little amplitude (table 2.1). Once the low pressure system is gone, significant sea level oscillations will last for additional several hours. These oscillations can be described as seichelike sea level variations, i.e. changes in inclination of sea surface deformation. It is observed as the positive phase of the surge within the Western Baltic (a sudden increase of up to a few tens cm above a tide gauge zero) and gravitational or wind-assisted falling of the water to the mean level in the North-Eastern Baltic (the negative phase in the north-eastern coasts of the Baltic Sea is not emphasized, since the baric low has already entered the land and does not influence on the sea surface anymore).

The entire process of creation of significant deformations in the Baltic Sea surface is possible because of two main reasons. First reason is explained by a geographical configuration of the Baltic Sea basin, which is elongated from the South-West to the North-East. Bay of Kiel and Bay of Mecklenburg are of easterly and north-easterly exposition, while the Gulf of Finland, Gulf of Riga and some parts of Gulf of Bothnia are south-westerly and westerly exposed. Such geographical configuration and relatively small depth of the Baltic Sea favour in some measure the mechanism of creating the surface deformations by fast-moving, deep low pressure systems. The surface of baric depression has to be similar to the non-disturbed surface of the Baltic in term of scale. The other reason should be associated with the velocity of the low pressure system, which is approximate to the velocity of propagation of induced and advancing wave that causes surface deformation (seichelike variations). It is worth mentioning that shallow basins located in the East, like Gulf of Riga with Bay of Pärnu or Gulf of Finland, are in impact range of such wave. Storm surge wave is enhanced in these basins due to a decrease in cross-section of these bays, what results in even higher sea levels and harmonic course of the oscillation.

The studies that were carried out and included herein led to the identification of spatial and temporal structure of the extreme sea levels of the Baltic Sea. The work explains the genesis of extreme sea levels and describes the processes responsible for their diversification along the Baltic coastline. In addition, a temporal variation of analysed extreme levels, which is ongoing in the condition of climatic changes, was shown for the last half century and for the earlier periods.

Within next few years there will be a need for an extension of the studies on extreme sea levels of the Baltic Sea which should base on measurement data and mareographic archival data of 19th and 20th century as well as on application of GPS and satellite measurements. A vast set of data derived from hydrodynamic forecasting models might be complement research material. Use of an increased variety of measurement data and knowledge on nature of occurrence of extreme sea levels in particular basins of the Baltic Sea included herein will allow to improve methods of operational forecasting of storm surges and falls, as well as it will contribute to precise evaluation of the influence of climatic changes on hazards related to the occurrence of extreme high sea levels and their impact on the coastal zone of the Baltic Sea.

Tłumaczył Michał Bugajny