The coastland changes of the black sea western part in connection with the climatic factors

Coastal research is becoming increasingly important because the impact of climate change is most felt here. The special system of natural complexes dynamically coupled in space and time is formed in the coastal zone. The sea level is an indirect indicator of processes in the ocean, at its bottom and in the atmosphere. Seacoasts are the most attractive regions for life and represent a natural zone of interaction of three environments: the first is geological (land and the bottom of the seas and oceans), the second is hydrological (the water layer of the seas and oceans) and the third is meteorological (the air with atmospheric impact).

Regional climate change and variability are the complex outcome of global climate change (including sea surface temperature characteristics, linked with large-scale changes in ocean circulation), large-scale atmospheric (e.g., jet stream, monsoon) circulation changes, and their interplay with local drivers (e.g., land surface, aerosols), local physical processes and the non-local responses to large- scale phenomena such as El Nino and the Southern Oscillation (ENSO) or other modes of climate variability. That's why the assessment of climatic changes and anthropogenic impact on the regional changes of the natural environment is one of the main tasks set in the AR6 Synthesis Report, due for release in 2022 (AR6 Synthesis Report: Climate Change 2022).

The global climatic changes occurring in the atmosphere were estimated in numerous works of the last 50 years, including those carried out by the authors of this paper (Voloshchuk, Boychenko, 1997; Efimov, Eremeev, 2003; Andrianova et al., 2005; Andrianova et al., 2015). Some authors (Voloshchuk, Boychenko, 1997) suggested that if the current trends in the development of the world economy persist, the planet's climate may be warmed by 3.0-3.5 °C by the second half of the 21st century. This can lead to unpredictable natural disasters.

The changes in sea level are also among the most important indicators of climate affecting human living conditions. Analysis of sea level changes on the coasts of various parts of the World Ocean (the Mediterranean, Black, Baltic, and North Seas, as well as on the Atlantic coasts in Brazil and France) over a long period of time have shown various fluctuations with an upward trend in recent decades (Jevrejeva et al., 2008; Andrianova, 2012; 2014; Andrianova et al., 2015; Navrotskaya et al., 2016).

In addition, the average annual (> 100 years) and average monthly (> 50 years) data on the rates of the rivers flowing into the northwestern part of the Black Sea are involved for the analysis (Table 1).

The series of sea level by stations were considered for estimating the intensity of fluctuations of the coastal land along the western part of the Black Sea (Fig.1): 6 Ukrainian points - Vylkove, Ilyichivsk, (since 2016 the city has been renamed to Chornomorsk), Odesa, Yuzhne, Ochakiv and Sevastopil; 2 Romanian points - Constanta and Sulina; and 2 Bulgarian points - Burgas and Varna.

Research results and discussion

The results of our calculations of the variability of the atmospheric impact on the coastal zone (by the example of Odesa) confirmed the general trend of global warming of the Earth's climate since the second half of the 20th century. The information in Table 1 clearly shows how climate norms have increased in all parameters over the past 70 years, relative to whole observation period. At the same time, the salinity of the water decreases accordingly. It shown also on the graphs presented in Figure

2. For the period considered, the air temperature in Odesa increased by 0,9 °C, the water temperature increased by 0,5 °C.

Fig. 1 Map of the coast of the western part of the Black Sea indicating the stations considered

A common feature of all parameters in Figure 2 in the interannual variability, is the presence of well-defined wave fluctuations of different scales which occurs against the background of alternating trends. The variability of the interannual course of precipitation was characterized by a wave-like rise throughout the observation period, with its increase in the last 50 years. The increase of the precipitation occurred mainly due to the autumn-winter time. The trends in the sea level and rivers flow rates had unidirectional tendency. The disturbance of the climatic seasonal variation and the appearance of a well-defined second extremum in the cold season with a maximum in December-January and a minimum in January-February was found in seasonal fluctuations in salinity, sea level in Odesa and the flow rates of the Danube and Dnieper rivers.

Trends in the sea level and land on the Black Sea coast were explored by using the average annual sea level data on the set of stations in the period from 1874 to 2015 (Fig. 3A). Our calculations of the general trend of the eustatic rise in the Black Sea level is consistent with its growth in the World Ocean (0,1-0,2 cm / year) (Jevrejeva et al., 2008).

FiЈ 2. Time series of annual mean (solid line) and moving average by 5-year (dashed line) values of air temperature (a), precipitation (b), sea level in Odesa (c) and flow rates of the Danube River (d).

The tendency of the wave-like growth of the level at Odesa-port station was monitored until 1994. Then from the second half of the 90s, a slight decrease even was noted, which can be traced to the present day. The exception was in 2010, when a sharp increase in the average annual level was observed, which was noted not only in Odesa, but also at other stations considered by us.

The value and intensity of interannual tectonic shifts of coastal land on the Black Sea were estimated during the same period with using the method of water leveling (Table 2).

The interannual variability of the coastal land subsidence is a mirroring of the fluctuations in sea-level rise. The tectonic subsidence of land at Ochakiv and Sevastopil stations, based on the materials of long-term observations (140-year series) and characterized by an insignificant negative trend (Table 2).

That's why the observations at Sevastopil station were used as a reference for assessing coastal land fluctuations on the Black Sea coasts based on the water leveling method (Fig. 3B). The short and sharp "discrete peaks" of sea level and similar "fallings" of the sea bottom were found in series of average annual data (Fig. 3). These disturbances were observed nearly simultaneously on all analyzed stations with quasi-decade frequency and had cycle time 3-4 years (sometimes up to 5 years) and amplitude up to 10-17 cm (usually 12 cm). The disturbances on the moving average by 5-year data had cycle time 10-12 years with an average amplitude of 5 cm. Sometimes the discrete peaks of sea level (and the same "fallings" of the sea bottom) were observed in pairs.

Fig. 3. Time series of annual mean (1) and moving average by 5-year data (2) A - sea level heights; B - calculated values of land subsidence at stations of the Black Sea.

Table 2.

Information about the stations on the Black Sea coast and averaged over the observation period the values and rates of the land dynamics

The estimation of the value and intensity of the land subsidence on the coast by data of the Odesa station showed that, in general, from 1875 to 2015 the land subsidence was about 50 cm, and its intensity was 0.35 cm / year (Table 2). At the same time, two stages are noted with different intensities of land subsidence - 0.5 cm/year from 1875 to 1962 and only - 0.2 cm/year from 1962 to 2015. At stations with a shorter duration of observations, land subsidence is also noted with different stages of its intensity, which have been synchronized in recent decades.

The analysis showed that the anomalous changes in the considered characteristics in the Black Sea region is consistent with the El Nino years (1953; 57-58; 65; 69; 72-73; 76; 82-83; 86-87; 97-98; 09-10). On Figure 4 is shown that the maximums in the average annual series at the near-equatorial stations of the Pacific Ocean is correlate with the same Black Sea data (shaded zones). The level maxima in these average annual series are global, has with approximately quasi-ten-year rhythm, and are consistent with the El Nino years. This phenomenon causes various cataclysms and has a significant impact on climate variations in different regions of the globe, including Europe.

Conclusions

The growth of all parameters (water and air temperature, precipitation, river flow rates, and sea level) in recent decades is a general trend in fluctuations in hydrometeorological characteristics. At the same time, the salinity of the water decreases accordingly. A common feature of all parameters in the interannual variability, is the presence of well-defined wave fluctuations of different scales which occurs against the background of alternating trends. The connection with the El Nino phenomenon is one of the reasons for the observed surges in changes in the water and air environments in the Black Sea region.

Fig. 4. Time series of annual mean (solid line) and moving average by 5-year (dashed line) sea level height on stations: a - La Libertad (Pacific), b - Balboa (Pacific), c - Ochakiv (Black Sea), d - Sevastopil (BlackSea), e - Tuapse (BlackSea).

Uneven subsidence of coastal land is a consequence of the growth of the sea level - its mirror image. The fluctuations in the Black Sea level are synchronous on low-frequency scales (their period is more than 5 years) because they are under the influence of global climate processes on our planet. The short-term changes in the Black Sea level are determined by regional characteristics and are created by local factors. The dynamics of the land motions are the result of the action of level fluctuations and tectonic processes.

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