On correlation of the ice cover of the European sector of the Arctic with surface heights of the Atlantic inner seas (as shown for the Black Sea)

An urgent problem of physical geography, oceanography and climatology is the improvement of methods for modeling long-term changes in the average water surface level (hereinafter referred to as AWSL) of the World Ocean regions. Its solution involves the development of existing concepts about the relationship of these changes with other natural processes. The solution of this problem is of greatest importance for the regions of the World Ocean, on the coasts of which there are large cities and ports. The inland seas of the Atlantic Ocean: Baltic, Mediterranean, Azov and Black seas take an important place among them.

It is known that among the most important factors of the year to year variability of the AWSL for such seas are the processes affecting the dynamics of their water balance, which are studied in the works of many domestic and foreign authors. They found that the dynamics of the water balance of the seas under consideration depends on the average intensity of precipitation and evaporation in their basins, which are caused by the corresponding changes in the total duration of atmospheric blockings (hereinafter referred to as TAB). 

The Arctic air entering the Europe area in the course of its incursions, as well as the air in the rear of the cyclones moving above Europe to the Arctic contributes to the occurrence of blockings. The characteristics of this air are largely determined by the peculiarities of its interaction with the atmosphere of the ice cover of the corresponding Arctic water areas, which depend on its total volume (hereinafter referred to as TVI). Consequently, the long-term sequence of the average annual values of the AWSL for each of the seas under consideration can be, to some extent, associated with variations in the average annual values of the  TVI in the waters of the European sector of the Arctic. AWSL may also be affected by warming of the regional climate, which causes the thermal expansion of the active layer of the sea and its average surface temperature (hereinafter AST).

Considering this, the authors put forward a hypothesis that the relationship between the TVI variations, characterizing the climate changes in the Arctic, and the long-term course of AWSL is statistically significant for some of the studied seas.  Confirmation of its adequacy would allow taking into account such relationships when modeling the dynamics of the AWSL of these seas, as well as planning economic activities on their coasts. However, this hypothesis has not been verified before.

The subject of study in this paper is the statistical relationships of the long–term course of the average annual values of the AWSL for the Black Sea, the TVI of the Arctic waters located in the European area, as well as TAB, which occur in this area.

The purpose of the work is to verify the adequacy of the proposed hypothesis.

The factual material used in the study of the long-term course of the average annual values of the AWSL for the Black Sea is the results of its mathematical modeling obtained by the ICDC (Integrated Climate Data Center) of the University of Hamburg, Germany. The information about the changes in AST is obtained from the GLORYS12.v1 (Global Ocean Physics Reanalysis) reanalysis maintained by Copernicus.

To confirm the adequacy of the specified factual material, it is compared with the archival information obtained during contact measurements at some points of Odessa, Ochakov, Sevastopol, Evpatoria, Feodosia, Kerch, Anapa, Novorossiysk, Gelendzhik and Sochi.

The information on the long-term course of the average annual TVI values in the waters of the European sector of the Arctic was obtained using information on variations in the average monthly values of the ice cover thickness and its concentration, which is also presented in the ICDC modeling results and the mentioned reanalysis.  

When assessing the average annual values of TAB in the European sector of the Northern Hemisphere, we applied the typification of macrocirculatory processes in the Northern hemisphere according to B.L. Dzerdzeevsky.

The research methodology involved the implementation of spectral analysis for the time series of the average annual values of the AWSL, which allowed us to determine the periods of the high-frequency modes of this process. When determining the average rate of each such process, it was smoothed over a time equal twice to the period of the longest period of the identified modes, after which the identified trend was equalized. The study of statistical relationships between the time series of the average annual values AWSL for the Black Sea, as well as TVI and TAB for the European sector of the Arctic, was carried out using the method of correlation analysis and Student's t-test.  The significance of the statistical relationships between the time series of the AWSL, TAB and TVI was estimated with shifts between them in time from 0 to 3 years and for the periods from 1979 to 2018, as well as from 1993 to 2018. The decision on the adequacy of the proposed hypothesis was to be made if the reliability of the statistical conclusion about the significance of the correlation of the considered time series was at least 0.95.

As a result of the research we tested the factual material and confirmed its adequacy. Considering the results of the spectral analysis, the smoothing of the considered time series when identifying their trends was carried out in a sliding window 11 years long. It has been established that, in the mean, the average annual values of the Black Sea AWSL increased at the rate of 0.048 m/year, and the average annual values of its AST increased at the average rate of 0.037^oC/year over the period from 1979 to 2018, which confirms the warming of the regional climate.

During the period 2010-2018 the rate of AST growth increased, which could have led to a relevant increase in the AWSL, but actually it didn't. The latter is confirmed by the presented distributions of the average rate of rise in the level of various sections of the Black Sea surface. Consequently, the factor that counteracts the increase in AWSL has increased.

Such factors could include a decrease in the annual amounts of atmospheric precipitation, as well as an increase in the intensity of evaporation in the Black Sea basin, which was associated with ongoing changes in the TAB.

The analysis of the trends in the TVI change in the European sector of the Arctic showed that in the period from 1993 to 2018 the value of this indicator decreased significantly, but in 2010-2018, it changed at a fixed level. The statistical relationships of interannual changes in the average annual values of the Black Sea AWSL, as well as the TVI in the waters of the European sector of the Arctic in the period under review, were statistically significant if the changes in the AWSL were lagging relative to the TVI variations by 1 year. In this case, these changes occur in phase. The significance of the relationship between the changes in AWSL and the variations in TVI is also maximum at the specified time shift. Therefore, the adequacy of the proposed hypothesis is confirmed.

When discussing the obtained results, we noted their compliance with the existing concepts about the causes of changes in the AWSL of the seas. The revealed relationships show that TVI variations in the European sector of the Arctic really affect the formation of blockings over the basins of the Black, Azov and Baltic Seas, and are also able to determine the dynamics of their water balances, and hence their AWSL.

The obtained results allow us to generalize the concept proposed by L.S. Berg for the Caspian Sea on the dependence of long-term changes in its level on climate variations in the Arctic and on the mentioned seas of the Atlantic Ocean. During the periods of time when the climate is warming in the Arctic and the TVI decreases, evaporation increases in the basins of these seas, and the average intensity of precipitation decreases.

It should be noted that this dependence is weaker for the seas under consideration than for the Caspian Sea, since they are not isolated from the Atlantic Ocean and the water exchange factor is significant.

The revealed relationships suggest that further changes in the levels of the Atlantic inland seas in the 21st century will depend to some extent on variations in the TVI of the European Arctic water areas and their climate. Modern changes in its TVI are the result of a confrontation between the factors that contribute to warming and cooling of the climate in this region.

As can be seen from the obtained results, a decrease in TVI in the European sector is no longer observed in the period after 2006, although before 2006 warming factors clearly prevailed there. Consequently, now the values of the resultant factors of warming and cooling of the Arctic climate change at a fixed level.

Since it is impossible to predict further changes in climate warming and cooling factors, future variations in the levels of the inland seas of the Atlantic are not predetermined, although the justification of forecasts of some of their components can probably be satisfactory if factors invariant to the uncertainties of the scenario of further climate changes are considered when developing them.