Long-term trends and variations in sea level of the Black Sea

The Black Sea is an almost enclosed basin interacted with the Mediterranean Sea through the Bosporus. It has a large catchment area receiving freshwater from the second longest river in Europe, the Danube, and other rivers spread over Europe and Asia. The total riverine discharge is 350 km³/year where the Danube contributes about 65% of the total discharge. Although evaporation rates (280 km³/year) exceed precipitation rates (200 km³/year), large riverine discharge makes the Black Sea an estuarine type basin.  The main feature of the Black Sea is a basin-wide cyclonic circulation, namely Rim Current. The cyclonic circulation causes a lower sea level in the inner part of the basin and a higher sea level in the shelf region. The freshwater budget and thermal expansion of the water are other factors affecting sea level of the Black Sea.  The North Atlantic Oscillation (NAO) could also influence sea level through changes in atmospheric pressure and the above-mentioned factors.  

 

In this study, firstly we investigated long term trends in sea level of the Black Sea on the basis of the tide gauge measurements, satellite altimetry, and gravity measurements from the Gravity Recovery and Climate Experiment (GRACE). Then, we assessed role of the wind curl, freshwater budget, and NAO on sea level variations through temporal and spatial data analysis. The tide gauge measurements suggest a positive sea level trend of about 1.05 – 2.37 mm/years, for a time period >50 years. Basin mean sea level derived from altimeter and GRACE (years between 2003-2019), does not exhibit a statistically significant trend (p<0.05) which might result from the shift towards a positive NAO condition in the last 30-years. We found that sea level variations both in the coastal and inner part of the basin are significantly correlated (p<0.05) with Danube discharge but these correlations are smaller in the inner part. The agreement between interannual variations of Danube discharge and the NAO index suggests that sea level variations are also associated with NAO index. An Empirical Orthogonal Function (EOF) analysis with associated time series (Principal Components, PC) is applied to the gridded altimeter data to capture space and time features of sea level variability. The first mode of the EOF explained about %81.9 of the total variability and showed the same sign over the basin indicating an in-phase oscillation of the whole Black Sea. The PC1 shows interannual variations in accordance with freshwater budget (r=0.76, p<0.05). The second mode of the EOF accounts for %5.7 of the total variability, has opposite signs in coastal and inner parts, the oscillation implied by this mode could be related to the Rim Current intensity governed by wind curl.