Keywords: water salinity, time series, river runoff, correlation analysis, spectral analysis, wavelet, interannual variability, climate change

Statistical methods were used to study the seasonal variation and long-term variability of salinity as an indicator of the interaction between riverine and marine waters on the shelf of the Northwestern part of the Black Sea. A comparison of statistical indicators for two consecutive climatic periods, 1960-1990 and 1991-2020, showed a decrease of average salinity in the port of Odessa by more than 0.4 psu and an increase of salinity in Ochakiv due to a decrease in the Dnieper River runoff by almost 0.4 psu. These effects occurred on the background of a general linear trend of decreasing salinity in the Black Sea due to a climatic decrease in evaporation from the sea surface. These effects manifested themselves on the background of a general linear trend of decreasing salinity in the Black Sea due to a climatic decrease in evaporation from the sea surface. The transformation of the seasonal salinity cycle in Odessa during the period 1991-2020 compared to the previous 30 years consists of a general decrease of salinity in the summer-autumn season and a shift of the maximum from July to September. Salinity in Ochakiv increased significantly in September (by 0.5 psu). Correlation analysis of the series of average monthly salinity and river water discharge values showed that the maximum correlations between salinity and Dnieper discharge were obtained for zero delay, since the movement of desalinated water from the mouth of the Dnieper to the Odessa Gulf and back takes several days, but less than a month. A significant correlation also persists at delays of 1 and 2 months, indicating strong inertia of salinity and its interaction with the Dnieper River flow. Spectral analysis of the longest series of average monthly salinity in Odessa (1951-2020) revealed four significant harmonics corresponding to the main periods of variability: semi-annual, annual, 4-year, and 35-year. The first two periods correspond to seasonal variability, and the 4-year period corresponds to interannual variability. Long-term changes in salinity with a period of 35 years are associated with corresponding fluctuations in the components of the climate system, which contribute to changes in evaporation from the sea surface. Wavelet analysis has shown that the increase in the power of the 4-year harmonic interannual salinity fluctuations in the port of Odessa occurs during periods of El Ni?o (EN) influence, with maximum power occurring between adjacent events or directly during EN. Accordingly, the decrease in this value began after the La Ni?a (LN) phenomenon, with a minimum between the previous LN and the following EN, or between two consecutive LN events.

1. Bol’shakov, V.S. (1970). Transformation of riverine waters in the Black Sea. Kiev: Naukova Dumka. [in Russian]

2. Ilyin, Y.P. (1999). Expansion of the riverine water. Natural conditions of the seaside of the Danube River and the Snake Island. Ivanov V.A., Goshovsky S.V. (eds). Sevastopol: MHI. 5-73. [In Russian]

3. Ilyin, Y.P. (2006). Hydrological regime of riverine waters expansion in the North-Western part of the Black Sea. Scientific works of UHMI, 255. 242-251. [In Russian]

4. Ilyin, Yu.P. (2023a). Average condition and seasonal variability of the structure and dynamics of transitional waters in the Dnieper-Bug estuary region. Ukrainian Hydrometeorological Journal, 32. 63-79. https://doi.org/10.31481/uhmj.32.2023.05 [In Ukrainian]

5. Ilyin, Yu. (2023b). Spreading of the extreme water discharge from the Dnipro-Buh estuary into the Black Sea in June 2023 by satellite observations data. Meteorology. Hydrology. Environmental monitoring, 2(4). 62-74. https://doi.org/10.15407/Meteorology2023.04.062. [In Ukrainian]

6. Ilyin, Yu. (2024). Manifestations of the interaction of riverine and marine waters in the statistical structure of salinity by the observations data at coastal stations of Ukraine. Meteorology. Hydrology. Environmental monitoring, 2(6). 70-79. https://doi.org/10.15407/Meteorology2024.06.072. [In Ukrainian]

7. Ilyin, Y.P., Repetin, L.N., Belokopytov, V.N. et al. (2012). Hydrometeorological conditions of the Ukrainian seas. Vol. 2: The Black Sea. Sevastopol: MB UHMI. 421. [In Russian]

8. Lipchenko A.E, Ilyin Y.P., Repetin L.N., Lipchenko M.M. (2006). Reduction of evaporation from the Black Sea surface in the second half of the twentieth century as a consequence of global climate change. Environmental safety of coastal and shelf areas and integrated use of shelf resources. 14. Sevastopol: Ekosi-Gidrofizika. 457-471.

9. Polonsky A.B. (2008). The role of the ocean in climate change. Kiev: Naukova Dumka. 183 с.

10. Tuchkovenko Yu.S., Kushnir D.V., Ovcharuk V.A., et al. (2023). Characteristics of Black Sea dispersion of freshened and polluted transitional waters from the Dnipro-Bug estuary after destruction of the Kakhovka reservoir dam. Ukrainian Hydrometeorological Journal, 32. 95-114. https://doi.org/10.31481/uhmj.32.2023.07 [In Ukrainian]

11. Tuchkovenko Yu.S., Kushnir D.V., Torgonskyi A.V., Komorin V.M. (2024). The impact of the destruction of the Kakhovka reservoir dam on the oceanographic conditions in the North-Western part of the Black Sea according to the results of modelling. Ukrainian Hydrometeorological Journal, 33. 66-80. https://doi.org/10.31481/uhmj.33.2024.05. [In Ukrainian]

12. Hammer, ?., Harper, D.A.T., Ryan, P.D. (2001). PAST: Paleontological Statistics Software Package for Education and Data Analysis. Palaeontologia Electronica, 4(1). 9 pp.

13. Maglietta R., Verri G., Saccotelli L. et al. (2025). Advancing estuarine box modeling: A novel hybrid machine learning and physics-based approach. Environmental Modelling and Software, 183, 106223. 14 pp. https://doi.org/10.1016/j.envsoft.2024.106223.

14. Torrence, C., Compo, G.P. (1998). A practical guide to wavelet analysis. Bulletin of the American Meteorological Society, 79. 61-78.

15. Yankovsky, A.E., Ilyin, Y.P. (2024). The Dnipro-Buh plume: A tale of high-volume freshwater discharge in a non-tidal sea. Continental Shelf Research, 282. 105345. 10 pp. https://doi.org/10.1016/j.csr.2024.105345

16. Yankovsky, A.E., Lemeshko E.M., Ilyin Y.P. (2004). The Influence of Shelfbreak Forcing on the Alongshelf Penetration of the Danube buoyant water, Black Sea. Continental Shelf Research, 24. 1083–1098. https://doi.org/10.1016/j.csr.2004.03.007.