Long-term assessment of forcing conditions affecting the hydrodynamic structure of a stratified sea strait

Sea straits are waterways characterized by stratified flow, in which the upper layer flow is driven by barotropic forcing while the lower layer flow is driven by the density difference between adjacent basins. The flow structures of a strait and the water bodies they are connected to are highly dependent on meteorological and hydrological conditions and change significantly as a result of extreme events in these conditions. The present study examines long-term variability in extreme meteorological and hydrological forcing conditions that control the hydrodynamic behavior of a sea strait, the Bosphorus, using 80-year (1931-2010) discharge data of the Danube River and 42-year (1979-2020) meteorological data. In addition to long-term trend analysis, extreme value analysis (EVA) was applied to both the maximum and minimum values of both parameters. In the Danube River discharges, trend directions have changed in successive 20-year periods from 1931 to 2010, but most of the trends are not significant except for the last 20 years of records. Extreme value analysis results indicate that both extreme maximum and minimum discharges are about 10 % greater in recent years (1971-2010) compared with earlier years (1931-1970). The wind conditions in the strait are less variable compared with freshwater discharges. However, the storm analysis indicates the occurrence of more energetic and short-lived storms in the Bosphorus in recent years with greater variability. The variation in extreme pressure differences is more notable, especially in the recent 20-year period. A 37 % decrease in minimum pressure difference, which means higher pressures at the southern entrance of the Bosphorus, indicates that extreme northward current events in the Bosphorus have decreased in recent years. This, combined with the increasing extreme river discharges, indicates a higher dominance of the brackish Black Sea water both in the hydrodynamics of the strait and the Sea of Marmara-northern Aegean Sea system in the future.