On flow variability in the Bosphorus Strait

Two bottom-mounted acoustic Doppler current profilers and a vertical string of temperature, conductivity, and pressure sensors were deployed at each end of the Bosphorus Strait in September 2008 and remained in place for over 5 months. These observations showed a two-layer structure of the exchange flow in the Bosphorus Strait with brackish waters originating in the Black Sea moving southward and more saline, denser waters from the Sea of Marmara flowing northward. Considerable differences in mean flow, current fluctuations, and layer thickness were also found. In the northern Bosphorus, the current variations were more pronounced in the lower layer than in the upper layer. The opposite situation was observed in the southern Bosphorus where the upper layer currents fluctuated more noticeably. The near-surface currents often exceeded 200 cm/s in the southern section and were generally below 30 cm/s in the northern section. Currents usually below 70 cm/s were observed in the lower layer in the southern part of the strait, while the lower layer outflow to the Black Sea in the northern part of the strait frequently reached 100 cm/s, with flow concentrated in a strong midlayer maximum core. The upper layer thickness displayed temporal variability and, on average, was about 39 m near its northern end and about 14 m near its southern end. Flow variability was found to be closely associated with variability of the bottom pressure difference and the atmospheric forcing on the synoptic time scale (2-10 days).