The 2 Meeting on the Physical Oceanography of Sea Straits, Villef ranche, 15th-19th April 2002 The current system of the Bosphorus Strait based on recent measurements Emin ¨ Ozsoy, Mohammed A. Latif and S ¸¨ ukr¨ u Bes ¸iktepe Institute of Marine Sciences, Middle East Technical University, Erdemli - ˙ Ic ¸el 33731 Turkey Abstract. A series of measurements in the Bosphorus Strait using ADCP and CTD profiling, current-meter and sea-level recordings are analysed to yield three-dimensional mapping of hydrography and currents as well as the statistical characteristics of sea-level and current variability. The results show time-dependent meandering of currents, with separated eddies formed in sheltered areas along the coast. Spatial correlation along the Strait reflects these changes. Sea-level differences respond rapidly to changes in flows, and sudden change in wind direction leads to blocking conditions. Introduction The Bosphorus is a passageway of heavy marine traffic between the two seas, as well as of land traffic between the two continents that it connects. Information on the nature of the currents in the Bosphorus is thus essential for safe navi- gation and transit through the Strait. Furthermore, the Turk- ish Straits System (consisting of the Dardanelles, Bospho- rus Straits and the Sea of Marmara) is sensitive to climatic changes, and potentially can induce such changes in the ad- jacent basins [ ¨ Ozsoy, 1999]. Acting as the limiting element of the The Turkish Straits System, the Bosphorus Strait con- trols the exchanges of mass and passive or active materials . 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 50 50 50 50 60 60 60 60 70 70 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' 0 5 km 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' CM2 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' CM1 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' SL2 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' SL1 Selvi Burnu Beylerbeyi Rumelikavagi Pilot Besiktas Figure 1. Bottom topography of the Bosphorus, and the locations of sea level (SL1, SL2) and current-meter (CM1, CM2) stations. . 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' BILIM 03 SEP 1998 GPS 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' 29˚ 00' 29˚ 00' 29˚ 05' 29˚ 05' 29˚ 10' 29˚ 10' 41˚ 00' 41˚ 00' 41˚ 05' 41˚ 05' 41˚ 10' 41˚ 10' 41˚ 15' 41˚ 15' 1 m/s SEP03 1999 Figure 2. (a) GPS positions of the ship collecting data along the Bosphorus, and (b) interpolated surface currents from continuous ADCP measurements, on 03 September 1998. transported between the Black and the Mediterranean Seas [ ¨ Ozsoy et al., 1995b; Polat and Tu˘ grul, 1995; ¨ Unl¨ uata et al., 1990]. Based on budgets calculated from average salinity the mass flux of the upper layer flow is about two times larger than that of the lower layer, yielding a net flux of about from the Black Sea to the Sea of Marmara [Latif et al., 1991; ¨ Unl¨ uata et al., 1990]. Geometrical features [O˘ guz et al., 1990; ¨ Unl¨ uata et al., 1990; ¨ Ozsoy et al., 1998] make the Bosphorus predisposed to ’maximal exchange’, with contraction and sill controls as in Farmer and Armi [1986]. Local topographic features have significant influ- ence on the flow, and determine its detailed structure [Gregg and ¨ Ozsoy, 2001]. The exchange flows respond dynam- 177