Ocean Sci., 14, 999–1019, 2018 https://doi.org/10.5194/os-14-999-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Circulation of the Turkish Straits System under interannual atmospheric forcing Ali Aydogdu˘ 1,2,3, Nadia Pinardi4,5, Emin Özsoy6,7, Gokhan Danabasoglu8, Özgür Gürses6,9, and Alicia Karspeck8 1Science and Management of Climate Change, Ca’ Foscari University of Venice, Venice, Italy 2Centro Euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy 3Nansen Environmental and Remote Sensing Center, Bergen, Norway 4Department of Physics and Astronomy, University of Bologna, Bologna, Italy 5Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy 6Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey 7Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey 8National Center for Atmospheric Research, Boulder, CO, USA 9Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Correspondence: Ali Aydogdu˘ ([email protected]) Received: 16 January 2018 – Discussion started: 31 January 2018 Revised: 31 May 2018 – Accepted: 25 July 2018 – Published: 11 September 2018 Abstract. A simulation of the Turkish Straits System 1 Introduction (TSS) using a high-resolution, three-dimensional, unstruc- tured mesh ocean circulation model with realistic atmo- The Turkish Straits System (TSS) connects the Marmara, spheric forcing for the 2008–2013 period is presented. The Black and Mediterranean seas through the Bosphorus and depth of the pycnocline between the upper and lower lay- Dardanelles straits. The near-surface layer of low salinity wa- ers remains stationary after 6 years of integration, indicat- ters originating from the Black Sea enters from the Bospho- ing that despite the limitations of the modelling system, the rus, flowing west and exiting into the Aegean Sea at the Dar- simulation maintains its realism. The solutions capture im- danelles. The deeper, more saline waters of Mediterranean portant responses to high-frequency atmospheric events such origin enter from the Dardanelles in the lower layer and as the reversal of the upper layer flow in the Bosphorus due eventually reach the Black Sea through the undercurrent of to southerly severe storms, i.e. blocking events, to the ex- the Bosphorus. The strongly stratified marine environment tent that such storms are present in the forcing dataset. The of the TSS is characterised by a sharp pycnocline positioned annual average circulations show two distinct patterns in the at a depth of 25 m (Ünlüata et al., 1990). The complex to- Sea of Marmara. When the wind stress maximum is localised pography of the Sea of Marmara consists of a wide shelf in in the central basin, the Bosphorus jet flows to the south and the south, a narrower one along the northern coast and three turns west after reaching the Bozburun Peninsula. In con- east–west deep basins separated by sills, connected to the two trast, when the wind stress maximum increases and expands shallow, elongated narrow straits providing passage to the ad- in the north–south direction, the jet deviates to the west be- jacent seas at the two ends, as shown in Fig.1. fore reaching the southern coast and forms a cyclonic gyre The TSS mass and property balances are mainly controlled in the central basin. In certain years, the mean kinetic energy by the Black Sea in the upstream. At the Bosphorus Black in the northern Sea of Marmara is found to be comparable to Sea entrance, the long-term salinity budget implies a ratio that of the Bosphorus inflow. of about 2 between the upper and lower layer volume fluxes (Peneva et al., 2001; Kara et al., 2008). The net flux is es- timated to be comparable to the Black Sea river runoff, as the annual average precipitation and evaporation over the sea surface are roughly of the same order (Özsoy and Ünlüata, Published by Copernicus Publications on behalf of the European Geosciences Union. 1000 A. Aydogdu˘ et al.: Circulation of the TSS 1997). Daily to seasonal variations in net fluxes through the conditions in adjacent basins, demonstrating the unique hy- TSS are driven by changes in Black Sea river runoff, baro- draulic controls in the maximal exchange regime that are to metric pressure and wind forcing. be established in the realistic case. The combined effects of Climatological means of water and tracer fluxes through the Bosphorus and the proposed parallel channel known as the TSS were initially estimated from long-term observa- Kanal Istanbul˙ have been investigated by Sözer and Özsoy tions of seawater properties at junctions of the straits and on (2017b), indicating weak coupling between the two chan- surface water fluxes (Ünlüata et al., 1990; Be¸siktepeet al., nels, which have very different characteristics, but this has 1994; Tugrul˘ et al., 2002; Maderich et al., 2015), followed been found to be of climatic significance in modifying the later by ship-borne and moored acoustic Doppler current pro- fluxes across the TSS. filer (ADCP) measurements at the straits (Özsoy et al., 1988, Very few studies have attempted to model the circulation 1998; Altıok et al., 2012; Jarosz et al., 2011b,a, 2012, 2013). in the Sea of Marmara, even as a stand-alone system exclud- Updated reviews of TSS fluxes based on combined data have ing the dynamical influences of the straits and atmospheric been provided by Schroeder et al.(2012), Özsoy and Altıok forcing. Chiggiato et al.(2012) modelled the Sea of Marmara (2016), Sannino et al.(2017) and Jordá et al.(2017). using realistic atmospheric forcing and open boundaries at The hydrodynamic processes of the TSS extend over a the junctions of the straits with the sea, indicating surface cir- wide range of interacting space scales and timescales. The culation changes in response to changes in the strength and complex topography of the straits and property distributions directional pattern of the wind force. have resulted in hydraulic controls being anticipated in both Similarly, the interannual variability of the Sea of Mar- straits (Özsoy et al., 1998, 2001), which can only partially mara has been examined by Demyshev et al.(2012) using be demonstrated by measurements at the northern sill of the open boundary conditions at the strait junctions in the ab- Bosphorus (Gregg and Özsoy, 2002; Dorrell et al., 2016). sence of atmospheric forcing. They reproduced the S-shaped Hydraulic controls have since been found by modelling at jet current traversing the basin under the isolated conditions the southern contraction-sill complex and the northern sill, of a net barotropic current, which with appropriate parame- confirming a unique maximal exchange regime adjusted to terisation successfully preserved the sharp interface between the particular topography and stratification (Sözer and Öz- the upper and lower layers when the model steady state was soy, 2017a; Sannino et al., 2017). These findings support reached after 18 years of simulation. The S-shaped upper the notion that the Bosphorus is the more restrictive of the layer circulation of the Sea of Marmara predicted by Demy- two straits in controlling the outflow from the Black Sea to shev et al.(2012) appears similar to what Be¸siktepeet al. the Mediterranean. The analysis of moored measurements by (1994) found in summer, when wind forcing is at its min- Book et al.(2014) demonstrated this and indicated a more re- imum or at least close to being in a steady state. An anti- strained sea level response transmitted across the Bosphorus cyclonic pattern has generally been identified in the central than in the Dardanelles. Sea of Marmara, like the cases reported by Be¸siktepeet al. Improvements in modelling have provided a better sci- (1994). entific understanding of the TSS circulation, and they can The challenges of modelling the entire TSS domain were now address the complex processes characterising the sys- recently undertaken by Gürses et al.(2016). The effects of at- tem. The initial step in this formidable task is to construct mospheric forcing were considered, excluding the effects of separate models of the individual compartments of the sys- the net flux through the TSS. The study used an unstructured tem, which are the two straits and the Marmara basin. The triangular mesh model, the Finite Element Sea-Ice Ocean first simplified models of the Bosphorus were by Johns and Model (FESOM), with a high horizontal resolution reaching Oguz˘ (1989), who solved the turbulent transport equations about 65 m in the straits in the horizontal. The water column in 2-D and found a two-layer stratification to develop. Sim- is discretised by 110 vertical levels. plified two-layer or laterally averaged models of the Dard- The study of Sannino et al.(2017) used curvilinear coordi- anelles and Bosphorus were later developed byO guz˘ and nate implementation of the MITgcm (Marshall et al., 1997) Sur(1989), Stashchuk and Hutter(2001) andO guz˘ et al. with a non-uniform grid in the horizontal, a minimum of (1990), respectively, while Hüsrevoglu˘ (1998) introduced a 65 m resolution in the narrowest part of Bosphorus and 100 2-D reduced gravity ocean model of the Dardanelles in- levels in the vertical. The model was used to investigate the flow into the Sea of Marmara. Similar 2-D laterally aver- circulation of the TSS under varying barotropic flow through aged models (Maderich and Konstantinov, 2002; Ilıcak et al., the system in the absence of atmospheric forcing. The overall 2009; Maderich et al., 2015) and 3-D models (Kanarska and circulation in the Sea of Marmara was found to differ signif- Maderich, 2008; Öztürk et al., 2012), which were of limited icantly in each of their experiments with variations in the net extent, have been used to construct simplified solutions for volume transport at the Bosphorus.