Date

2016-4-15

Author

Gürses, Özgür

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This thesis presents the application of the finite element ocean model to the Turkish Strait System (TSS) which connects the two nearly-enclosed marginal water bodies of the Black and Mediterranean Seas through the narrow, non-straight and elongated Bosphorus and Dardanelles Straits and the Marmara Sea in between. The Turkish Strait System is a key gateway between the Black and the Mediterranean Seas and includes narrow channels where two-layer exchange flow develops. Being the only salt source in the Black Sea and additional brackish water source in the North Aegean Basin, TSS poses wide range of physical processes in temporal and spatial scale. Adequate and effective numerical representation of this large spectrum is a grand challenge and still beyond today’s classical Ocean General Circulation Models (OGCM). In this work, we focused on a new regional implementation of an unstructured mesh based multi-resolution ocean model (the Finite Element Ocean Model, FEOM) with refinement up to _65 m in its connecting straits while keeping a coarse resolution no more than _1,6 km in the Marmara Sea and _5 km setup in the adjacent reservoirs. This model forms the basis of hindcast simulations in conjunction with sensitivity experiments for the year 2008. Nonlinear hydraulic transitions and stratified turbulent exchange flow through the narrow straits and over steep topography exemplifies the complexity in the Turkish Strait System. Correct reproduction of such processes in this region strictly depends on the accuracy of the topography as well as the complex coastal geometry. Consequently, a new bathymetry is produced by merging available data sets of different sources, such as multi-beam surveys or the digitized information from the bathymetric charts. ArcGIS software provided necessary tools for the extraction of the consistent coastal information. Unlike a few previous studies, this is the first comprehensive study enabling the representation of the entire TSS without nesting. Thanks to its that multi-resolution flexibility and numerically efficient algorithm, FEOM is a versatile alternative not only for global climate studies but also for regional oceanographic applications. We perform a-year long simulations that were forced with the realistic atmosphere of the year 2008, the period of available data. The results are assessed by focusing mostly on stability of the pycnocline and the variability of the volume transports across the Bosphorus and Dardanelles. The Black Sea freshwater input is a key element to obtain correct stratification in the Marmara Sea as well as the Bosphorus volume fluxes. The model captures the flow reversal of the upper layer, so-called Orkoz, during the passage of southwesterly storms over the region. Hindcast results are compared to observational datasets (partly independent) collected over two months (April and October 2008). Hindcast simulations includes almost all forcing functions in the system. It is of importance to assess the model response to various coefficients and forcing perturbations. Therefore, series of simulations are presented which are based on idealized lock-exchange initial conditions. Numerical calculations show that the main circulation patterns are produced well and consistent with previous modeling and observational studies. The interface which is the region of rapid salinity and temperature change between quasi-homogeneous upper and lower layers is sloped steeply throughout the Bosphorus and the Dardanelles Straits indicating that mass and momentum transfer between the these layers are important.

Subject Keywords

Turkish Strait System, Finite Element Ocean Modeling, Unstructured Grid

URI

https://hdl.handle.net/11511/95094

Collections

Graduate School of Marine Sciences, Thesis