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This study deals with turbulent flow below a stress-driven interface with stable density stratification, as a simplified configuration of a turbulent flow below a calm gas-liquid interface with wind forcing. A simple shear stress, which is constant in time and uniform at the interface, is enforced, hence only turbulence below the interface is studied. A flat interface is assumed to simplify transport phenomena beneath the interface by limiting the strength of the enforced wind stress and avoiding violent wave motions. A direct numerical simulation technique is employed to obtain three-dimensional turbulence structures below the interface. Results from the present study indicate two major changes of turbulence structures below the sheared interface in the presence of stable density stratification; one is a reduction of the Reynolds stress at the near-interface region, which leads to a coincidental increase of mean velocity gradient. The other effect of stable stratification is the quantitative variation of the surface-streak structures and associated microscale turbulence structures. Examination of the autocorrelation coefficients of the streamwise velocity fluctuations in the subinterface region exhibited that the surface-streak structures in the velocity field are established in both neutral and stably stratified fluid. These streak structures are, however, partially disorganized in the region very close to the interface when strong stable stratification is imposed. The spacing between the streaks tends to decrease with increasing Richardson number in the subinterfacial region −30Ͻ x 3 + Ͻ −20, where the disorganization of the streaks is not encountered. A flow visualization technique verifies the establishment of these streaks in every Richardson number case and their partial disorganization under the effect of strong density stratification. A statistical three-dimensional relation between the streaks and the vortical structures are explored by applying a two-point correlation technique to instantaneous turbulent flow realizations. The presence of a counter-rotating vortex pair below the high-momentum fluid at the interface can be found in these examinations. Such presence of the counter-rotating vortex pair are confirmed by the visualization of the vortical structures in instantaneous turbulent flow realizations, whose three-dimensional shape is rather a hairpin-like configuration than two separated vortex tubes.

The main focus of this thesis is the effect of topographic features on the transport of tracers in the bottom boundary layer. The thesis covers two specific studies; transport of CO 2 which is directly introduced into the deep ocean and transport of passive food particles to cold-water corals. The study is performed using Reynolds Averaged Navier Stokes Models (RANS) on a relative small scale with horizontal grid sizes ranging from 75 meter to approximately 1 meter. A general background for the study is given in Part I, and Part II consists of the four included papers. Papers included in this thesis The following papers are included in this thesis; Paper A: Dissolution of a CO 2 lake, modeled by using an advanced vertical turbulence mixing scheme, L.

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The ocean takes up approximately 2 GT carbon per year due to the enhanced CO 2 concentrations in the atmosphere. Several options have been suggested in order to reduce the emissions of CO 2 into the atmosphere, and among these are CO 2 storage in the deep ocean. Topographic effects of dissolution and transport from a CO 2 lake located at 3,000-m depth have been studied using the z-coordinate model Massachusetts Institute of Technology general circulation model (MITgcm) and the σ -coordinate model Bergen ocean model (BOM). Both models have been coupled with the general ocean turbulence model (GOTM) in order to account for vertical subgrid processes. The chosen vertical turbulence mixing scheme includes the damping effect from stable stratification on the turbulence intensity. Three different topographic scenarios are presented: a flat bottom and the CO 2 lake placed within a trench with depths of 10 and 20 m. The flat case scenario gives good correlation with previous numerical studies of dissolution from a CO 2 lake. When topography is introduced, it is shown that the z-coordinate model and the σ -coordinate model give different circulation patterns in the trench. This leads to different dissolution rates, 0.1 μmol cm −2 s −1 for the scenario of a 20-m-deep trench using BOM and 0.005-0.02 μmol cm −2 s −1 for the same scenario using the MITgcm. The study is also relevant for leakages of CO 2 stored in geological formations and to the ocean.