Publication Date

2009-05-09

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2009-03-31

First Committee Member

Tamay M. Özgökmen - Committee Chair

Second Committee Member

William E. Johns - Committee Member

Third Committee Member

Mohamed Iskandarani - Committee Member

Fourth Committee Member

Hartmut Peters - Committee Member

Fifth Committee Member

Helmut Z. Baumert - Outside Committee Member

Abstract

Mixing between overflows and ambient water masses is a crucial problem of deep-water formation in the down-welling branch of the meridional overturning circulation of the ocean. In this dissertation work, performance of second-order turbulence closures in reproducing mixing of overflows is investigated within both hydrostatic and non-hydrostatic models. First, a 2D non-hydrostatic model is developed to simulate the Red Sea overflow in the northern channel. The model results are compared to the Red Sea Outflow Experiment. It is found that the experiments without sub-grid scale models cannot reproduce the basic structure of the overflow. The k-ε model yields unrealistically thick bottom layer (BL) and interfacial layer (IL). A new technique so-called very large eddy simulation (VLES) which allows the use of k-ε model in non-hydrostatic models is also employed. It is found that VLES results the most realistic reproduction of the observations. Furthermore, the non-hydrostatic model is improved by introducing laterally average terms, so the model can simulate the constrictions not only in the z-direction but also in the y-direction. Observational data from the Bosphorus Strait is employed to test the spatially average 2D non-hydrostatic model (SAM) in a realistic application. The simulations from SAM with a simple Smagorinsky type closure appear to be excessively diffusive and noisy. We show that SAM can benefit significantly from VLES turbulence closures. Second, the performance of different second-order turbulence closures is extensively tested in a hydrostatic model. Four different two-equation turbulence closures (k-&epsilon, k-&omega, Mellor-Yamada 2.5 (MY2.5) and a modified version of k- &epsilon) and K-Profile Parameterization (KPP) are selected for the comparison of 3D numerical simulations of the Red Sea overflow. All two-equation turbulence models are able to capture the vertical structure of the Red Sea overflow consisting of the BL and IL. MY2.5 with Galperin stability functions produce the largest salinity deviations from the observations along two sections across the overflow and the modified k-&epsilon exhibits the smallest deviations. The rest of the closures fall in between, showing deviations similar to one another. Four different closures (k- &epsilon, k-&omega, MY2.5KC and KPP) are also employed to simulate the Mediterranean outflow. The numerical results are compared with observational data obtained in the 1988 Gulf of Cadiz Expedition. The simulations with two-equation closures reproduce the observed properties of the overflow quite well, especially the evolution of temperature and salinity profiles. The vertically integrated turbulent salt flux displays that the overflow goes under significant mixing outside the west edge of the Strait of Gibraltar. The volume transport and water properties of the outflow are modified significantly in the first 50 km after the overflow exits the strait. The k-&epsilon and k-&omega cases show the best agreement with the observations. Finally, the interaction between the Red Sea overflow and Gulf of Aden (GOA) eddies has been investigated. It is found that the overflow is mainly transported by the undercurrent at the west side of the gulf. The transport of the overflow is episodic depending strength and location of GOA eddies. The most crucial finding is that the Red Sea overflow leaves the Gulf of Aden in patches rather than one steady current. Multiple GOA eddies induce lateral stirring, thus diapycnal mixing of the Red Sea outflow.

Keywords

Red Sea Overflow; Gravity Currents; VLES; K-epsilon

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