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From Forcing to Dissipation : Kinetic and Available Potential Energy Pathways in Idealized Models of Ocean Circulation

Abstract

The general circulation of the ocean is forced by surface fluxes of momentum, heat, and freshwater at basin scales. The kinetic (E_k) and available potential (E_a) energy sources associated with these external forces drive a circulation which exhibits flow features that vary on a wide range of spatial and temporal scales. Understanding how the different forcing mechanisms lead to the observed large-scale ocean circulation patterns and to what degree do the various smaller scale processes modify them have been long standing problems for oceanographers. The purpose of this dissertation is, first, to examine the role of buoyancy forcing and the associated E_a source in maintaining the observed meridional overturning circulation (MOC) and overall thermal structure of the ocean and, second, to understand the possible E_k pathways in the ocean, from forcing to dissipation scales. We first derive an exact positive definite available potential energy density e_a that is connected to well-known temporal evolution equations for both E_k and E_a. e_a is easily linked to the dynamics of a fluid flow and can be interpreted in a similar manner to the commonly used E_k density. Next, we apply the E_a framework to the horizontal convection (HC) model, a simple physical construct used to investigate the role of buoyancy forcing in driving the MOC. The basic HC model refers to the flow resulting from a buoyancy variation imposed along a horizontal boundary of a fluid. We study and quantify the effects of rotation on three-dimensional HC with respect to the overall thermal structure and buoyancy transport mechanisms, the overturning circulation, and the flow energetics. Our numerical results show that the steady state solution of rotating horizontal convection (RHC) is substantially different that that of HC. In RHC geostrophic eddies dominate the vertical and horizontal buoyancy fluxes as well as the energy reservoirs and exchange terms, leading to enhanced stratification and a deeper thermal boundary layer compared with HC. Finally, we examine the kinetic energy pathways and cascades in the RHC model as well as in a model externally forced by wind stress. In both models the simulated flow is allowed to reach a statistical steady state at which point it exhibits both a forward and an inverse E_k cascade. We show that the E_k of the 'balanced' geostrophic eddies (EKE) is dissipated preferentially at small scales near the surface via frontal instabilities associated with loss of balance' and a forward energy cascade rather than by bottom drag after an inverse energy cascade, typical of geostrophically turbulent flows. This is true both with and without forcing by the wind. These results suggest that submesoscale instabilities near the ocean surface could efficiently dissipate EKE, independent of boundary effects

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