Search

A deterministic model is developed to evaluate and explain the rate of dissipation of momentum in eddying oceanic flows. Theory is based on a classical conceptualization of mesoscale variability -- Stern's modon-sea solution -- which represents a closely packed array of steady compact dipolar vortices on the barotropic beta-plane. In our model, the periodic modon-sea pattern is subjected to a large-scale perturbation, weakly modulating the amplitude of the individual modons. The asymptotic multiscale analysis makes it possible to explicitly describe the interaction between the modon-sea eddies and the perturbing flow. This interaction results in a systematic weakening of the large-scale perturbation. The eddy viscosity in the model is found to be only weakly dependent on the explicit dissipation but rapidly decreases with increased separation of the modons. The estimates based on the modon-sea model are comparable to, but less than, the values of viscosity typically used in coarse resolution numerical ocean models. The eddy diffusivity of passive tracers is also evaluated and discussed in terms of a combination of analytical and numerical methods. The asymptotic theories are successfully tested by direct numerical simulations. [ABSTRACT FROM AUTHOR]

This study examines the importance of the Southern Ocean (SO) stratification in determining the upper cell of the Atlantic meridional overturning circulation (MOC) and stratification. Main results are based on a suite of idealized numerical simulations of the Atlantic with the prescribed density structure at the Atlantic southern boundary, intended to explore the importance of various factors. The results demonstrate that the density distribution at the SO-Atlantic boundary is the key factor controlling the Atlantic stratification and MOC, whereas the main importance of the Ekman and eddy (parameterized) exchanges is in setting the SO stratification. Among all aspects of the SO stratification, the position of the deep isopycnals near the western boundary of the Atlantic basin appears to determine the strength of the MOC for given isopycnal outcrop positions in the North Atlantic. The interplay between the SO stratification and surface density in the North Atlantic is, however, important for the MOC. In particular, the steady-state AMOC response to a negative North Atlantic density anomaly is shown to be amplified if the SO stratification is not allowed to change. [ABSTRACT FROM AUTHOR]

A two-dimensional analytical residual-mean model of the meridional overturning in the upper ocean is presented which illustrates dynamics of the interaction between the Northern and Southern hemispheres. The theory is based on the semi-adiabatic approximation in which all diabatic processes are confined to the upper mixed layer. The overturning circulation is driven directly by the wind forcing which, in our model, is affected by the sea-surface temperature distribution. The surface boundary conditions are symmetric with respect to the equator, and therefore one of the steady state solutions represents a symmetric flow characterized by the absence of the inter-hemispheric buoyancy fluxes. However, linear stability analysis, which takes into account both mechanical and thermodynamic forcing at the sea surface, indicates that the symmetric configuration such as this is unstable. The instability results in transition to the asymmetric regime with finite cross-equatorial exchange flows and heat transfer. Weakly nonlinear instability theory makes it possible to estimate the equilibrium fluxes in the new asymmetric steady states; for the oceanographically relevant range of parameters our model predicts the meridional overturning of about 10 Sv. While earlier studies considered the role of salt advection in spontaneous symmetry breaking, our study relies on a positive feedback between atmospheric winds and the oceanic meridional circulation. [ABSTRACT FROM AUTHOR]

An explicit analytical model of the Antarctic Circumpolar Current (ACC) is presented in which the key feature is the balance between the Eulerian circulation acting to overturn the isopycnals and geostrophic eddies which tend to flatten them. Solutions for the stratification and overturning circulation are obtained by expanding the governing residual mean equations of motion in a small parameter (ϵ) which measures the relative strength of the surface buoyancy flux and mechanical forcing by winds. Our balanced asymptotic model extends and reconciles the earlier views on the dynamics of the (streamwise-averaged) ACC by demonstrating that the zero order balance between the eddy-induced circulation and the mean flow determines gross features of the buoyancy distribution in the interior of the ACC. The diabatic buoyancy fluxes are essential in driving the secondary (residual) circulation which is, however, largely steered along the pathways set by the dominant adiabatic balance. Model assumptions are supported by a close agreement of the analytical solutions and fully nonlinear numerical calculations. [ABSTRACT FROM AUTHOR]

Focuses on the numerical solutions for salt fingers in an unbounded thermocline with uniform overall vertical temperature-salinity gradients. Use of Navier-Stokes-Boussinesq equations in a finite computational domain; Extension of two-dimensional heat-salt calculations for density ratio; Assumption of an analytic theory.

Presents three-dimensional numerical calculations of the fluxes and statistics of salt finger for a vertically unbounded fluid with uniform vertical gradients of sugar and salt. Instability of the lateral S-agent in the salt fingers; Calculations for moderate aspect ratio; Mechanism of amplitude equilibration; Comparison with results obtained from rigid boundary conditions.

Provides information on finite amplitude numerical calculations made for a completely unbounded salt finger domain whose overall vertical property gradients T and S are uniform and remain unaltered in time. Information on asymptotic formulations; Detailed information on subharmonic instability; Indepth look at numerical solutions of the 2D-large Prandtl number equations at small finite e.