Eddy Potential Vorticity Flux Research Articles

Eddy-Potential vorticity Fluxes in the Gulf Stream Recirculation

The local effect of the mesoscale eddy field on the mean potential vorticity distribution of the Gulf Stream recirculation region is determined from the quasi-geostrophic eddy potential vorticity flux. This flux is calculated by finite difference of current and temperature time series from the Local Dynamics Experiment. This long-term array of moorings is the only experimental data from which the complete eddy flux can be calculated. The total eddy flux is dominated by the term due to the time variation in the thickness of isopycnal layers. This thickness flux is an order of magnitude larger than the relative vorticity flux. The total flux is statistically significant and directed 217°T to the southwest with a magnitude of 1.57 × 10−5 cm s−1. The direction of the eddy flux with respect to the mean large-scale potential vorticity gradient from hydrographic data indicates that eddies in this region tend to reduce the mean potential vorticity gradient.

Waves and circulation driven by oscillatory winds in an idealized ocean basin

Abstract We examine, via direct numerical integration, the transient and rectified response of a flat-bottomed barotropic ocean to a spatially localized oscillatory wind-stress pattern. These experiments exemplify in many respects the dynamics which drive the deep motion in recent eddy-resolving ocean circulation studies [e.g., Holland and Rhines (1980)], and may be contrasted with the results of Pedlosky (1965) and Veronis (1966) for spatially broad, time-dependent forcing. By considering doubly re-entrant (periodic) and closed basin geometries, the structure and magnitude of the induced circulation is shown to depend most critically on the form of the mean quasi-geostrophic contours (which are closed and blocked, respectively, in the periodic and basin geometries). In both situations, however, the forced primary wave field may be usefully understood by appeal to the radiation pattern of a time-periodic Green's function, and (in a basin) its image in the western boundary. The dynamics of the prograde and retrograde rectified circulation is seen to relate most directly to the mean eddy potential vorticity flux v′ q′ (and not to the Reynolds stress u′ v′). In particular: Eulerian vorticity budgets indicate the dominance of the turbulent Sverdrup balance [βv ∼—▿ · v′q′] in nearly all parts of the flow; Lagrangian (particle-wise) balances clearly emphasize the regions of counter-gradient q fluxes in providing the propulsion necessary for fluid particles to cross the mean quasigeostrophic contours. Although these flows do not strictly comply with the assumptions of recent q-transport theories, nonetheless all the qualitative (and some of the quantitative) features predicted by these theories are confirmed in the simulations.

A theoretical discussion of eddy-driven mean flows

This paper gives fairly detailed account of our recent results on the induction of mean circulation by eddy processes {whether wave-like or turbulent). The relevance of the work to climatology is through the role of oceans in climate, as well as the need to understand and parameterize eddies in the atmospheric circulation itself. In Section 1 we discuss the vorticity constraints that are likely to dominate the wind-driven mean circulation, in the absence of strong eddy effects. In Section 2 we describe the principle formulation for Eulerian-mean flow, rewriting the eddy-induction effects in the spirit of early work by Taylor {1915). Both transient and statistically steady problems, wavelike and turbulent flows, and oceanic and a tmospher ic geometry can be treated, although the details are distinctly different in each case. The key quantities that express the eddy effects are the Lagrangian diffusivity of marked fluid particles (~) and the large-scale geometry of Q, where Q is the large-scale potential vorticity field. The non-dissipative theory stems from work of Taylor, but most problems are sensitive to the nature of potential vorticity dissipation by the enstrophy cascade. These new effects are summarized. The meanflow equation is solved by integrating along characteristics. The principal restriction is that the lateral scale of the Q-field is supposed to far exceed the displacement of fluid particles, over few eddy periods. The important role played by the eddy flux of potential vorticity, (q'u' }, is

An Observational Test of the Quasi-Geostrophic Relation between Eddy Momentum Flux Convergence and Eddy Fluxes of Potential Vorticity and Potential Temperature

The quasi-geostrophic relation between the fluxes of momentum, potential vorticity and potential temperature is tested with atmospheric data by computing the convergence of momentum flux as a residual of of the potential temperature and potential vorticity flux and comparing it to the momentum flux convergence computed directly. It is shown that in the troposphere between 18 and 74°N the observed momentum flux convergence differs from the quasi-geostrophic convergence by 25–60% depending on tropospheric level and season.

The Vertical Scale of an Unstable Baroclinic Wave and Its Importance for Eddy Heat Flux Parameterizations

Abstract Linear, quasi-geostrophic waves destabilized by a surface temperature gradient produce eddy potential vorticity fluxes which characteristically extend above the surface to a height where the vertical shear ∂u¯/∂z, static stability N2 and potential vorticity gradient ∂q/∂y of the zonal flow are evaluated at the surface. Utilizing this result and a simple scaling analysis, we argue that the time averaged, vertically integrated, poleward eddy heat flux is proportional to the fifth power of the meridional temperature gradient when h0 is much less than the scale height of the atmosphere.

Trajectory Slopes, Countergradient Heat Fluxes and Mixing by Lower Stratospheric Waves

Abstract In large-scale wave disturbances in the lower stratosphere, the poleward and upward velocity components are positively correlated so that typical air trajectories, when projected onto the meridional plane, slope upward toward the pole. The slope of the air trajectories can be reconciled with the observed poleward, countergradient eddy heat flux at these levels if one takes into account the poleward acceleration of warm air in the wave troughs and the equatorward acceleration of cold air in the ridges. These temperature anomalies are produced by subsidence in the wave troughs and ascent in the ridges. The same processes are capable of producing poleward and downward eddy fluxes of potential vorticity, ozone, and other tracers whose values or concentrations increase rapidly with height.