The relation between eddy-induced transport and isopycnic gradients of potential vorticity
Journal of Physical Oceanography 29:7 (1999) 1571-1578
Abstract:
The dynamical control of the eddy-induced transport is investigated in a series of idealized eddy-resolving experiments. When there is an active eddy field, the eddy-induced transport is found to correlate with isopycnic gradients of potential vorticity, rather than gradients of layer thickness. For any unforced layers, the eddy transfer leads to a homogenization of potential vorticity and a vanishing of the eddy-induced transport in the final steady state.Do we require adiabatic dissipation schemes in Eddy-resolving ocean models?
Journal of Physical Oceanography 28:10 (1998) 2050-2063
Abstract:
Use of horizontal diffusion of temperature and salinity in numerical ocean models causes spurious diapycnal transfers - the "Veronis effect" - leading to erosion of the thermocline and reduced poleward heat transports. The authors derive a relation between these diapycnal transfers and the dissipation of vorticity gradients. An increase in model resolution does not significantly reduce the diapycnal transfers since vorticity gradients cascade to smaller scales and must ultimately be dissipated to maintain numerical stability. This is confirmed in an idealized primitive equation ocean model at a variety of resolutions between 1° and 1/8°. Thus, the authors conclude that adiabatic dissipation schemes are required, even in eddy-resolving ocean models. The authors propose and implement a new biharmonic form of the Gent and McWilliams scheme, which adiabatically dissipates at the grid scale while preserving larger-scale features.How slippery are piecewise-constant coastlines in numerical ocean models?
Tellus, Series A: Dynamic Meteorology and Oceanography 50:1 (1998) 95-108
Abstract:
Coastlines in numerical ocean models are oriented at various finite angles to the model grid. The true coastline is usually replaced by a piecewise-constant approximation in which the model coastline is everywhere aligned with the model grid. Here we study the consequences of the piecewise-constant approximation in an idealised shallow-water ocean model. By rotating the numerical grid at various finite angles to the physical coastlines, we are able to isolate the impact of piecewise-linear boundaries on the model circulation. We demonstrate that piecewise-constant coastlines exert a spurious form stress on model boundary currents, dependent on both the implementation of the slip boundary condition and the form of the viscous stress tensor. In particular, when free-slip boundary conditions are applied, the character of the circulation can be reduced to no-slip in the presence of a piecewise-constant boundary. The spurious form stress can be avoided in a free-slip limit if the viscous stress tensor is written in terms of vorticity and divergence.On the eddy transfer of tracers: Advective or diffusive?
Journal of Marine Research 55:3 (1997) 483-505
Abstract:
Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses and tracers in a down-gradient manner. However, eddies also have an advective role that may lead to an up-gradient transfer of tracers, as has been recognized in atmospheric tracer studies and recent eddy parameterizations developed for the ocean. Eddies provide an advective transfer or "bolus" velocity through the secondary circulation formed by the slumping of density surfaces in baroclinic instability. Here we use an eddy-resolving isopycnal ocean model to investigate the meridional transfer across a zonal jet. The jet undergoes baroclinic instability, forming a vibrant eddy field and inducing a meridional bolus velocity. The bolus velocity is found to be correlated with gradients of potential vorticity rather than thickness. A transient tracer is released with high and low values at the southern and northern boundaries respectively. Over the first few years, the tracer spreads diffusively in a down-gradient manner. The implied eddy diffusivity of the passive tracer is found to be reassuringly similar to that of the dynamic tracer, potential vorticity. On the decadal time scale, however, the eddy-induced advection dominates and leads to a poleward spreading of tracer in the upper layer, and equatorward spreading of tracer in the lower layer. This eddy-induced advection is likely to be important in controlling the water-mass distribution wherever the time-mean meridional flow is weak. Observationally, the transport velocity is difficult to measure directly, but we argue might be inferred from the spreading of transient tracers, such as CFCs, before they reach a statistically-steady state.Subduction of water masses in an eddying ocean
Journal of Marine Research 55:2 (1997) 201-222