David Marshall

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.

Abstract:

Mesoscale eddies modify the rate at which a water mass transfers from the surface mixed layer of the ocean into the interior thermocline, in particular in regions of intense baroclinic instability such as the Antarctic Circumpolar Current, open-ocean convective chimneys, and ocean fronts. Here, the time-mean subduction of a water mass, evaluated following the meandering surface density outcrops, is found to incorporate a rectified contribution from eddies, arising from correlations between the area over which the water mass is outcropped at the sea surface and the local subduction rate. Alternatively, this eddy subduction can be interpreted in terms of an eddy-driven secondary circulation associated with baroclinic instability. The net subduction rate, incorporating both Eulerian-mean and eddy contributions, can be further related to buoyancy forcing of the surface mixed layer using a formula by Walin (1982). Solutions from an idealized two-dimensional ocean model are presented to illustrate the eddy contribution to subduction rates in the Southern Ocean and in an open-ocean convective chimney. In the Southern Ocean, the net subduction rate is the residual of the Eulerian-mean and eddy contributions, which cancel at leading order; given plausible patterns of surface buoyancy forcing, one can obtain subduction of Antarctic Intermediate Water and Antarctic Bottom Water, with entrainment of North Atlantic Deep Water in between. In a convective chimney, in contrast, the Eulerian-mean subduction rate is vanishingly small and the subduction is contributed entirely by mesoscale eddies.