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Free Content On the eddy transfer of tracers: Advective or diffusive?

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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.
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Document Type: Research Article

Publication date: May 1, 1997

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  • The Journal of Marine Research, one of the oldest journals in American marine science, publishes peer-reviewed research articles covering a broad array of topics in physical, biological and chemical oceanography. Articles that deal with processes, as well as those that report significant observations, are welcome. Biological studies involving coupling between ecological and physical processes are preferred over those that report systematics. The editors strive always to serve authors and readers in the academic oceanographic community by publishing papers vital to the marine research in the long and rich tradition of the Sears Foundation for Marine Research. We welcome you to the Journal of Marine Research.
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