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K. J. Richards, Y. Jia, and C. F. Rogers

Abstract

The dispersion of a tracer by a two-dimensional gyre circulation is studied using simple numerical models. Two approaches are taken: a random walk model formulated in a streamline coordinate system and the numerical solution of the advection-diffusion equation. A number of different gyres are considered. Attention is focused on the characteristics of the gyre that determine the spreading and mixing time of the tracer. The authors find that the dispersion by a given gyre can be characterized in terms of a bulk Péclet number and the three length scales: L the horizontal width of the gyre, l the width of the boundary current, and L the length of the boundary current. By taking into account the length of the boundary layer, gyre dispersion is found to conform moderately well with previous analytic models, in particular the partitioning between weak and strong diffusive regimes, even though the shear characteristics may be quite variable across the gyre. The analytic models become less valid as the length of the boundary layer increases. Simple expressions are given for the cross-streamline diffusion coefficient and mixing time in terms of the characteristics of the gyre. An important conclusion coming from the present study is the importance of the structure of the recirculation region in determining the shape of the tracer distribution. The results highlight the need for care in comparing model tracer fields with observed tracer distributions.

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A. L. New, R. Bleck, Y. Jia, R. Marsh, M. Huddleston, and S. Barnard

Abstract

This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.

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