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Chris Wilson and Richard G. Williams

Abstract

The mechanisms controlling the direction of eddy tracer fluxes are examined using eddy-resolving isopycnic experiments for a cyclic zonal channel. Eddy fluxes are directed downgradient on average when either (i) there is a Lagrangian increase in tracer variance or (ii) there is strong dissipation of tracer variance. The effect of the eddies on the mean tracer evolution can be described through an ensemble of eddies that each have a particular life cycle. Local examination of the eddy behavior, such as fluxes, eddy kinetic energy, and tracer variance appears complex, although the cumulative time-mean picture has coherence: eddies are preferentially formed in localized regions with downstream growth and increase in tracer variance concomitant with downgradient eddy tracer fluxes, while eventually the eddies decay with a decrease in tracer variance and upgradient eddy tracer fluxes. During spinup, tracer deformation through flow instability leads to an area-average increase in tracer variance (although locally it is increasing and decreasing with the individual eddy life cycles) and therefore an implied area-average, downgradient tracer flux. At a steady state, part of the pattern in eddy fluxes simply reflects advection of background tracer variance by the time-mean and eddy flows. The eddy flux becomes biased to being directed downgradient if there is a strong sink in the tracer, which is likely to be the case for eddy heat fluxes along isopycnals outcropping in the mixed layer or for eddy nitrate fluxes along isopycnals intersecting the euphotic zone.

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Chris Wilson and Richard G. Williams

Abstract

Eddy fluxes systematically affect the larger-scale, time-mean state, but their local behavior is difficult to parameterize. To understand how eddy fluxes of potential vorticity (PV) are controlled, the enstrophy budget is diagnosed for a five-layer, 1/16°, eddy-resolving, isopycnic model of a wind-driven, flat-bottom basin. The direction of the eddy flux across the mean PV contours is controlled by the Lagrangian evolution of enstrophy, including contributions from the temporal change and mean and eddy advection, as well as dissipation of enstrophy. During the spinup, an overall increase in enstrophy is consistent with eddy fluxes being directed downgradient on average and homogenization of PV within intermediate layers. Enstrophy becomes largest along the flanks of the gyre, where PV gradients are large, and becomes smallest in the interior. At a statistically steady state, there is a reversing pattern of up- and downgradient eddy PV fluxes, which are locally controlled by the advection of enstrophy. A downgradient eddy PV flux occurs only on the larger scale over the gyre flanks and part of the western boundary. These larger-scale patterns are controlled by the eddy advection of enstrophy, which becomes significant in regions of high eddy enstrophy. As a consequence, at a statistically steady state, the eddy PV fluxes are not simply related to the mean fields, and their local, finescale pattern is difficult to parameterize.

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Richard E. Thomson and Robert E. Wilson

Abstract

Cape St. James is an extensive triangular-shaped promontory located in a tidally energetic region at the southern tip of the Queen Charlotte Islands approximately 150 km off the mainland coast or British Columbia. Several years of oceanographic data collected in vicinity of the cape reveal a regional circulation characterized by a strong (0.50 m s−1) coastal current along the western continental margin and respective clockwise and counterclockwise rotating mesoscale baroclinic eddies to the west and south of the cape. The coastal current flows counter to the prevailing winds while the anticyclonic eddy to the west of the cape is a particularly intense feature that appears consistently in AVHRR imagery of the region. The structure of the mean flow, combined with the marked O(0.1 0 m s−1) low-frequency current variability at fortnightly and monthly tidal periods plus significant coherence at fortnightly periods between low-frequency currents and demodulated tidal flow, suggests that rectification of the strong diurnal and semidiurnal tidal currents is the principal cause of the residual circulation in the vicinity of the cape.

Results from an analytical model indicate that generation of the mean residual circulation is due primarily to the M2 tidal current constituent and that maximum countercurrent velocities occur over the inner portion of the continental shelf. The fortnightly modulation of the mean flow is effected by both diurnal and semidiurnal currents but with a tendency for semidiurnal contributions to dominate in regions of greatest counterflow. Generic depth-dependent numerical simulations for nondimensional frictional parameters typical of the region verify that the asymmetry in the observed location and intensity of the eddy field, together with the presence of the strong coastal countercurrent on the west side of the cape and a narrow jet to the south of the cape, are associated with tidal rectification. These models also suggest that residual vertical motion due to topographic lifting and Ekman suction are responsible for the observed tilting of the isopycnals and thereby the development of baroclinicity in the residual horizontal motion.

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Richard G. Williams, Chris Wilson, and Chris W. Hughes

Abstract

Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

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