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Richard G. Williams

Abstract

Air–Sea interaction influences the ventilated thermocline by forcing the mixed layer to deepen and cool poleward. When there is flow from the mixed layer into the interior, the mixed-layer depth and density fields help to set the potential vorticity of the subducted fluid. The importance of this process is assessed by incorporating a depth-varying mixed layer in a ventilation model which is forced by Ekman pumping and implied surface heating. The formulation of the ventilation problem is simplified by only allowing density surfaces to outcrop along latitude circles, and by assuming that there is no zonal inflow along the eastern boundary. The surface heating enables a cross-isopycnal flow within the mixed layer. The volume of ventilated fluid within the subtropical gyre is increased by including the depth-varying mixed layer, and this fluid partly originates from the western boundary, as well as from the Ekman layer. The depth-varying mixed layer increases the depth at which isopycnals are subducted and changes the value of the potential vorticity injected into the main thermocline. However, the mixed layer only alters the detail of the general streamline pattern, with an increase in the subducted potential vorticity leading to the surface flow strengthening and the deeper flow weakening

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Richard G. Williams

Abstract

A steady ventilation model is used to assess the effect of the mixed layer on the structure of the main thermocline; the potential vorticity is found in a subtropical gyre after imposing the thickness and density of the mixed layer, the Ekman pumping, and the hydrography on the eastern boundary.

The modeled potential vorticity becomes comparable in value to observations in the North Atlantic if the mixed layer deepens poleward as is observed in winter. The isopycnal gradients in potential vorticity are reduced on the denser ventilated surfaces if the mixed-layer outcrops deviate from latitude circles and, more realistically, sweep southward along the eastern boundary; the age of the subducted fluid is also in reasonable agreement with observations of the tritium-helium age by Jenkins.

This study suggests that ventilation may form much of the extensive region of nearly uniform potential vorticity observed on the σθ = 26.75 surface in the North Atlantic, with lateral mixing by eddies being required only in the unventilated pool on the western side of the gyre.

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Richard G. Williams
and
Vassil Roussenov

Abstract

The role of sidewalls in determining the interior distribution of potential vorticity (PV) is investigated using eddy-resolving isopycnic experiments. The layer model is integrated at 1 16° resolution for a wind-driven double gyre with either vertical or sloping sidewalls. If there are vertical sidewalls, eddy stirring leads to PV homogenization within unforced, interior density layers. If there are sloping sidewalls, frictional torques lead to bands of low and high PV being formed along the western boundary of the subpolar and subtropical gyres, respectively. These regions of low and high PV are transferred into the interior by a separated jet at the intergyre boundary. Over a limited domain, this injection of the PV contrast can prevent eddy homogenization from occurring. However, over a larger-scale domain, eddies provide a downgradient transfer of PV, reducing the PV contrast downstream along the jet and enabling homogenization to occur for intermediate layers within the basin interior. Diabatic mixing along the slope can introduce low PV for intermediate layers and even mask the frictional contributions.

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Gualtiero Badin
and
Richard G. Williams

Abstract

The effect of buoyancy forcing on the residual circulation in the Southern Ocean is examined in two different ways. First, the rates of water-mass transformation and formation are estimated using air–sea fluxes of heat and freshwater in the isopycnal framework developed by Walin, which is applied to two different air–sea flux climatologies and a reanalysis dataset. In the limit of no diabatic mixing and at a steady state, these air–sea flux estimates of water-mass transformation and formation are equivalent to estimating the residual circulation and the subduction rates in the upper ocean, respectively. All three datasets reveal a transformation of dense to light waters between σ = 26.8 and 27.2, as well as positive formation rates peaking at σ = 26.6, versus negative rates peaking at σ = 27. The transformation is achieved either by surface heating or freshwater inputs, although the magnitude of the formation rates varies in each case. Second, an idealized model of a mixed layer and adiabatic thermocline for a channel is used to illustrate how changes in ocean dynamics in the mixed layer and freshwater fluxes can modify the buoyancy fluxes and, thus, alter the residual circulation. Increasing the Ekman advection of cold water northward enhances the air–sea temperature difference and the surface heat flux into the ocean, which then increases the residual circulation; an increase in wind stress of 0.05 N m−2 typically increases the surface heat flux by 8 W m−2 and alters the peaks in formation rate by up to 8 Sv (1 Sv ≡ 106 m3 s−1). Conversely, increasing the eddy advection and diffusion leads to an opposing weaker effect; an increase in the eddy transfer coefficient of 500 m2 s−1 decreases the surface heat flux by 3 W m−2 and alters the peaks in formation rate by 1 Sv.

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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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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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Alison J. McLaren
and
Richard G. Williams

Abstract

Subduction requires a buoyancy input into the mixed layer, which over the gyre scale can either be achieved by an atmospheric input or a wind-induced Ekman redistribution of buoyancy. The buoyancy budget for subduction is diagnosed over the North Atlantic using monthly fields from 1950 to 1992. The climatological-mean budget suggests that subduction over the subtropical gyre occurs through an Ekman redistribution of buoyancy from the Tropics, rather than a surface buoyancy flux from the atmosphere. In contrast, interannual variations in subduction are controlled by the variations in the surface buoyancy flux, which are generally greater than the variations in the Ekman redistribution of buoyancy. However, over the Tropics and southern part of the subtropical gyre, there is a partial cancellation in the opposing contributions from the surface and Ekman buoyancy fluxes, which acts to reduce the interannual variations in subduction.

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A. J. George Nurser
and
Richard G. Williams

Abstract

The effect of cooling on the separated boundary current predicted by the model of Parsons is studied. The separating current is found to strengthen and to move southwards and eastwards. The model is also robust to limited heating. in which case the separating current weakens and moves northwards.

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Chris Wilson
,
Bablu Sinha
, and
Richard G. Williams

Abstract

The control of atmospheric storm tracks by ocean dynamics, orography, and their interaction is investigated using idealized experiments with a simplified coupled atmosphere–ocean climate model. The study focuses on the quasi–steady state for the storm tracks in the Northern Hemisphere winter mean. The experiments start with a background state without mountains and ocean dynamics, and in separate stages include orography and a dynamic ocean to obtain a more realistic control simulation. The separate effects of ocean dynamics, orography, and their nonlinear interaction are identified for the storm tracks and the surface thermodynamic forcing over the ocean.

The model study suggests that atmospheric storm tracks are a robust feature of the climate system, occurring at midlatitudes even if there is no orographic forcing or ocean heat transport. Ocean dynamics generally lead to a poleward shift in both the storm track and the maximum in atmospheric northward heat transport and induce a northeastward tilt over the Atlantic. This poleward shift is linked to the extra heat transport by the ocean and the tightening of sea surface temperature gradients on the western side of ocean basins. Orographic forcing causes along-track variability with a weakening of the storm track over the continents and induces a northeastward tilt over the western Pacific, which is associated with a stationary planetary wave train generated by the Tibetan Plateau. The interaction between ocean dynamics and orographic forcing plays a localized role, enhancing the contrast between the Atlantic and Pacific. Much of the response to the forcing is eddy mediated and transient eddies help to spread the influence of orographic and ocean forcing.

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Jane O’Dwyer
and
Richard G. Williams

Abstract

Climatological maps of the large-scale potential vorticity field Q along isopycnals are diagnosed for the abyssal waters over the global ocean. The inferred patterns of Q vary with density, the basin, and hemisphere. At middepths, the distribution of Q is controlled by the background planetary vorticity gradient. On deeper isopycnals, there are regions where Q contours deviate from latitude circles and even regions of nearly uniform Q. These different regimes appear to be robust features over the interior of ocean basins, as the standard error is found to be relatively small there. The nearly uniform Q occurs in the deep waters of the North Pacific and, possibly, in the bottom waters of the western North Atlantic and North Pacific. The nearly uniform Q has a low magnitude in each case, as well as a relatively low variability for the deep waters of the North Pacific. This nearly uniform Q signal appears to be formed when a single water mass enters the basin from a low latitude source.

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