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J. A. Whitehead

Abstract

As a driving parameter is slowly altered, thermohaline ocean circulation models show either a smooth evolution of a mode of flow or an abrupt transition of temperature and salinity fields from one mode to another. An abrupt transition might occur at one value or over a range of the driving parameter. The latter has hysteresis because the mode in this range depends on the history of the driving parameter. Although assorted ocean circulation models exhibit abrupt transitions, such transitions have not been directly observed in the ocean. Therefore, laboratory experiments have been conducted to seek and observe actual (physical) abrupt thermohaline transitions. An experiment closely duplicating Stommel’s box model possessed abrupt transitions in temperature and salinity with distinct hysteresis. Two subsequent experiments with more latitude for internal circulation in the containers possessed abrupt transitions over a much smaller range of hysteresis. Therefore, a new experiment with even more latitude for internal circulation was designed and conducted. A large tank of constantly renewed freshwater at room temperature had a smaller cavity in the bottom heated from below with saltwater steadily pumped in. The cavity had either a salt mode, consisting of the cavity filled with heated salty water with an interface at the cavity top, or a temperature mode, in which the heat and saltwater were removed from the cavity by convection. There was no measurable hysteresis between the two modes. Possible reasons for such small hysteresis are discussed.

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J. A. Whitehead
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J. A. Whitehead

Abstract

A laboratory experiment is conducted where hot water is cooled by exposure to air in a cylindrical rotating tank with a flat shallow outer “continental shelf” region next to a sloping “continental slope” bottom and a flat “deep ocean” center. It is taken to be a model of wintertime cooling over a continental shelf. The flow on the shelf consists of cellular convection cells descending from the top cooled surface into a region with very complicated baroclinic eddies. Extremely pronounced fronts are found at the shelf break and over the slope. Associated with these are sizable geostrophic currents along the shelf and over shelf break contours. Eddies are particularly energetic there. Cooling rate of the hot water is determined and compared with the temperature difference between the continental shelf and deep ocean. The results are compared with scaling arguments to produce an empirical best-fit formula that agrees with the experiment over a wide range of experimental parameters. A relatively straight trend of the data causes a good collapse to a regression line for all experiments. These experiments have the same range of governing dimensionless numbers as actual ocean continental shelves in some Arctic regions. Therefore. this formula can be used to estimate how much temperature decrease between shelf and offshore will be produced by a given cooling rate by wintertime cooling over continental shelves. The formula is also generalized to include brine rejection by ice formation. It is found that for a given ocean cooling rate, shelf water will be made denser by brine rejection than by thermal contraction. Estimates of water density increase implied by these formulas are useful to determine optimum conditions for deep-water formation in polar regions. For instance, shelves longer than the length scale 0.09 fW 5/3/B 1/3 (where f is the Coriolis parameter, W is shelf width, and B is buoyancy flux) will produce denser water than shorter shelves. In all cases, effects of earth rotation are very important, and the water will be much denser than if the fluid was not rotating.

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J. A. Whitehead

Abstract

Velocity, surface height profiles, and volume flux are calculated for critically controlled flow of a layer of rotating fluid from a channel to an exit passage. The upstream fluid possesses constant potential vorticity. These are models of an internal layer of ocean water flowing out of a basin through a passage. An analysis is used that allows general passage bottom shapes. A number of features differ from those of nonrotating critically controlled flow. First, sizeable gyres appear for a range of upstream conditions. Second, more than one critical flow (maximum flux) is possible at the control point for the same upstream condition, but only one of these is allowed with continuous laminar flow from the channel to the passage. Third, a bottom that slopes away from the right-hand side (Northern Hemisphere rotation) and that is at right angles to flow direction produces small volume flux at high rotation rates. Fourth, although there is a rigorous bound for flux out of a passage, this is exceeded for some cases with multiple exits.

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C. Cenedese and J. A. Whitehead

Abstract

The authors discuss laboratory experiments that elucidate the mechanism of formation and westward drift of anticyclonic baroclinic vortices from a buoyant surface current flowing along a lateral boundary and around a cape. Experiments were carried out with a sloping bottom in order to simulate the topographic β effect. They showed how a vortex can be generated from the current where it separates and reattaches to the cape and that, under some conditions, the eddy is able to detach from the cape and drift westward following isobaths. Two important timescales regulate the flow: the time t f that the current takes to generate a vortex and the time t d that the vortex takes to drift westward for a distance equal to its radius. When these two timescales are either of the same order of magnitude or t f < t d, the eddy was observed to translate westward. For t f > t d the vortex was able to form at the cape but it did not detach and drift westward. The influence of the depth of the lower layer, h 0, on the flow was investigated. The theoretical westward speed U 2d depends on the depth of the lower layer:the deeper the lower layer the slower the drift. The values of the slope s required in the experiments in order to obtain the detachment and drift of the vortex indicate that the phenomena will occur on a planetary β plane only when the variation of the Coriolis parameter with latitude is reinforced by a topographic β effect. A good agreement between the laboratory experiments and the observations of meddies in the Canary Basin, where the Mediterranean Outflow from the Strait of Gibraltar flows along the coast of Spain and around Cape St. Vincent, suggests that the eddy-shedding process is similar to that observed in the laboratory experiments.

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J. A. Whitehead and Wei Wang

Abstract

A model of deep ocean circulation driven by turbulent mixing is produced in a long, rectangular laboratory tank. The salinity difference is substituted for the thermal difference between tropical and polar regions. Freshwater gently flows in at the top of one end, dense water enters at the same rate at the top of the other end, and an overflow in the middle removes the same amount of surface water as is pumped in. Mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with Reynolds numbers >500. A stratified upper layer (“thermocline”) deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (“deep ocean overflow”) from a dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper-layer deepening. Data are taken over a wide range of parameters and compared to scaling theory, energetic considerations, and simple models of turbulently mixed fluid. There is approximate agreement with a simple theory for Reynolds number >1000 in experiments with a tank depth less than the thermocline depth. A simple argument shows that mixing and plume potential energy flux rates are equal in magnitude, and it is suggested that the same is approximately true for the ocean.

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Claudia Cenedese, John Marshall, and J. A. Whitehead

Abstract

A laboratory experiment has been constructed to investigate the possibility that the equilibrium depth of a circumpolar front is set by a balance between the rate at which potential energy is created by mechanical and buoyancy forcing and the rate at which it is released by eddies. In a rotating cylindrical tank, the combined action of mechanical and buoyancy forcing builds a stratification, creating a large-scale front. At equilibrium, the depth of penetration and strength of the current are then determined by the balance between eddy transport and sources and sinks associated with imposed patterns of Ekman pumping and buoyancy fluxes. It is found that the depth of penetration and transport of the front scale like [(fwe)/g′] L and w e L 2, respectively, where w e is the Ekman pumping, g′ is the reduced gravity across the front, f is the Coriolis parameter, and L is the width scale of the front. Last, the implications of this study for understanding those processes that set the stratification and transport of the Antarctic Circumpolar Current (ACC) are discussed. If the laboratory results scale up to the ACC, they suggest a maximum thermocline depth of approximately h = 2 km, a zonal current velocity of 4.6 cm s−1, and a transport T = 150 Sv (1 Sv ≡ 106 m3 s−1), not dissimilar to what is observed.

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Young-Gyu Park and J. A. Whitehead

Abstract

Convection experiments were carried out in a rectangular tank as a model of oceanic meridional overturning circulation. The objective was finding a relation between the meridional heat flux and thermal forcing. To make the meridional heat flux estimate possible, the heat flux was fixed at one bottom end of the tank using an electrical heater. Temperature was fixed at the other end using a cooling plate. All other boundaries were insulated. In equilibrium, the heat input to the fluid H was the same as the meridional heat flux (heat flux from the source to the sink), so it was possible to find a scaling law relating H to the temperature difference across the tank ΔT and rotation rate f. The experimental result suggests that the meridional heat transport in the experiment was mostly due to geostrophic flows with a minor correction caused by bottom friction. When the typical values of the North Atlantic are introduced, the geostrophic scaling law predicts meridional heat flux comparable to that estimated in the North Atlantic when the vertical eddy diffusivity of heat is about 1 cm2 s−1.

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Melinda M. Hall, Michael McCartney, and J. A. Whitehead

Abstract

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssal currents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from the Brazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv ≡ 106 m3 s−1), comprising organized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated by a quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transports occur in September–October, while minimum transports occur in February–March. Allowing for this quasi-annual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated mean AABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water (LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speed through the array, 9.9 cm s−1 into the Brazil Basin. The LNADW records also have a quasi-annual cycle with strong LNADW flow episodes occurring in April–May. Time series of temperature indicate that the LNADW/AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (uplifted transition layer during strong AABW transport periods). An observed overall warming trend appears to be accompanied by a decline in AABW transport.

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M-L. Timmermans, P. Winsor, and J. A. Whitehead

Abstract

The Arctic Ocean likely impacts global climate through its effect on the rate of deep-water formation and the subsequent influence on global thermohaline circulation. Here, the renewal of the deep waters in the isolated Canadian Basin is quanitified. Using hydraulic theory and hydrographic observations, the authors calculate the magnitude of this renewal where circumstances have thus far prevented direct measurements. A volume flow rate of Q = 0.25 ± 0.15 Sv (Sv ≡ 106 m3 s−1) from the Eurasian Basin to the Canadian Basin via a deep gap in the dividing Lomonosov Ridge is estimated. Deep-water renewal time estimates based on this flow are consistent with 14C isolation ages. The flow is sufficiently large that it has a greater impact on the Canadian Basin deep water than either the geothermal heat flux or diffusive fluxes at the deep-water boundaries.

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