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Peter G. Baines

Abstract

The reasons for the large-amplitude tidal motion observed in oceanic submarine canyons have been explored with a laboratory experiment. A barotropic tide was forced in a stratified tank, containing continental shelf-slope topography into which a narrow canyon was incised. Large-amplitude tidal motions were observed in the canyon; it is shown that these were forced by the large horizontal pressure gradient existing on the continental shelf near the canyon head. Another significant feature of this experiment was that internal waves inside the canyon were partially reflected from the open boundary at the mouth of the canyon, like sound waves from the open end of an organ pipe. This enabled energy to propagate down the canyon in the form of leaky modes.

The character of the flow in the canyon was strongly dependent on the ratio of bottom slope α to ray (or characteristic) slope c. For α/c Lt; 1 the stratification had little effect on the motion, and the largest displacements were nearly barotropic and occurred near the canyon head; for α/c ≈ 1 the motion was baroclinic and had the same pattern at all depths. For α/c > 1 the energy propagated down the canyon in the form of leaky modes; because of reflection at the bottom, large amplitudes may occur near there in some cases.

The analysis also suggests a mechanism for the large amplitudes of high-frequency internal waves observed in submarine canyons. For a narrow canyon, wave motion in the canyon will be forced at the mouth by the pressure field of an incident wave from the deep sea, plus that of the wave reflected from the eternal continental slope; this will result in a wave with up to twice the amplitude (and hence four times the energy) inside the canyon.

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Peter G. Baines
and
Peter C. Manins

Abstract

Laboratory modeling provides a reasonably quick and relatively inexpensive method for investigating stratified air Row around mesoscale topography. Quantitative results for stratified flow over complex terrain may be obtained from suitably designed experiments. This paper collects together information about this technique that has been obtained in recent years by various authors. The associated dimensionless parameters and their relative importance are discussed. The principal parameter is Nh/U, which determines whether the flow is lee-wavelike or blocked-flowlike, and this in turn determines the importance and scaling of the various dynamical terms. The techniques are now readily applicable to quantitative environmental and engineering fluid transport problems.

Two examples of practical applications with real complex topography are briefly described. The first involves a separation eddy in stable conditions in light winds near Melbourne, Australia. The second describes the investigation of effect of lee waves on the suitability of a particular site for an international airport in Hong Kong.

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Peter G. Baines
and
Roger L. Hughes

Abstract

In regards to the recent paper by Pedlosky and Spall (Journal of Physical Oceanography, November 2015), this comment maintains that the steady-state solutions of Rossby waves in a uniform eastward current past an island have waves on the upstream side that are not caused by the island because of inappropriate boundary conditions and the assumed form of the solution. The solutions are interesting but are not the solutions to the problem as posed in their paper. Similar upstream waves in a time-dependent numerical model are also inconsistent with linear Rossby wave theory, though the reasons for their presence are uncertain.

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Peter G. Baines
and
Chris K. Folland

Abstract

It is shown that a number of important characteristics of the global atmospheric circulation and climate changed in a near-monotonic fashion over the decade, or less, centered on the late 1960s. These changes were largest or commonest in tropical regions, the Southern Hemisphere, and the Atlantic sector of the Northern Hemisphere. Some, such as the decrease in rainfall in the African Sahel, are well known. Others appear to be new, but their combined extent is global and dynamical linkages between them are evident. The list of affected variables includes patterns of SST; tropical rainfall in the African Sahel and Sudan, the Amazon basin, and northeast Brazil; pressure and SST in the tropical North Atlantic and the west and central Pacific; various branches of the southern Hadley circulation and the southern subtropical jet stream; the summer North Atlantic Oscillation; south Greenland temperature; the Southern Hemisphere storm track; and, quite likely, the Antarctic sea ice boundary. These changes are often strongest in the June–August season; changes are also seen in December–February but are generally smaller. In Greenland, annual mean temperature seems to be affected strongly, reflecting similar changes in SST throughout the year in the higher latitudes of the North Atlantic. Possible causes for these coordinated changes are briefly evaluated. The most likely candidates appear to be a likely reduction in the northward oceanic heat flux associated with the North Atlantic thermohaline circulation in the 1950s to 1970s, which was nearly in phase with a rapid increase in anthropogenic aerosol emissions during the 1950s and 1960s, particularly over Europe and North America.

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Peter G. Baines
and
Wenju Cai

Abstract

An interactive atmosphere–ocean instability mechanism that reproduces some salient properties of the observed Antarctic Circumpolar Wave and also its manifestation in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 coupled model is analyzed with a more complete treatment than that studied by others. It is suggested that this interaction mechanism is important in maintaining this phenomenon in both the model and the real atmosphere–ocean but is not strong enough to initiate it. Through use of a simple model consisting of a zonally periodic midlatitude beta plane with a uniform mean north–south temperature gradient, a barotropic atmosphere, and a two-layer ocean with an inactive lower layer, the stability of uniform zonal flow to small perturbations was analyzed. The perturbation equations describe the velocity and temperature fields in both the atmospheric and oceanic layers and include the exchange in momentum and heat between them by surface fluxes. The interaction occurs between long (most notably wavenumbers 2 and 3) barotropic Rossby waves in the atmosphere forced by surface heat flux from the ocean and similarly long waves in the upper layer of the ocean forced by the wind stress curl. Growth times are long—on the order of several decades—indicating that modes can be sustained by the interaction process but that they may need to be energized by other mechanisms to reach realistic amplitudes in a reasonable time.

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Peter G. Baines
and
Klaus P. Hoinka

Abstract

This paper describes some laboratory experiments with two-dimensional stratified flow over isolated topography, in which a novel configuration simulating a radiating upper boundary condition is employed. Several experimental tests show that the upper boundary is quite effective in absorbing energy. The properties of flow over five different obstacle shapes were obtained for a merge of values of the parameter Nh/U(h is obstacle height, N Brunt-Väisälä frequency and U towing velocity) from 0 to 4 (approximately). The main results of the study are 1) for 0 ≤ Nh/U ≤ 0.5 (±0.2), the flow is consistent with linear theory and Longs model; 2) for 0.5 ≲ Nh/U ≲ 2.0, upstream columnar disturbances are found which apparently propagate arbitrarily far upstream in an inviscid system. 3) overturning and rotors in the lee wave field occur for Nh/U ∼ 1.5; and 4) for Nh/U ∼ 2.0, blocked fluid is present upstream, and in some cases is also apparent downstream. This upstream blocking is due to the super-position of the propagating columnar disturbances; it will similarly extend arbitrarily far upstream given sufficient time.

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Peter G. Baines
and
Klaus Fraedrich

Abstract

Topographically induced flows around Antarctica in a rotating tank experiment with both homogeneous and stratified fluid are analyzed and compared with the mean tropospheric circulation. A circular tank of fluid was brought to a state of near-rigid clockwise rotation, and a topographic model of the Antarctic continent was then rotated counterclockwise to simulate a mean westerly zonal wind. Stratification was chosen to give the same ratios of topographic and dynamical length scales as in the atmosphere, as was the Rossby number based on the ratio of rotation rates. After the onset of the relative rotation of the Antarctic model, cyclonic eddies evolved in the coastal areas with, in the homogeneous case, anticyclonic eddies over the Antarctic dome. After about ten tank rotation periods, a dominant wavenumber 3 structure with cyclonic eddies in the Ross and Weddell seas and Prydz Bay is observed as an approximately steady state. Flow over the topography is relatively stagnant, with weak anticyclonic circulation. Variation of the Rossby number by a factor of 4 about the mean atmospheric value showed that the same general behavior was obtained, although there were differences in detail.

These flows show remarkable similarity to the observed mean 700 mb height and 850 rob wind fields around Antarctica. This strongly suggests that the same dynamical factors are operating, namely conservation of potential vorticity and strong coupling in the vertical, so that these motions are virtually baratropic. The large cyclonic eddies am then forced by flow separation around prominent coastal irregularities such as the Antarctic Peninsula.

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Peter G. Baines
and
Roger L. Hughes

Abstract

Observations of a laboratory model of a western boundary current, and its separation and subsequent meandering, are described. The current is established by pumping fluid through a rotating channel that contains a topographic β effect and continental slope topography. The observations are compared with a theoretical model of all three aspects of the current: the structure of the attached current, the process of separation, and the dynamics and path of the meandering jet. This model includes a viscous boundary layer for the attached current, with a thickness of order [ν/(dv I/dy)]1/2, where ν is kinematic viscosity and dv I/dy is the velocity gradient of the inviscid (free slip) flow along the boundary.

Comparison between the observations and the model show that the attached boundary current is governed by potential vorticity conservation and the Bernoulli equation, and the pressure decreases along its length. The separation of this current from the sidewall is then caused by the minimum pressure level that is set by the downstream conditions in the tank, which forces the current into deeper water. The process is analogous to the separation of a boundary layer from a surface in an adverse pressure gradient in nonrotating flows. This process has implications for the separation of ocean boundary currents, where the details are more complex but clear analogies exist. Meanders in the separated current are qualitatively consistent with an inertial jet model, although detached eddies attributable to instability are also observed.

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Gregory F. Lane-Serff
and
Peter G. Baines

Abstract

The formation of eddies by dense overflows in stratified water is examined by laboratory experiments. The dense fluid initially flows down the slope but turns (under the influence of rotation) to flow along the slope. The inviscid alongslope flow is continuously drained by a viscous Ekman layer that flows more directly downslope. In some cases this Ekman layer flow becomes unstable to growing waves. Under certain conditions, strong cyclonic vortices form in the ambient fluid above the alongslope flow due to vortex stretching, causing the dense fluid to break up into a series of domes. There are three main mechanisms for this: first, the initial downslope flow of the current (before it turns under the influence of rotation) may take “captured” upper-layer fluid with it out into deeper water; second, adjustment of the current to geostrophic balance stretches the fluid column above the current; and, finally, the continuous viscous draining from the current (and later from the domes) also causes stretching in the ambient fluid.

The vertical extent of the influence of the overflow (and thus the initial effective height of these columns of ambient fluid) is controlled by the stratification in the ambient fluid. Two types of stratification are examined: a two-layer ambient fluid with an interface above the overflow and a linearly stratified ambient fluid. For the two-layer ambient fluid the relevant vertical scale is simply the height of the interface above the overflow, d l , while for the linearly stratified case a height scale based on the strength of the stratification is derived, d N. The stretching of the columns of ambient fluid is measured by the parameter Γ l = /d l or ΓN = /d N, where L is the Rossby deformation radius and α is the bottom slope. The frequency at which the eddy/dome structures are produced increases with the stretching parameter Γ, while the speed at which the structures propagate along the slope depends on viscous effects. The behavior is very similar to that for flow into an unstratified ambient fluid where the stretching parameter Γ = /D, where D is the total fluid depth, except that the propagation speed of the eddies along the slope is slower in the stratified case by a factor of approximately 0.7. The flow of dense fluid on slopes is a very important part of the global ocean circulation system, and the implications of the laboratory experiments for oceanographic flows are discussed particularly for Denmark Strait.

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Wenju Cai
,
Peter G. Baines
, and
Hal B. Gordon
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