Barry A. Klinger

Meridional Circulation Cells and the Source Waters of the Pacific Equatorial Undercurrent

Abstract A 3½-layer model is used to study the meridional circulation cells that provide the source waters of the Pacific Equatorial Undercurrent (EUC). Its three active layers represent tropical, thermocline, and upper-intermediate waters, respectively, and across-interface flow between the layers parameterizes the processes of upwelling, subduction, and diapycnal mixing. Solutions are driven by climatological winds in a domain resembling the Pacific basin from 35°S to 55°N. An additional forcing mechanism is a specified inflow into layer 3 across the open southern boundary and a compensating outflow from layers 1 and 2 along the western boundary just north of the equator; the resulting circulation simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific and the same amount of thermocline and tropical waters exit via the Indonesian Throughflow. Five meridional cells contribute to the EUC in the main-run solution: north and south Subtropical Cells (STCs), no...

Representation of convective plumes by vertical adjustment

Open-ocean deep-water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large-scale models? If so, is it important that the mixing occurs over a finite time tmix, or would instantaneous mixing produce the same effect on large-scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends on tmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume-resolving simulations of open-ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value for tmix is explored by setting tmix = 12 hours in some experiments, in accordance with scaling considerations, and tmix = 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the water column over a time tmix.

Meridional Heat Transport by the Subtropical Cell

The wind-driven circulation adds a significant contribution to poleward meridional heat transport. Numerical models indicate that equatorward of f 0, the zero wind stress latitude (308 lat), most of the wind-induced heat transport is due to the meridional overturning circulation known as the subtropical cell. The volume transport of this overturning is approximately given by the surface Ekman transport. By combining this fact with the assumption that Ekman-downwelled water approximately follows isotherms except near the equator, the authors derive an expression for the meridional heat transport that depends only on wind stress and surface temperature. The expression is confirmed in numerical models with simplified geometry and forcing. Numerical results indicate that peak heat transport due to the subtropical cell is about 0.1 3 1015 W for the North Atlantic and 0.3 3 1015 W for the North Pacific.

Behavior of Double-Hemisphere Thermohaline Flows in a Single Basin

A coarse resolution, three-dimensional numerical model is used to study how external parameters control the existence and strength of equatorially asymmetric thermohaline overturning in a large-scale, rotating ocean basin. Initially, the meridional surface density gradient is directly set to be larger in a ‘‘dominant’’ hemisphere than in a ‘‘subordinate’’ hemisphere. The two-hemisphere system has a broader thermocline and weaker upwelling than the same model with the dominant hemisphere only. This behavior is in accord with classical scaling arguments, providing that the continuity equation is employed, rather than the linear vorticity equation. The dominant overturning cell, analogous to North Atlantic Deep Water formation, is primarily controlled by the surface density contrast in the dominant hemisphere, which in turn is largely set by temperature. Consequently, in experiments with mixed boundary conditions, the dominant cell strength is relatively insensitive to the magnitude QS of the salinity forcing. However, QS strongly influences subordinate hemisphere properties, including the volume transport of a shallow overturning cell and the meridional extent of a tongue of low-salinity intermediate water reminiscent of Antarctic Intermediate Water. The minimum QS is identified for which the steady, asymmetric flow is stable; below this value, a steady, equatorially symmetric, temperature-dominated overturning occurs. For high salt flux, the asymmetric circulation becomes oscillatory and eventually gives way to an unsteady, symmetric, salt-dominated overturning. For given boundary conditions, it is possible to have at least three different asymmetric states, with significantly different large-scale properties. An expression for the meridional salt transport allows one to roughly predict the surface salinity and density profile and stability of the asymmetric state as a function of QS and other external parameters.

Sensitivity of basinwide meridional overturning to diapycnal diffusion and remote wind forcing in an idealized atlantic-southern Ocean geometry

Abstract Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the wind-forced component. The predicted overturning is compared with coarse-resolution numerical experiments with an idealized Atlantic Ocean–Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and “layer thickness.” A layer model produces similar results, increasing confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highl...

The Dynamics of Equatorially Asymmetric Thermohaline Circulations

The three-dimensional dynamics of equatorially asymmetric thermohaline flow are investigated using an ocean general circulation model in a highly idealized configuration with no wind forcing and nearly fixed surface density. Small asymmetries in surface density lead to strongly asymmetric meridional overturning patterns, with deep water formed in the denser (northern) hemisphere filling the abyss. The poleward deep transport in the lighter hemisphere implies that the deep zonal-mean zonal pressure gradient reverses across the equator. Density along the eastern boundary and the zonally averaged density are nearly symmetric about the equator except at very high latitudes; the Southern Hemisphere western boundary thermocline, in contrast, is balanced by weaker upwelling and is hence broader than its northern counterpart. This pattern is explained through the spinup of the asymmetric circulation from a symmetric one, the timescale of which is set through advection by the mean deep western boundary current. For the strength of the interhemispheric transport, a lower bound of one-half the one-hemisphere overturning strength is derived theoretically for small finite forcing asymmetries, implying that the symmetric circulation is unlikely to be realized. Under asymmetric surface forcing, enhanced mixing in the denser hemisphere suppresses interhemispheric transport. Conversely, very strong cross-equatorial transport is caused by enhanced mixing in the lighter hemisphere. These results indicate that, once the surface densities determine that North Atlantic Deep Water is the dominant ventilating source, its export rate from the North Atlantic is controlled by mixing and upwelling in the rest of the World Ocean.

Inviscid Current Separation from Rounded Capes

Abstract Laboratory experiments have suggested that the separation of a coastal surface current from a cape of radius of curvature ρ in a system rotating with Coriolis parameters f occurs when ρ < u/f, where u is the characteristic flow speed of the current. Using an inviscid, reduced gravity model in the limit of ρ large compared to the current width, separation criteria are derived for various coastal currents of uniform potential vorticity. In this model, separation occurs when centrifugal forces at the cape raise the density interface bounding the current to the surface. For a current bounded on the offshore edge by a density front, the critical radius of curvature ρc is approximately that found in the laboratory. For a current bounded by a zero-velocity contour, the expression for ρc must be multiplied by a factor of W/R, where W is the upstream current width and R is the Rossby radius.

Baroclinic eddy generation at a sharp corner in a rotating system

Laboratory experiments were conducted to investigate the generation of anticyclonic gyres by separation of a surface current from a coast in a rotating, two layer system. The experiments were motivated by the hypothesis that the flow of coastal currents around capes can generate oceanic eddies, as well as by the observation of gyres at the mouths of various straits. In the experiments the gyre is formed when the current, which flows with the coast to its right if one is oriented in the downstream direction, encounters a sharp convex corner. The current overshoots the corner, loops to the right, and reattaches to the coast downstream of the corner. Between the current loop and the coast is an anticyclone whose width grows with time. If a countercurrent flows under the surface current, a similar separation in the lower layer results in the generation of a cyclone as well; under some circumstances the cyclone and anticyclone advect each other away from the coast as a heton. Previous studies on related systems found that the corner must be sufficiently sharp for a gyre to form. I show that for a very sharp corner the angle made by the corner must be above a critical value of between 40° and 45° for a gyre to form. This is in contrast to nonrotating flows of comparable Rayleigh number, which will separate from a sharp corner at virtually any angle. For angles below the critical value, the current profile downstream of the corner changes as a function of corner angle, indicating that it is the stagnation of the flow nearest the wall which causes the anticyclone to form. This stagnation is reminiscent of the two-dimensional, nonrotating picture of viscous boundary layer dynamics forcing separation of a boundary current. However, the gyre grows more slowly when the lower layer is much thicker than the upper layer, indicating that baroclinic processes are at least quantitatively important in the generation of the gyre. By varying the initial condition of the current, it is shown that the gyre formation is not a product of the interaction of the nose of the current with the corner. In conclusion, the experiments indicate that the basic mechanism of gyre formation may be viscous boundary effects as in nonrotating systems, but that rotation tends to inhibit eddy generation while baroclinic effects tend to enhance it.

Regimes and scaling laws for rotating deep convection in the ocean

Numerical experiments are presented which explore the dependence of the scale and intensity of convective elements in a rotating fluid on variations in external parameters in a regime relevant to open ocean deep convection. Conditions inside a convection region are idealized by removing buoyancy at a uniform rate B from the surface of an initially homogeneous, motionless, incompressible ocean of depth H with a linear equation of state, at a latitude where the Coriolis parameter is tf. The key nondimensional parameters are the natural Rossby number Ro∗ =(B/f3H2)12 and the flux Rayleigh number Raf = BH4/(κ2ν), where κ and ν are (eddy) diffusivities of heat and momentum. Ro∗ is set to values appropriate to open ocean deep convection (0.01 < Ro∗ < 1), and moderately high values of Raf (104 < Raf < 1013) were chosen to produce flows in which nonlinear effects are significant. The experiments are in the ‘geostrophic turbulence’ regime. As Ro∗ and Raf are reduced the convective elements become increasingly quasi-two-dimensional and can be described as a field of interacting ‘hetons’. The behavior of the flow statistics—plume horizontal length scale L, speed scale U and buoyancy scale G, and the magnitude of the mean adverse density gradient measured by the stratification parameter H—are studied as a function of Ro∗ and Raf. Physically motivated scaling laws are introduced, which, when appropriate, employ geostrophic and hydrostatic contraints. They are used to interpret the experiments. In the heton regime, in which the motion is predominantly geostrophic and hydrostatic, the observed scales are sensitive to moderate variations in Ro∗ and large variations in Raf. We demonstrate broad agreement between our numerical experiments and previous laboratory studies. The lateral scale of the convective elements and the (adverse) stratification in which they exist adjust to one another so that NH/fL≈ 1; the horizontal scale of the hetons is thus controlled by a pseudo Rossby radius based on the unstable stratification parameter N, the scale at which the overturning forces associated with N are balanced by the counter-overturning forces associated with rotation.

The Relationship between Oscillating Subtropical Wind Stress and Equatorial Temperature

Abstract An earlier study showed that an atmosphere–ocean model of the Pacific develops a midlatitude oscillation that produces decadal sea surface temperature (SST) variability on the equator. The authors use the ocean component of this model to understand better how subtropical wind stress oscillations can cause such SST variability. The model ocean consists of three active layers that correspond to the mixed layer, the thermocline, and intermediate water, all lying above a motionless abyss. For a steady wind, the model develops a subtropical cell (STC) in which northward surface Ekman transport subducts, flows equatorward within the thermocline, and returns to the surface at the equator. Analytic results predict the models equatorial temperature, given some knowledge of the circulation and external forcing. A prescribed subtropical wind stress anomaly perturbs the strength of the STC, which in turn modifies equatorial upwelling and equatorial SST. The transient response to a switched-on wind perturbat...