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Barry A. Klinger

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 ρ c is approximately that found in the laboratory. For a current bounded by a zero-velocity contour, the expression for ρ c must be multiplied by a factor of W/R, where W is the upstream current width and R is the Rossby radius.

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Barry A. Klinger

Abstract

A kinematic model of flow in the subtropical gyre is used to estimate the upper-ocean temperature distribution and heat transport that is forced by the wind-driven circulation. The temperature is idealized as a passive tracer forced by a zonally constant Haney condition and advected by a barotropic gyre. The simplicity of the model allows for analytical solutions, which are in fairly good quantitative agreement with the results obtained by Wang et al. using a numerical model. In particular, the maximum heat transport Q occurs when the restoring timescale for temperature is about one-tenth of the time it takes a particle to flow around the gyre. Estimates for the actual heat transport carried by the subtropical gyre are 0.2–0.4 PW for the North Atlantic and 0.3–0.6 PW for the North Pacific. Finally, the model shows that when the restoring timescale is short compared to the gyre advection timescale, the heat transport is sensitive to the width of the western boundary current. Potentially this could lead coarse-resolution numerical models to underestimate meridional heat transport, but the actual restoring timescale of the ocean may be too long for this to be an important effect.

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Barry A. Klinger
and
Jochem Marotzke

Abstract

The wind-driven circulation adds a significant contribution to poleward meridional heat transport. Numerical models indicate that equatorward of ϕ0, the zero wind stress latitude (30° 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 × 1015 W for the North Atlantic and 0.3 × 1015 W for the North Pacific.

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Barry A. Klinger
and
Carlos Cruz

Abstract

A substantial component of North Atlantic Deep Water formation may be driven by westerly wind stress over the Southern Ocean. Variability of this wind stress on decadal time scales may lead to circulation variability far from the forcing region. The Hybrid Coordinate Ocean Model (HYCOM), a numerical ocean model, is used to investigate the spatial patterns and the time scales associated with such wind variability. The evolution of circulation and density anomalies is observed by comparing one 80-yr simulation, forced in part by relatively strong Southern Hemisphere westerlies, with a simulation driven by climatological wind.

The volume transport anomaly takes about 10 yr to reach near-full strength in the entire Southern Hemisphere; however, in the Northern Hemisphere, it grows for the duration of the run. The Southern Hemisphere Indo-Pacific volume transport anomaly is about twice the strength of that found in the Atlantic. In the thermocline, water exits the southern westerlies belt in a broad flow that feeds a western boundary current (WBC) in both the Atlantic and Pacific Oceans. These WBCs in turn feed an Indonesian Throughflow from the Pacific and cyclonic gyres in the far north, which are broadly consistent with the Stommel–Arons theory. The deep return flow in each hemisphere is strongly affected by deep-sea ridges, which leads to a number of midocean “WBCs.” The wind perturbation causes isopycnals to sink over most of the basin. After about 20 yr, this sinking is very roughly uniform with latitude, though it varies by basin.

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Jochem Marotzke
and
Barry A. Klinger

Abstract

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.

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Barry A. Klinger
and
Jochem Marotzke

Abstract

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 Q S of the salinity forcing. However, Q S 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 Q S 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 Q S and other external parameters.

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Oluwayemi A. Garuba
and
Barry A. Klinger

Abstract

Surface flux perturbations (heat, freshwater, and wind) due to an increase of atmospheric CO2 cause significant intermodel spread in ocean heat uptake; however, the mechanism underlying their impact is not very well understood. Here, we use ocean model experiments to isolate the impact of each perturbation on the ocean heat uptake components, focusing on surface heat flux anomalies caused directly by atmospheric CO2 increase (passive) and indirectly by ocean circulation change (active). Surface heat flux perturbations cause the passive heat uptake, while all the surface flux perturbations influence ocean heat uptake through the active component. While model results have implied that the active component increases ocean heat uptake because of the weakening of the Atlantic meridional overturning circulation (AMOC), we find that it depends more on the shallow circulation change patterns. Surface heat flux perturbation causes most of the AMOC weakening, yet it causes a net global active heat loss (12% of the total uptake), which occurs because the active heat loss in the tropical Pacific through the subtropical cell weakening is greater than the active heat gain in the subpolar Atlantic through AMOC weakening. Freshwater perturbation weakens the AMOC a little more, but increases the subpolar Atlantic heat uptake a great deal through a large weakening of the subpolar gyre, thereby causing a large global active heat gain (34% of the total uptake). Wind perturbation also causes an active heat loss largely through the poleward shift of the Southern Hemisphere subtropical cells.

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Oluwayemi A. Garuba
and
Barry A. Klinger

Abstract

Global warming induces ocean circulation changes that not only can redistribute ocean reservoir temperature stratification but also change the total heat content anomaly of the ocean. Here all consequences of this process are referred to collectively as “redistribution.” Previous model studies of redistributive effects could not measure the net global contribution to the amount of ocean heat uptake by redistribution. In this study, a global ocean model experiment with abrupt increase in surface temperature is conducted with a new passive tracer formulation. This separates ocean heat uptake into contributions due to redistribution temperature and surface heat flux anomalies and those due to the passive advection and mixing of surface heat flux anomalies forced in the atmosphere. For a decline in the Atlantic meridional overturning circulation of about 40%, redistribution nearly doubles the Atlantic passive anomalous surface heat input and depth penetration of temperature anomalies. However, smaller increases in the Indian and Pacific Oceans cause the net global redistributive contribution to be only 25% of the passive contribution. Despite the much larger anomalous surface heat input in the Atlantic, the Pacific gains heat content anomaly similar to that in the Atlantic because of export from the Atlantic and Indian Oceans via the global conveyor belt. Of this interbasin heat transport, most of the passive component comes from the Indian Ocean and the redistributive component comes from the Atlantic.

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Barry A. Klinger
,
Julian P. McCreary Jr.
, and
Richard Kleeman

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 model's 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 perturbation is used to predict the ocean response to an oscillating wind. This method correctly predicts the results of several numerical experiments, and extends these results to a wide range of forcing periods. For an oscillating wind, there is a more complicated relationship between perturbations to equatorial SST and the various branches of the STC. The thermocline-branch anomalies are generally weaker than those in the surface and equatorial-upwelling branches. Equatorial SST anomalies lead, follow, and are roughly coincident with, variations in the thermocline, surface, and upwelling branches, respectively. Thus, while recent studies have suggested using the subsurface branch variations as a predictor of tropical–subtropical interactions, the surface branch may be a better predictor.

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Barry A. Klinger
,
Sybren Drijfhout
,
Jochem Marotzke
, and
Jeffery R. Scott
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