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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, is performed for both coupled and ocean-only models. Spontaneous internal variability (primarily on the decadal timescale) is found to exist in the higher-resolution cases (with the exception of one of the restoring experiments). The decadal oscillation (whose period varies slightly between cases) is described as an advective–convective mechanism that is thermally driven and linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity in the high-resolution cases presented in this paper is enough to destroy the variability, whereas decreasing the diffusivity in the moderately coarse-resolution cases is capable of inducing decadal-scale variability. As the resolution is increased still further, baroclinic instability within the western boundary current adds a more stochastic component to the solution such that the variability is less regular and more chaotic (giving rise to intradecadal timescales). These results point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer variability in models that require less parameterized diffusion.

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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, are performed. The coupled atmosphere–ocean model results contradict earlier studies, which showed that the heat transport associated with time-varying circulations counteracted increases in the time mean so that the total remained unchanged as resolution was increased. Even though the total oceanic heat transport has not converged, the net planetary heat transport has essentially converged owing to the strong constraint of energy balance at the top of the atmosphere. Consequently, the atmospheric heat transport is reduced to offset the increasing oceanic heat transport.

To interpret these results, the oceanic heat transport is decomposed into its baroclinic overturning (related to the meridional overturning and Ekman transports), barotropic gyre (that in the horizontal plane), and baroclinic gyre (associated with the jet core within the western boundary current) components. The increase in heat transport occurs in the steady currents. In particular, the baroclinic gyre transport increases by a factor of 5 from the coarsest- to the finest-resolution case, equaling the baroclinic overturning transport at mid- to high latitudes.

To further assess the results, a parallel series of experiments under restoring conditions are performed to elucidate the differences between heat transport in coupled versus uncoupled models and models driven by temperature and salinity or equivalent buoyancy. Although heat transport is more strongly constrained in the restoring experiments, results are similar to those in the coupled model. Again, the total heat transport is increased due to an increasing baroclinic gyre component.

These results point to the importance of higher resolution in the oceanic component of current coupled climate models. These results also stress the need to adequately represent the heat transport associated with the “warm core” region of the Gulf Stream (the baroclinic gyre transport) in order to adequately represent oceanic poleward heat transport.

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Paul G. Myers
,
Augustus F. Fanning
, and
Andrew J. Weaver

Abstract

A diagnostic, finite element, barotropic ocean model has been used to simulate the mean circulation in the North Atlantic. With the inclusion of the joint effect of baroclinicity and relief (JEBAR), the Gulf Stream is found to separate at the correct latitude, ∼35°N, off Cape Hatteras. Results suggest that the JEBAR term in three key regions (offshore of the separation point in the path of the main jet, along the slope region of the North Atlantic Bight, and in the central Irminger Sea) is crucial in determining the separation point. The transport driven by the bottom pressure torque component of JEBAR dominates the solution, except in the subpolar gyre, and is also responsible for the separation of the Gulf Stream. Excluding high latitudes (in the deep-water formation regions) density variations in the upper 1000 m of the water column govern the generation of the necessary bottom pressure torque in our model.

Examination of results from the World Ocean Circulation Experiment-Community Modelling Effort indicates that the bottom pressure torque component of JEBAR is underestimated by almost an order of magnitude, when compared to our diagnostic results. The reason for this is unclear but may be associated with the diffuse nature of the modeled thermocline in the CME as suggested by our model’s sensitivity to the density field above 1000 m.

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Augustus F. Fanning
,
Richard J. Greatbatch
,
Arlindo M. Da Silva
, and
Sydney Levitus

Abstract

A linear, barotropic model of the North Atlantic at 1° ×1° resolution is employed to investigate the effect of using different wind-stress climatologies on the model response at the Florida Straits. The wind-stress climatologies are those of da Silva et al. (DS), Hellerman and Rosenstein (HR), Isemer and Hasse (IH), and Trenberth et al. (TR). For each climatology, the model shows maximum northward transport in the summer and minima in the fall and late winter, in general agreement with transport measurements from cable data (Larsen). However, the amplitude of the model response differs considerably between the climatologies. In the case of DS the range (maximum transport minus minimum transport) is 2.8 Sv (1 Sv=1 × 106 m3 s−1); HR: 3.6 Sv, TR: 5.2 Sv, and IH: 5.9 Sv, compared to a range of 4.6 ± 0.3 Sv derived from cable data. The increased range in the IH case compared to HR is in general agreement with the finding of Böning et al. using the Kiel version of the model that forms the WOCE Community Modelling Effort. However, whereas Böning et al. claim that winds north of 35°N have little influence on the seasonal response of their model at the Florida Straits, it is found that winds north of 35°N play an important role in the model presented here. The reason for the behavior of the community model is not clear but may be associated with advection by the western boundary current, an effect not present in the linear model discussed here. In the case of the present model, the importance of forcing by the meridional component of the wind is shown, although forcing through the zonal component also plays some role in explaining the differences between the cases run under the different climatologies. The importance in the model of forcing associated with the meridional component of the wind along the continental slope region to the north of the straits is emphasized.

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