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Andrew J. Weaver and Sophie Valcke

Abstract

The ocean component of the Geophysical Fluid Dynamics Laboratory coupled model is used to investigate whether or not the interdecadal variability found in Delworth et al. is an ocean-only mode or a mode of the full coupled system. In particular, it has been previously suggested that the variability in the full coupled model is either 1) an internal ocean mode driven by fixed fluxes (atmospheric annual mean plus flux adjustment), which is made less regular through forcing from atmospheric noise; 2) a consequence of the use of flux adjustments; or 3) an ocean-only mode that is excited by atmospheric noise. Through a series of experiments conducted under fixed-flux boundary conditions it is shown that none of these three hypotheses holds and therefore it is concluded that the interdecadal variability found in Delworth et al. is a mode of the full coupled system.

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Andrew J. Weaver and Michael Eby

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The results from ocean model experiments conducted with isopycnal and isopycnal thickness diffusion parameterizations for subgrid-scale mixing associated with mesoscale eddies are examined from a numerical standpoint. It is shown that when the mixing tensor is rotated, so that mixing is primarily along isopycnals, numerical problems may occur and non-monotonic solutions, which violate the second law of thermodynamics, may arise when standard centered difference advection algorithms are used. These numerical problems can be reduced or eliminated if sufficient explicit (unphysical) background horizontal diffusion is added to the mixing scheme. A more appropriate solution is the use of more sophisticated numerical advection algorithms, such as the flux- corrected transport algorithm. This choice of advection scheme adds additional mixing only where it is needed to preserve monotonicty and so retains the physically desirable aspects of the isopycnal and isopycnal thickness diffusion parameterizations, while removing the undesirable numerical noise. The price for this improvement is a computational increase.

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Hannah Hickey and Andrew J. Weaver

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A coupled model of intermediate complexity is used to examine the effect of the Southern Ocean on tropical Atlantic variability. Model experiments with positive surface temperature, freshwater, and wind forcings in the Southern Ocean show an increased advection of polar water into the Atlantic basin, resulting in anomalously cold freshwater in the eastern Atlantic Ocean at 25°S. With time, these anomalies spread northward and away from the coast, into the tropical thermocline. The magnitude of the response increases linearly with the strength of the forcing. The results of these experiments show that southeastern Atlantic Ocean temperature and salinity are particularly sensitive to changes in the Southern Ocean. These findings suggest that this link could be the oceanic branch of a mode of variability linking the Southern and tropical Atlantic Oceans—a possible mechanism for the as-yet-unexplained decadal mode of tropical Atlantic variability.

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J. Paul Spence and Andrew J. Weaver

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The influence of ENSO-related changes in the Atlantic-to-Pacific freshwater budget on the North Atlantic meridional overturning is examined using the University of Victoria (UVic) Earth System Climate Model. The initial analysis of freshwater fluxes in the 50-yr NCEP–NCAR (NCEP50) reanalysis product and Global Precipitation Climatology Project (GPCP) dataset reveals that the transport of water vapor out of the tropical Atlantic drainage basin is enhanced during El Niño phases and reduced during La Niña phases; a one standard deviation in the Southern Oscillation index alters the tropical Atlantic freshwater balance by about 0.09 Sv (Sv ≡ 106 m3 s−1). A weaker link with ENSO is found in the 40-yr ECMWF Re-Analysis (ERA-40), although its usefulness is severely limited by a strong and spurious trend in tropical precipitation. Model results suggest that tropical Atlantic salinity anomalies generated with the frequency and amplitude of ENSO tend not to impact deep-water formation as they are diluted en route to the North Atlantic. Lower frequency, decadal time-scale anomalies, however, do have an impact, albeit weak, on the rate of North Atlantic Deep Water formation. In addition, and contrary to earlier results, it is found that even a shift of the tropical Atlantic freshwater balance toward permanent El Niño conditions only slightly mitigates the transient reduction of North Atlantic Deep Water formation associated with the increase of anthropogenic greenhouse gases. Taken together, the results suggest that the poleward propagation of salinity anomalies from the tropical Atlantic, associated with changes in ENSO, should not be considered a significant mechanism for the variability of the North Atlantic meridional overturning in the present and foreseeable future climate.

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Andrew J. Weaver and E. S. Sarachik

Abstract

In centered difference models of ocean circulation, two grid-point computational modes can be excited if grid Reynolds and Peclet numbers are greater than two. The Bryan-Cox General Circulation Model (GCM) is used to show the dramatic effect that this instability has on the equatorial thermohaline circulation. In many recent numerical calculations researchers have used 12 vertical levels. It is shown that this resolution produces an artificial cell at the equator when typical values of the vertical diffusivity and viscosity parameters are used. This artifical cell rotates counter to the primary cell driven by deep water formation at high latitudes, is driven by downwelling at the eastern boundary near the equator and is 40% the strength of the primary cell for the parameters used in the present study. When the vertical resolution is increased the cell vanishes. It is suggested therefore that higher vertical resolution should be used in Bryan-Cox GCM deep-ocean modeling studies when current values of the vertical diffusivity and viscosity parameters are used.

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Salil Kumar Das and Andrew J. Weaver

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The semi-Lagrangian adveation scheme is applied to the meridional-plane model of the thermohaline circulation developed by Marotzke et al., whose governing equations are solved under a variety of boundary conditions. To determine the extent to which the accuracy and efficiency of the calculations depends on the numerical integration scheme, the test problem is solved independently using an explicit finite-difference (leapfrog in time, centered difference in space) method and three implicit methods: a finite difference, a finite element, and an upwind scheme. Integrations of the model to several equilibria are performed to determine the accuracy, efficiency, and stability of each integration scheme as a function of time step. For the same level of accuracy the time step used in the semi-Lagrangian scheme is found to be at least five times greater than that employed in the case of the implicit methods. The time step used in the implicit methods in turn are at least six times greater than those needed in the explicit integration of the governing equations. It is further shown that Dirichlet, Neumann, and mixed boundary conditions can be handled efficiently with the semi-Lagrangian method.

The semi-Lagrangian method is applied in the usual three-time-level and two-time-level interpolating versions as well as in a noninterpolating, three-time-level version. The two-time-level scheme further doubles the speed of the time integration step for the same level of accuracy, beyond that which is achieved using the three-time-level scheme. The noninterpolating scheme does not eliminate the damping introduced by the interpolation, as pointed out by Ritchic. Hence, the two-time-level, semi-Lagrangian advection method stands out as a viable time integration scheme for climate models that are normally run for hundreds of years and is best suited for ocean climate studies.

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Augustus F. Fanning and Andrew J. Weaver

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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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Andrew J. Weaver and E. S. Sarachik

Abstract

No abstract available.

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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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Andrew J. Weaver and E. S. Sarachik

Abstract

Several simple numerical experiments are conducted, using both single- and double-hemisphere ocean basins under symmetric steady forcing to study de ocean's thermohaline circulation. It is shown that a stable steady state obtained under a restoring surface boundary condition on salinity becomes unstable upon a switch to a flux boundary condition. The polar halocline catastrope of F. Bryan occurs. It is shown that further integration of this collapsed state ultimately yields a steady, stable one-cell circulation with the approach being essentially chaotic but with significant energy at decadal period. The two-hemisphere ocean passes through many stages in which violent overturning occurs O(80 × 101 m3 a−1). These flushes occurs in both hemispheres and are of one-cell structure. The time period between them Bushes varies from seveal hundred to about one thousand years.

A single 12-vertical-level hemispheric basin, spun up from an initial state of rest under mixed boundary conditions (restoring boundary condition on temperature and flux boundary condition on salinity), never reaches a study gate. Three characteristic stages are observed in the integration: a stage where the system oscillates with decadal time scale, a stage when the system undergoes a violent overturning flush, and a Quiescent stage in which either deep water is forming or the themohaline circulation is in a collapsed state. These three characteristic stage are also present in 33 level single- and double-hemisphere runs. The decadal time wide is associated primarily with the advection of positive salinity anomalies into the region of deep-water formation from the midocean region between the subtropical and subpolar gyres. Upon increasing the resolution to 33 levels a steady is reached. The resulting steady state is fundamentally different from the one obtained under the same resolution and restoring boundary conditions in that it is more energetic and has much warmer basin mean temperature. These differences are due to a change in the location of deep-water formation.

The dependence of the results on the type a convection scheme used, vertical resolution and time-stepping procedure (synchronous or asynchronous integration) is also studied in order to separate physical processes from those that might be numerical artifacts. Sufficient vertical resolution is shown to be important in obtaining realistic models of the thermohaline circulation. It is shown that a steady state, which is stable under asynchronous integration and mixed boundary conditions may become unstable upon a switch to synchronous integration. It is also shown that the steady state obtained under restoring boundary conditions only changes slightly upon a switch to synchronous integration. Under mixed boundary conditions the steady state is shown to be very sensitive to the choice of surface tracer time step even while integrating asynchronously. Upon a Switch in this time step a polar halocline catastrophe way be induced.

The implications of the present study for future ocean climate modles are discussed.

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