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John T. Fasullo
and
Peter R. Gent

Abstract

An accurate diagnosis of ocean heat content (OHC) is essential for interpreting climate variability and change, as evidenced for example by the broad range of hypotheses that exists for explaining the recent hiatus in global mean surface warming. Potential insights are explored here by examining relationships between OHC and sea surface height (SSH) in observations and two recently available large ensembles of climate model simulations from the mid-twentieth century to 2100. It is found that in decadal-length observations and a model control simulation with constant forcing, strong ties between OHC and SSH exist, with little temporal or spatial complexity. Agreement is particularly strong on monthly to interannual time scales. In contrast, in forced transient warming simulations, important dependencies in the relationship exist as a function of region and time scale. Near Antarctica, low-frequency SSH variability is driven mainly by changes in the circumpolar current associated with intensified surface winds, leading to correlations between OHC and SSH that are weak and sometimes negative. In subtropical regions, and near other coastal boundaries, negative correlations are also evident on long time scales and are associated with the accumulated effects of changes in the water cycle and ocean dynamics that underlie complexity in the OHC relationship to SSH. Low-frequency variability in observations is found to exhibit similar negative correlations. Combined with altimeter data, these results provide evidence that SSH increases in the Indian and western Pacific Oceans during the hiatus are suggestive of substantial OHC increases. Methods for developing the applicability of altimetry as a constraint on OHC more generally are also discussed.

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Peter R. Gent
and
Gokhan Danabasoglu

Abstract

Results from two perturbation experiments using the Community Climate System Model version 4 where the Southern Hemisphere zonal wind stress is increased are described. It is shown that the ocean response is in accord with experiments using much-higher-resolution ocean models that do not use an eddy parameterization. The key to obtaining an appropriate response in the coarse-resolution climate model is to specify a variable coefficient in the Gent and McWilliams eddy parameterization, rather than a constant value. This result contrasts with several recent papers that have suggested that coarse-resolution climate models cannot obtain an appropriate response.

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Byron A. Boville
and
Peter R. Gent

Abstract

The NCAR Climate System Model, version one, is described. The spinup procedure prior to a fully coupled integration is discussed. The fully coupled model has been run for 300 yr with no surface flux corrections in momentum, heat, or freshwater. There is virtually no trend in the surface temperatures over the 300 yr, although there are significant trends in other model fields, especially in the deep ocean. The reasons for the successful integration with no surface temperature trend are discussed.

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Peter R. Gent
and
Gokhan Danabasoglu

Abstract

Ocean heat uptake and the thermohaline circulation are analyzed in present-day control, 1% increasing CO2, and doubled CO2 runs of the Community Climate System Model, version 2 (CCSM2). It is concluded that the observed 40-yr trend in the global heat content to 300 m, found by Levitus et al., is somewhat larger than the natural variability in the CCSM2 control run. The observed 40-yr trend in the global heat content down to a depth of 3 km is much closer to trends found in the control run and is not so clearly separated from the natural model variability. It is estimated that, in a 0.7% increasing CO2 scenario that approximates the effect of increasing greenhouse gases between 1958 and 1998, the CCSM2 40-yr trend in the global heat content to 300 m is about the same as the observed value. This gives support for the CCSM2 climate sensitivity, which is 2.2°C.

Both the maximum of the meridional overturning streamfunction and the vertical flow across 1-km depth between 60° and 65°N decrease monotonically during the 1% CO2 run. However, the reductions are quite modest, being 3 and 2 Sv, respectively, when CO2 has quadrupled. The reason for this is that the surface potential density in the northern North Atlantic decreases steadily throughout the 1% CO2 run. In the latter part of the doubled CO2 run, the meridional overturning streamfunction recovers in strength back toward its value in the control run, but the deep-water formation rate across 1-km depth between 60° and 65°N remains at 85% of the control run value. The maximum northward heat transport at 22°N is governed by the maximum of the overturning, but the transport poleward of 62°N appears to be independent of the deep-water formation rate.

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Jeffrey T. Kiehl
and
Peter R. Gent

Abstract

The Community Climate System Model, version 2 (CCSM2) is briefly described. A 1000-yr control simulation of the present day climate has been completed without flux adjustments. Minor modifications were made at year 350, which included all five components using the same physical constants. There are very small trends in the upper-ocean, sea ice, atmosphere, and land fields after year 150 of the control simulation. The deep ocean has small but significant trends; however, these are not large enough that the control simulation could not be continued much further. The equilibrium climate sensitivity of CCSM2 is 2.2 K, which is slightly larger than the Climate System Model, version 1 (CSM1) value of 2.0 K.

Several aspects of the control simulation's mean climate and interannual variability are described, and good and bad properties of the control simulation are documented. In particular, several aspects of the simulation, especially in the Arctic region, are much improved over those obtained in CSM1. Other aspects, such as the tropical Pacific region simulation, have not been improved much compared to those in CSM1. Priorities for further model development are discussed in the conclusions section.

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Peter R. Gent
and
James C. McWilliams

Abstract

An analysis is made of the linear waves of the Balance Equations and the global Balance Equations on an equatorial β-plane. We consider both finite and infinite meridional domains and show the effect of different choices of boundary conditions in a finite domain. The infinite domain is similar to a complete spherical domain, a problem studied by Moura. We find analogies to several of his results: for example, the Balance Equations have no eastward traveling waves, whereas the global Balance Equations do. We also make an extensive study of the long-wave limit, which is relevant for ocean domains whose width greatly exceeds the Rossby radius of deformation. This limit is singular for many of the wave solutions. In general, however, the balanced models provide reasonably good approximations to the low-frequency waves of the primitive equations. The global Balance Equations do have high-frequency waves, but they are very different from those of the primitive equations.

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Esther C. Brady
and
Peter R. Gent

Abstract

The seasonal heat transport mechanisms important in the Pacific equatorial upwelling zone are investigated using the primitive equation, reduced gravity model developed by Gent and Cane. Mechanisms of meridional heat transport are shown and discussed with respect to the heat budget of a box about the equator containing the upwelling. There is a horizontal cell in which warm water enters the upwelling box in the west in strong equatorward currents located near the, western boundary, which feed the eastward flowing undercurrent. To compensate, water leaves the section as a colder and weaker poleward thermocline flow in the eastern basin. The meridional-vertical cell comprises additional equatorward geostrophically balanced inflow in the upper thermocline, which is compensated by the warmer poleward outflow by Ekman divergence in the surface layer.

In the annual mean, the magnitude of the net heat exported by the meridional-vertical cell exceeds the net heat import due to the gyre exchange so that the net heat transport is poleward. This annual mean net heat export is compensated by the surface heat flux. The transient eddy heat transport is equatorward and much smaller. It is noted that in the winter seasons, boreal December–February and austral June–August, a large amount of heat is lost by a net excess of heat transport by meridional overturning. In the transition seasons, March–May and September–November, there is an equatorward heat transport anomaly in the upwelling box, either related to an excess of heat equatorward transport by gyre exchange in March–May or a reduction in the poleward heat transport by meridional overturning in September–November. March-May is the season during which the undercurrent has its maximum transport, and the strength of the gyre exchange is largest. During September–November, the season of strongest zonal wind when maximum overturning transport is expected, the poleward heat transport by meridional overturning is a minimum. This is partly because the temperature difference between the divergent surface water and convergent subsurface water is smallest in this season, which is the season of lowest SST in the cold tongue and the shallowest and warmest subsurface flow. Seasonally, the variations in the surface heat flux are much smaller than the variations in the heat transport. Thus, the seasonal heat content changes are compensated by the heat transport anomalies.

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Eric P. Chassignet
and
Peter R. Gent

Abstract

The separation Point of a midlatitude jet from the western boundary in ocean numerical models depends upon both the governing equations and the vertical coordinate used. Systematic differences in the point of separation between level and layer models are shown. In level models, the separation usually occurs poleward of the zero wind-stress curl line, whereas, in layer models, it usually occurs equatorward. These differences are caused by two aspects of the numerical implementation. First, the wind forcing is usually assumed to act as a body force over the upper layer or level in the models, and this corresponds to a different physical assumption. Second, the free-slip boundary condition is imposed as zero vorticity in both models. This is an inconsistency because vorticity is not the same quantity when the governing equations are formulated in physical (level model) and isopycnal (layer model) coordinates. The effects on separation of these numerical implementation differences are illustrated using analytical solutions of linear models and numerical solutions of several nonlinear models.

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James C. McWilliams
and
Peter R. Gent

Abstract

Numerical solutions are examined for the nearly adiabatic, large-scale ocean circulation in a midlatitude, rectangular domain with steady wind driving. The model used is the balance equations and its various subsets; hence, dynamical effects at finite Rossby number are included. The solutions are steady ones, and the necessary transports by the missing mesoscale eddies are parameterized, in part, using the authors’ previous proposal for isopycnally oriented mixing of tracers and isopycnal thickness or static stability. This yields qualitatively credible, quasi-adiabatic solutions for realistic magnitudes for the subgrid-scale transport coefficients. Among these solutions are ones with nearly homogeneous fields of potential vorticity on upper-thermocline isopycnal surfaces, even though the parameterized eddy mixing does not act directly to this end.

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Peter R. Gent
and
James C. Mcwilliams

Abstract

A subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models. The mixing is applied in isopycnal coordinates to isopycnal layer thickness, or inverse density gradient, as well as to passive scalars, temperature and salinity. The transformation of these mixing forms to physical coordinates is also presented.

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