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## Abstract

The response of a rectangular, flat-bottom, eddy-resolving, quasigeostrophic ocean to a steady, double-gyre wind stress is studied to assess the sensitivity of the solutions to a partial-slip lateral boundary condition in which tangential stress is proportional to tangential velocity. The constant of proportionality (Î±) has limiting values of zero and infinity, corresponding to free-slip (no-stress) and no-slip conditions, respectively. Seven numerical solutionsâ€”corresponding to the Î± values 0.0, 2.0, 3.5, 5.0, 6.5, 8.0, and 100.0â€”are obtained, which span the free-slip and no-slip limits.

Significant qualitative changes in the time-mean behavior of the solutions are observed to occur with increasing Î±. These changes include a gradual retreat of the separation points of the western boundary currents in the subtropical and subpolar gyres, a dramatic reduction in the basin-integrated reservoirs of mean and eddy kinetic energy, a weakening of bottom dissipation and its replacement by lateral dissipation as the dominant sink of kinetic energy, and the emergence of secondary pools of homogenized potential vorticity within the interiors of the time-mean gyres. Similar dependencies on Î± are found to apply across a broad dynamical regime encompassing alternate type and strengths of lateral friction, asymmetric wind forcing, and dynamically more complete governing equations.

Despite the considerable complexity of the solutions as a function of Î±, a single dynamical interpretation of the process of boundary current separation is found to apply equally well in the no-stress and no-slip limits. In particular, irrespective of the value of Î±, we find separation to be associated with the occurrence of an adverse value of the higher-order pressure gradient term in the time-mean momentum budget just upstream of the point of separation. The results, therefore, strongly indicate that separation in this model is most easily understood diagnostically as the consequence of boundary current deceleration due to an adverse, along-boundary pressure gradient.

## Abstract

The response of a rectangular, flat-bottom, eddy-resolving, quasigeostrophic ocean to a steady, double-gyre wind stress is studied to assess the sensitivity of the solutions to a partial-slip lateral boundary condition in which tangential stress is proportional to tangential velocity. The constant of proportionality (Î±) has limiting values of zero and infinity, corresponding to free-slip (no-stress) and no-slip conditions, respectively. Seven numerical solutionsâ€”corresponding to the Î± values 0.0, 2.0, 3.5, 5.0, 6.5, 8.0, and 100.0â€”are obtained, which span the free-slip and no-slip limits.

Significant qualitative changes in the time-mean behavior of the solutions are observed to occur with increasing Î±. These changes include a gradual retreat of the separation points of the western boundary currents in the subtropical and subpolar gyres, a dramatic reduction in the basin-integrated reservoirs of mean and eddy kinetic energy, a weakening of bottom dissipation and its replacement by lateral dissipation as the dominant sink of kinetic energy, and the emergence of secondary pools of homogenized potential vorticity within the interiors of the time-mean gyres. Similar dependencies on Î± are found to apply across a broad dynamical regime encompassing alternate type and strengths of lateral friction, asymmetric wind forcing, and dynamically more complete governing equations.

Despite the considerable complexity of the solutions as a function of Î±, a single dynamical interpretation of the process of boundary current separation is found to apply equally well in the no-stress and no-slip limits. In particular, irrespective of the value of Î±, we find separation to be associated with the occurrence of an adverse value of the higher-order pressure gradient term in the time-mean momentum budget just upstream of the point of separation. The results, therefore, strongly indicate that separation in this model is most easily understood diagnostically as the consequence of boundary current deceleration due to an adverse, along-boundary pressure gradient.

## Abstract

The *f*-plane linear shallow-water equations support coastal Kelvin waves. These waves propagate along the coast and have zero velocity normal to the coast. It is shown that the balance equations also support coastal Kelvin waves, but these waves differ depending upon the boundary conditions imposed. Three different boundary conditions and resulting Kelvin wave approximations are examined. It is shown that one set of boundary conditions gives balance-model Kelvin waves that are closer to those of the shallow-water equations than the other two boundary conditions.

## Abstract

The *f*-plane linear shallow-water equations support coastal Kelvin waves. These waves propagate along the coast and have zero velocity normal to the coast. It is shown that the balance equations also support coastal Kelvin waves, but these waves differ depending upon the boundary conditions imposed. Three different boundary conditions and resulting Kelvin wave approximations are examined. It is shown that one set of boundary conditions gives balance-model Kelvin waves that are closer to those of the shallow-water equations than the other two boundary conditions.

## Abstract

The dynamics of the lower cell of the meridional overturning circulation (MOC) in the Southern Ocean are compared in two versions of a global climate model: one with high-resolution (0.1Â°) ocean and sea ice and the other a lower-resolution (1.0Â°) counterpart. In the high-resolution version, the lower cell circulation is stronger and extends farther northward into the abyssal ocean. Using the water-mass-transformation framework, it is shown that the differences in the lower cell circulation between resolutions are explained by greater rates of surface water-mass transformation within the higher-resolution Antarctic sea ice pack and by differences in diapycnal-mixing-induced transformation in the abyssal ocean.

While both surface and interior transformation processes work in tandem to sustain the lower cell in the control climate, the circulation is far more sensitive to changes in surface transformation in response to atmospheric warming from raising carbon dioxide levels. The substantial reduction in overturning is primarily attributed to reduced surface heat loss. At high resolution, the circulation slows more dramatically, with an anomaly that reaches deeper into the abyssal ocean and alters the distribution of Southern Ocean warming. The resolution dependence of associated heat uptake is particularly pronounced in the abyssal ocean (below 4000 m), where the higher-resolution version of the model warms 4.5 times more than its lower-resolution counterpart.

## Abstract

The dynamics of the lower cell of the meridional overturning circulation (MOC) in the Southern Ocean are compared in two versions of a global climate model: one with high-resolution (0.1Â°) ocean and sea ice and the other a lower-resolution (1.0Â°) counterpart. In the high-resolution version, the lower cell circulation is stronger and extends farther northward into the abyssal ocean. Using the water-mass-transformation framework, it is shown that the differences in the lower cell circulation between resolutions are explained by greater rates of surface water-mass transformation within the higher-resolution Antarctic sea ice pack and by differences in diapycnal-mixing-induced transformation in the abyssal ocean.

While both surface and interior transformation processes work in tandem to sustain the lower cell in the control climate, the circulation is far more sensitive to changes in surface transformation in response to atmospheric warming from raising carbon dioxide levels. The substantial reduction in overturning is primarily attributed to reduced surface heat loss. At high resolution, the circulation slows more dramatically, with an anomaly that reaches deeper into the abyssal ocean and alters the distribution of Southern Ocean warming. The resolution dependence of associated heat uptake is particularly pronounced in the abyssal ocean (below 4000 m), where the higher-resolution version of the model warms 4.5 times more than its lower-resolution counterpart.

## Abstract

Different parameterizations for vertical mixing and the effects of ocean mesoscale eddies are tested in an eddy-permitting ocean model. It has a horizontal resolution averaging about 0.7Â° and was used as the ocean component of the parallel climate model. The old ocean parameterizations used in that coupled model were replaced by the newer parameterizations used in the climate system model. Both ocean-alone and fully coupled integrations were run for at least 100 years. The results clearly show that the drifts in the upper-ocean temperature profile using the old parameterizations are substantially reduced in both sets of integrations using the newer parameterizations. The sea-ice distribution in the fully coupled integration using the newer ocean parameterizations is also improved. However, the sea-ice distribution is sensitive to both sea-ice parameterizations and the atmospheric forcing, in addition to being dependent on the ocean simulation. The newer ocean parameterizations have been shown to improve considerably the solutions in non-eddy-resolving configurations, such as in the climate system model, where the horizontal resolution of the ocean component is about 2Â°. The work presented here is a clear demonstration that the improvements continue into the eddy-permitting regime, where the ocean component has an average horizontal resolution of less than 1Â°.

## Abstract

Different parameterizations for vertical mixing and the effects of ocean mesoscale eddies are tested in an eddy-permitting ocean model. It has a horizontal resolution averaging about 0.7Â° and was used as the ocean component of the parallel climate model. The old ocean parameterizations used in that coupled model were replaced by the newer parameterizations used in the climate system model. Both ocean-alone and fully coupled integrations were run for at least 100 years. The results clearly show that the drifts in the upper-ocean temperature profile using the old parameterizations are substantially reduced in both sets of integrations using the newer parameterizations. The sea-ice distribution in the fully coupled integration using the newer ocean parameterizations is also improved. However, the sea-ice distribution is sensitive to both sea-ice parameterizations and the atmospheric forcing, in addition to being dependent on the ocean simulation. The newer ocean parameterizations have been shown to improve considerably the solutions in non-eddy-resolving configurations, such as in the climate system model, where the horizontal resolution of the ocean component is about 2Â°. The work presented here is a clear demonstration that the improvements continue into the eddy-permitting regime, where the ocean component has an average horizontal resolution of less than 1Â°.

## Abstract

The Linear Balance Equations (LBE) are intermediate between the more familiar Quasi-geostrophic (QG) and Primitive Equations (PE) in both physical completeness and computational efficiency. We first present a consistent boundary-value problem for the LBE and its numerical implementation. Then we examine its solutions for equilibrium, adiabatic, wind-driven, midlatitude circulation in a rectangular ocean basin. They differ from analogous QG solutions in many respects, even for the moderately small Rossby number appropriate to this problem. The LBE solutions have 1) less total transport; 2) a broader, weaker, more surface-intensified mean Gulf Stream with sizable standing menders and a shorter penetration length into the basin interior; 3) an enhancement of the mean thermocline circulation and its associated mesoscale eddies in the subtropical gyre relative to the subpolar gyre; 4) an enhanced eddy generation rate by Reynolds stress work and a diminished generation rate by potential energy conversion; and 5) greater eddy energy in the Gulf Stream and neighboring recirculation zone but diminished eddy energy in the far field. These solution differences are due to the combined dynamical influences of both a more general form of the Coriolis force and the ageostrophic buoyancy advection in the LBE, with the former effect the more sizable in this problem. One can partially summarize the relations between LBE and QG solutions by stating that the dynamics of the former have an intrinsic meridional asymmetry, absent in the latter, which is the general source of many of the more specific differences.

## Abstract

The Linear Balance Equations (LBE) are intermediate between the more familiar Quasi-geostrophic (QG) and Primitive Equations (PE) in both physical completeness and computational efficiency. We first present a consistent boundary-value problem for the LBE and its numerical implementation. Then we examine its solutions for equilibrium, adiabatic, wind-driven, midlatitude circulation in a rectangular ocean basin. They differ from analogous QG solutions in many respects, even for the moderately small Rossby number appropriate to this problem. The LBE solutions have 1) less total transport; 2) a broader, weaker, more surface-intensified mean Gulf Stream with sizable standing menders and a shorter penetration length into the basin interior; 3) an enhancement of the mean thermocline circulation and its associated mesoscale eddies in the subtropical gyre relative to the subpolar gyre; 4) an enhanced eddy generation rate by Reynolds stress work and a diminished generation rate by potential energy conversion; and 5) greater eddy energy in the Gulf Stream and neighboring recirculation zone but diminished eddy energy in the far field. These solution differences are due to the combined dynamical influences of both a more general form of the Coriolis force and the ageostrophic buoyancy advection in the LBE, with the former effect the more sizable in this problem. One can partially summarize the relations between LBE and QG solutions by stating that the dynamics of the former have an intrinsic meridional asymmetry, absent in the latter, which is the general source of many of the more specific differences.

## Abstract

Horizontal momentum flux in a global ocean climate model is formulated as an anisotropic viscosity with two spatially varying coefficients. This friction can be made purely dissipative, does not produce unphysical torques, and satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriate for the parameterization of missing mesoscale eddies wherever possible and to use other values only where required by the numerics. These other viscosities control numerical noise from advection and generate western boundary currents that are wide enough to be resolved by the coarse grid of the model. Noise on the model gridscale is tolerated provided its amplitude is less than about 0.05 cm s^{âˆ’1}. Parameter tuning is minimized by applying physical and numerical principles. The potential value of this line of model development is demonstrated by comparison with equatorial ocean observations.

In particular, the goal of producing model equatorial ocean currents comparable to observations was achieved in the Pacific Ocean. The Equatorial Undercurrent reaches a maximum magnitude of nearly 100 cm s^{âˆ’1} in the annual mean. Also, the spatial distribution of near-surface currents compares favorably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improved, but still too weak, and the northward flow along the coast of South America may be too shallow. Equatorial Pacific upwelling has a realistic pattern and its magnitude is of the same order as diagnostic model estimates. The necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than about 1 cm^{2} s^{âˆ’1}, and a mesoscale eddy viscosity of order 1000 m^{2} s^{âˆ’1} acting on meridional shear of zonal momentum. Model resolution is not critical, provided these three elements remain unaltered. Thus, if the scatterometer winds are accurate, the model results are consistent with observational estimates of these two coefficients. These winds have larger westward stress than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow.

In the Indian Ocean the seasonal cycle of equatorial currents does not appear to be overly attenuated by the horizontal viscosity, with differences from observations attributable to interannual variability. However, in the Atlantic, the numerics still require too large a meridional viscosity over too much of the basin, and a zonal resolution approaching 1Â° may be necessary to match observations. Because of this viscosity, increasing the background vertical viscosity slowed the westward surface current; opposite to the response in the Pacific.

## Abstract

Horizontal momentum flux in a global ocean climate model is formulated as an anisotropic viscosity with two spatially varying coefficients. This friction can be made purely dissipative, does not produce unphysical torques, and satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriate for the parameterization of missing mesoscale eddies wherever possible and to use other values only where required by the numerics. These other viscosities control numerical noise from advection and generate western boundary currents that are wide enough to be resolved by the coarse grid of the model. Noise on the model gridscale is tolerated provided its amplitude is less than about 0.05 cm s^{âˆ’1}. Parameter tuning is minimized by applying physical and numerical principles. The potential value of this line of model development is demonstrated by comparison with equatorial ocean observations.

In particular, the goal of producing model equatorial ocean currents comparable to observations was achieved in the Pacific Ocean. The Equatorial Undercurrent reaches a maximum magnitude of nearly 100 cm s^{âˆ’1} in the annual mean. Also, the spatial distribution of near-surface currents compares favorably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improved, but still too weak, and the northward flow along the coast of South America may be too shallow. Equatorial Pacific upwelling has a realistic pattern and its magnitude is of the same order as diagnostic model estimates. The necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than about 1 cm^{2} s^{âˆ’1}, and a mesoscale eddy viscosity of order 1000 m^{2} s^{âˆ’1} acting on meridional shear of zonal momentum. Model resolution is not critical, provided these three elements remain unaltered. Thus, if the scatterometer winds are accurate, the model results are consistent with observational estimates of these two coefficients. These winds have larger westward stress than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow.

In the Indian Ocean the seasonal cycle of equatorial currents does not appear to be overly attenuated by the horizontal viscosity, with differences from observations attributable to interannual variability. However, in the Atlantic, the numerics still require too large a meridional viscosity over too much of the basin, and a zonal resolution approaching 1Â° may be necessary to match observations. Because of this viscosity, increasing the background vertical viscosity slowed the westward surface current; opposite to the response in the Pacific.

## Abstract

It is shown that the effects of mesoscale eddies on tracer transports can be parameterized in a large-scale model by additional advection and diffusion of tracers. Thus, tracers are advected by the effective transport velocity, which is the sum of the large-scale velocity and the eddy-induced transport velocity. The density and continuity equations are the familiar equations for adiabatic, Boussinesq, and incompressible flow with the effective transport velocity replacing the large-scale velocity. One of the main points of this paper is to show how simple the parameterization of Gent and McWilliams appears when interpreted in terms of the effective transport velocity. This was not done in their original 1990 paper. It is also shown that, with the Gent and McWilliams parameterization, potential vorticity in the planetary geostrophic model satisfies an equation close to that for tracers. The analogy of this parameterization with vertical mixing of momentum is then described.

The effect of the Gent and McWilliams parameterization is illustrated by applying it to a strong, sloping two-dimensional front. The final state is that the front is flat, corresponding to a state of minimum potential energy. However, the amount of water of a given density has not been changed and there has been no flow across isopycnals. These properties are not preserved with horizontal diffusion of tracer. Finally, the Levitus dataset is used to estimate the effects of the Gent and McWilliams parameterization. The zonal mean meridional overturning streamfunction for the eddy-induced transport velocity has a maximum of 18 Sverdrups near the Antarctic Circumpolar Current. The associated poleward heat transport is 0.4 petawatts. The maximum poleward heat transport in the Northern Hemisphere is 0.15 petawatts at 40Â°N. These values are the same order of magnitude as estimates from observations and regional eddy-resolving ocean models.

## Abstract

It is shown that the effects of mesoscale eddies on tracer transports can be parameterized in a large-scale model by additional advection and diffusion of tracers. Thus, tracers are advected by the effective transport velocity, which is the sum of the large-scale velocity and the eddy-induced transport velocity. The density and continuity equations are the familiar equations for adiabatic, Boussinesq, and incompressible flow with the effective transport velocity replacing the large-scale velocity. One of the main points of this paper is to show how simple the parameterization of Gent and McWilliams appears when interpreted in terms of the effective transport velocity. This was not done in their original 1990 paper. It is also shown that, with the Gent and McWilliams parameterization, potential vorticity in the planetary geostrophic model satisfies an equation close to that for tracers. The analogy of this parameterization with vertical mixing of momentum is then described.

The effect of the Gent and McWilliams parameterization is illustrated by applying it to a strong, sloping two-dimensional front. The final state is that the front is flat, corresponding to a state of minimum potential energy. However, the amount of water of a given density has not been changed and there has been no flow across isopycnals. These properties are not preserved with horizontal diffusion of tracer. Finally, the Levitus dataset is used to estimate the effects of the Gent and McWilliams parameterization. The zonal mean meridional overturning streamfunction for the eddy-induced transport velocity has a maximum of 18 Sverdrups near the Antarctic Circumpolar Current. The associated poleward heat transport is 0.4 petawatts. The maximum poleward heat transport in the Northern Hemisphere is 0.15 petawatts at 40Â°N. These values are the same order of magnitude as estimates from observations and regional eddy-resolving ocean models.

## Abstract

This paper describes the global ocean component of the NCAR Climate System Model. New parameterizations of the effects of mesoscale eddies and of the upper-ocean boundary layer are included. Numerical improvements include a third-order upwind advection scheme and elimination of the artificial North Pole island in the original MOM 1.1 code. Updated forcing fields are used to drive the ocean-alone solution, which is integrated long enough so that it is in equilibrium. The ocean transports and potential temperature and salinity distributions are compared with observations. The solution sensitivity to the freshwater forcing distribution is highlighted, and the sensitivity to resolution is also briefly discussed.

## Abstract

This paper describes the global ocean component of the NCAR Climate System Model. New parameterizations of the effects of mesoscale eddies and of the upper-ocean boundary layer are included. Numerical improvements include a third-order upwind advection scheme and elimination of the artificial North Pole island in the original MOM 1.1 code. Updated forcing fields are used to drive the ocean-alone solution, which is integrated long enough so that it is in equilibrium. The ocean transports and potential temperature and salinity distributions are compared with observations. The solution sensitivity to the freshwater forcing distribution is highlighted, and the sensitivity to resolution is also briefly discussed.

## Abstract

New features that may affect the behavior of the upper ocean in the Community Climate System Model version 3 (CCSM3) are described. In particular, the addition of an idealized diurnal cycle of solar forcing where the daily mean solar radiation received in each daily coupling interval is distributed over 12 daylight hours is evaluated. The motivation for this simple diurnal cycle is to improve the behavior of the upper ocean, relative to the constant forcing over each day of previous CCSM versions. Both 1- and 3-h coupling intervals are also considered as possible alternatives that explicitly resolve the diurnal cycle of solar forcing. The most prominent and robust effects of all these diurnal cycles are found in the tropical oceans, especially in the Pacific. Here, the mean equatorial sea surface temperature (SST) is warmed by as much as 1Â°C, in better agreement with observations, and the mean boundary layer depth is reduced. Simple rectification of the diurnal cycle explains about half of the shallowing, but less than 0.1Â°C of the warming. The atmospheric response to prescribed warm SST anomalies of about 1Â°C displays a very different heat flux signature. The implication, yet to be verified, is that large-scale airâ€“sea coupling is a prime mechanism for amplifying the rectified, daily averaged SST signals seen by the atmosphere. Although the use of upper-layer temperature for SST in CCSM3 underestimates the diurnal cycle of SST, many of the essential characteristics of diurnal cycling within the equatorial ocean are reproduced, including boundary layer depth, currents, and the parameterized vertical heat and momentum fluxes associated with deep-cycle turbulence. The conclusion is that the implementation of an idealized diurnal cycle of solar forcing may make more frequent ocean coupling and its computational complications unnecessary as improvements to the airâ€“sea coupling in CCSM3 continue. A caveat here is that more frequent ocean coupling tends to reduce the long-term cooling trends typical of CCSM3 by heating already too warm ocean depths, but longer integrations are needed to determine robust features. A clear result is that the absence of diurnal solar forcing of the ocean has several undesirable consequences in CCSM3, including too large ENSO variability, much too cold Pacific equatorial SST, and no deep-cycle turbulence.

## Abstract

New features that may affect the behavior of the upper ocean in the Community Climate System Model version 3 (CCSM3) are described. In particular, the addition of an idealized diurnal cycle of solar forcing where the daily mean solar radiation received in each daily coupling interval is distributed over 12 daylight hours is evaluated. The motivation for this simple diurnal cycle is to improve the behavior of the upper ocean, relative to the constant forcing over each day of previous CCSM versions. Both 1- and 3-h coupling intervals are also considered as possible alternatives that explicitly resolve the diurnal cycle of solar forcing. The most prominent and robust effects of all these diurnal cycles are found in the tropical oceans, especially in the Pacific. Here, the mean equatorial sea surface temperature (SST) is warmed by as much as 1Â°C, in better agreement with observations, and the mean boundary layer depth is reduced. Simple rectification of the diurnal cycle explains about half of the shallowing, but less than 0.1Â°C of the warming. The atmospheric response to prescribed warm SST anomalies of about 1Â°C displays a very different heat flux signature. The implication, yet to be verified, is that large-scale airâ€“sea coupling is a prime mechanism for amplifying the rectified, daily averaged SST signals seen by the atmosphere. Although the use of upper-layer temperature for SST in CCSM3 underestimates the diurnal cycle of SST, many of the essential characteristics of diurnal cycling within the equatorial ocean are reproduced, including boundary layer depth, currents, and the parameterized vertical heat and momentum fluxes associated with deep-cycle turbulence. The conclusion is that the implementation of an idealized diurnal cycle of solar forcing may make more frequent ocean coupling and its computational complications unnecessary as improvements to the airâ€“sea coupling in CCSM3 continue. A caveat here is that more frequent ocean coupling tends to reduce the long-term cooling trends typical of CCSM3 by heating already too warm ocean depths, but longer integrations are needed to determine robust features. A clear result is that the absence of diurnal solar forcing of the ocean has several undesirable consequences in CCSM3, including too large ENSO variability, much too cold Pacific equatorial SST, and no deep-cycle turbulence.

## Abstract

An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented.

The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural variability in the 1870 control runs. The heat uptake down to 300 m between 1957 and 1996 varies by a factor of 2 across the ensemble. Some possible reasons for this large spread are discussed. There is much less spread in the heat uptake down to 3 km. On average, the CCSM3 twentieth-century ensemble runs take up 25% more heat than the recent estimate from ocean observations. Possible explanations for this are that the model heat uptake is calculated over the whole ocean, and not just in the regions where there are many observations and that there is no parameterization of the indirect effects of aerosols in CCSM3.

## Abstract

An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented.

The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural variability in the 1870 control runs. The heat uptake down to 300 m between 1957 and 1996 varies by a factor of 2 across the ensemble. Some possible reasons for this large spread are discussed. There is much less spread in the heat uptake down to 3 km. On average, the CCSM3 twentieth-century ensemble runs take up 25% more heat than the recent estimate from ocean observations. Possible explanations for this are that the model heat uptake is calculated over the whole ocean, and not just in the regions where there are many observations and that there is no parameterization of the indirect effects of aerosols in CCSM3.