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P. R. Gent

Abstract

Eady's model of baroclinic instability is extended by allowing the basic velocity profile to vary slowly in the meridional direction. The eigenfunctions are found by simple use of the WKB technique, and the meridional scale of the growing baroclinic waves is found to be influenced by the width of the jet in the basic velocity profile and by the radius of deformation. Calculation of the eigenvalues reveals that the waves with the longest meridional wavelength are the most unstable. The same method is then applied to the two-layer model, and the same qualitative results are found as for the continuous model. These results are compared with some recent numerical work by Simmons. The present method extends the work of Stone who used a similar technique and included the β-effect in his two-layer model. He assumed that the scale of the most unstable waves was always the Rossby radius of deformation, so that the meridional wave-number was proportional to the width of the channel considered. For disturbances between the pole and equator, he found that the meridional wavenumber of the most unstable waves was usually either 3 or 4.

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P. R. Gent

Abstract

Further results are presented using Eady's model of baroclinic instability with a zonal flow varying slowly in the meridional direction. Now, however, the basic flow is more in keeping with observations and this means the WKB method used to solve the ensuing linear eigenvalue problem has to be considered in the complex plane. The overall effect of meridional shear is a slight stabilization and the meridional scale of the growing waves is governed by the scale of the jet in the basic zonal flow and by the radius of deformation. Further results are also presented for the two-layer model including the β-effect, and far from neutral stability they complement the findings of the continuous model. Close to neutral stability, however, the method can be used to explain the differences between the previous solutions of Simmons and Stone.

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J. C. McWilliams and P. R. Gent

Abstract

Several sets of model equations are presented which represent coupled processes in the tropical atmosphere and ocean. The distribution of ocean surface temperature generates large-scale convective motions in the atmosphere. These winds in turn drive ocean currents which advect ocean temperatures. Under most parametric circumstances, the model solutions have the character of moderately damped oscillations of several year period. This period is characteristic of either ocean particle advection across the zonal extent of the basin or potential energy release associated with the ocean temperature distribution. Less stable model solutions can also occur—limit cycle oscillations, alternative mean climatic balances for fixed parameters—but these are not typical of the parameters selected for application to the tropical Pacific. Simulations of possible El Niño sequences are discussed; in general the responses seem weaker than observed.

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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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C. M. Bitz, P. R. Gent, R. A. Woodgate, M. M. Holland, and R. Lindsay
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C. M. Bitz, P. R. Gent, R. A. Woodgate, M. M. Holland, and R. Lindsay

Abstract

Two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing CO2 are investigated with Community Climate System Model version 3 (CCSM3): a deep warming below ∼500 m and extending down several kilometers in the Southern Ocean and warming in a ∼200-m layer just below the surface in the Arctic Ocean. Ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone. This integration has a climate response with strong ocean heat uptake in the Southern Ocean and modest ocean heat uptake in the subsurface Arctic Ocean.

The Arctic Ocean warming results from enhanced ocean heat transport from the northern North Atlantic. At the time of CO2 doubling, about 1/3 of the heat transport anomaly results from advection of anomalously warm water and 2/3 results from strengthened inflow. At the same time the overturning circulation is strengthened in the northern North Atlantic and Arctic Oceans. Wind stress changes cannot explain the circulation changes, which instead appear related to strengthened convection along the Siberian shelves.

Deep ocean warming in the Southern Ocean is initiated by weakened convection, which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport. Below about 500 m, changes in convection reduce the vertical and meridional temperature gradients in the Southern Ocean, which significantly reduce isopycnal diffusion of heat upward around Antarctica. The geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients, making sea ice an important player in deep ocean heat uptake in the Southern Ocean.

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Peter R. Gent, Anthony P. Craig, Cecilia M. Bitz, and John W. Weatherly

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 0.7° 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 2°. 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 1°.

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Gokhan Danabasoglu, William G. Large, Joseph J. Tribbia, Peter R. Gent, Bruce P. Briegleb, and James C. McWilliams

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 1°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 0.1°C of the warming. The atmospheric response to prescribed warm SST anomalies of about 1°C displays a very different heat flux signature. The implication, yet to be verified, is that large-scale air–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 air–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.

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Stuart P. Bishop, Peter R. Gent, Frank O. Bryan, Andrew F. Thompson, Matthew C. Long, and Ryan Abernathey

Abstract

The Southern Ocean’s Antarctic Circumpolar Current (ACC) and meridional overturning circulation (MOC) response to increasing zonal wind stress is, for the first time, analyzed in a high-resolution (0.1° ocean and 0.25° atmosphere), fully coupled global climate simulation using the Community Earth System Model. Results from a 20-yr wind perturbation experiment, where the Southern Hemisphere zonal wind stress is increased by 50% south of 30°S, show only marginal changes in the mean ACC transport through Drake Passage—an increase of 6% [136–144 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1)] in the perturbation experiment compared with the control. However, the upper and lower circulation cells of the MOC do change. The lower cell is more affected than the upper cell with a maximum increase of 64% versus 39%, respectively. Changes in the MOC are directly linked to changes in water mass transformation from shifting surface isopycnals and sea ice melt, giving rise to changes in surface buoyancy forcing. The increase in transport of the lower cell leads to upwelling of warm and salty Circumpolar Deep Water and subsequent melting of sea ice surrounding Antarctica. The MOC is commonly supposed to be the sum of two opposing components: a wind- and transient-eddy overturning cell. Here, the transient-eddy overturning is virtually unchanged and consistent with a large-scale cancellation of localized regions of both enhancement and suppression of eddy kinetic energy along the mean path of the ACC. However, decomposing the time-mean overturning into a time- and zonal-mean component and a standing-eddy component reveals partial compensation between wind-driven and standing-eddy components of the circulation.

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C. M. Bitz, K. M. Shell, P. R. Gent, D. A. Bailey, G. Danabasoglu, K. C. Armour, M. M. Holland, and J. T. Kiehl

Abstract

Equilibrium climate sensitivity of the Community Climate System Model, version 4 (CCSM4) is 3.20°C for 1° horizontal resolution in each component. This is about a half degree Celsius higher than in the previous version (CCSM3). The transient climate sensitivity of CCSM4 at 1° resolution is 1.72°C, which is about 0.2°C higher than in CCSM3. These higher climate sensitivities in CCSM4 cannot be explained by the change to a preindustrial baseline climate. This study uses the radiative kernel technique to show that, from CCSM3 to CCSM4, the global mean lapse-rate feedback declines in magnitude and the shortwave cloud feedback increases. These two warming effects are partially canceled by cooling because of slight decreases in the global mean water vapor feedback and longwave cloud feedback from CCSM3 to CCSM4.

A new formulation of the mixed layer, slab-ocean model in CCSM4 attempts to reproduce the SST and sea ice climatology from an integration with a full-depth ocean, and it is integrated with a dynamic sea ice model. These new features allow an isolation of the influence of ocean dynamical changes on the climate response when comparing integrations with the slab ocean and full-depth ocean. The transient climate response of the full-depth ocean version is 0.54 of the equilibrium climate sensitivity when estimated with the new slab-ocean model version for both CCSM3 and CCSM4. The authors argue the ratio is the same in both versions because they have about the same zonal mean pattern of change in ocean surface heat flux, which broadly resembles the zonal mean pattern of net feedback strength.

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