Climate Modelling Group
School of Earth and Ocean Sciences
Climate Research Network
Collaborative Research Agreement at the University of Victoria
on Behalf of the Canadian Institute for Climate Studies and Environment Canada
(#11 CICS-Global Oceans)

Progress Report:

April 1, 1997

1. Principal Investigator
Andrew Weaver

2. Institution
School of Earth & Ocean Sciences
University of Victoria
PO Box 1700
Victoria, BC, V8W 2Y2

3. Executive Summary

One of the fundamental questions in the debate on global warming concerns the role of the oceans. Several international centres have undertaken coupled ocean-atmosphere modelling studies to address the transient climatic response to anthropogenic increases in atmospheric greenhouse gases. With respect to the ocean, they have suggested that, during the next century or so, the ocean will act to slow global warming through the uptake of vast quantities of heat. The Canadian Centre for Climate Modelling and Analysis (CCCma) has also recently developed a coupled ocean-atmosphere model as Canada's contribution to international modelling efforts in this area.

The purpose of the CICS Global Oceans Project is to fund research into the development of increasingly more sophisticated ocean models which will be coupled to the CCCma Atmospheric Model. Initial work funded through this project has lead to first and second generation ocean models. We have also recently developed a new ocean model which incorporates improved parameterizations of processes which occur at scales less than those resolvable by the ocean model. Associated with this new model are more sophisticated numerical solution techniques.

4. Scientific Report

This progress report highlights the work conducted during the 1996 fiscal year. It is available on the world wide web at:

4.1 Advection schemes and Mixing Parameterizations in the GFDL Ocean Model -- A Hierarchy of Models

The results from ocean model experiments conducted with isopycnal and isopycnal thickness diffusion parameterizations for subgrid scale mixing associated with mesoscale eddies were examined from a numerical standpoint (Weaver and Eby, 1997). It was 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 centred 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.

We have also been analysing the effects of numerous advection schemes and sub grid scale mixing parameterizations in the global version of the ocean model used in the CCCma. We expect to write a manuscript on this analysis during the next year.

Following numerous discussions with members of the CCCma coupling group we decided to publish the climatology of the CCCma OGCM as part of the paper being written to describe the climatology of the full coupled model. Dr. G. Flato will act as the lead author of this multi-authored paper and I will write the section detailing the ocean model and the sensitivity tests we performed with it.

4.2 The Role of Flux Adjustments in Global Warming Experiments

The effect of employing flux adjustments on the climatic response of an idealized coupled model to an imposed radiative forcing was investigated with two coupled models, one of which employs flux adjustments (Fanning and Weaver, 1997a). A linear reduction (to the planetary longwave flux) of 4W/m2 was applied over a 70 year period and held constant thereafter. Similar model responses were found (during the initial 70 year period) for global-scale diagnostics of hemispheric air temperature due to the nearly linear surface air temperature response to the radiative forcing. Significant regional scale differences did exist, however. As the perturbation away from the present climate grew, basin-scale diagnostics (such as meridional overturning rates) began to diverge between flux adjusted and non-flux adjusted models. Once the imposed radiative forcing was held constant, however, differences in global mean air temperature of up to 1/2 oC were found, with large regional-scale differences in air temperature and overturning rates within the North Atlantic and Southern Ocean.

Two additional experiments with the flux adjusted model (beginning from points further along the control integration) suggest that the elimination of much of the coupling shock before the radiative forcing is applied leads to results slightly closer to the non-flux adjusted case, although large differences still persist. In particular, a dipole structure indicating an enhanced warming within the Pacific sector of the Southern Ocean, and cooling within the Atlantic sector is not reproduced by the flux adjusted models. This disparity is intimately linked to the Southern Ocean overturning cell along with the flux adjustments employed as well as the drift arising from coupling shock. If a similar form of sensitivity exists in more realistic coupled models, our results suggest: 1) perturbation experiments should not be undertaken until after the coupled model control experiment is carried out for several hundred years (thereby minimizing the coupling shock); 2) care should be exercised in the interpretation of regional-scale results (over the ocean) in coupled models which employ flux adjustments; 3) care should also be taken in interpreting even global-scale diagnostics in flux adjusted models for large perturbations about the present climate.

4.3 The Effect of Momentum Dissipation Parameterization in Ocean Models

An ocean circulation model was developed for a Cartesian coordinate flat-bottomed beta-plane, based on the planetary geostrophic (PG) equations, in order to test different parameterizations of the momentum dissipation (Laplacian, biharmonic, Rayleigh and none) and associated boundary conditions (Huck et al., 1997). It was used at coarse-resolution (160 km) for a mid-latitude basin, with restoring boundary conditions for the surface density and with no wind stress. Comparison with the GFDL ocean model for the same geometry and forcing validates the PG dynamics and confirms the negligible effects of vertical viscosity and the total derivatives in the momentum equations at coarse resolution. The surface temperature fields and poleward heat transports are quite similar for the equilibrium states obtained using different momentum dissipation parameterizations. However, a comparison of the velocity fields and bottom water properties shows large discrepancies. The traditional Laplacian friction produces a more satisfying interior circulation, in better agreement with geostrophy and Sverdrup balance, but generates excessively large vertical transports along the lateral boundaries: especially upwelling in the western boundary current (the Veronis effect) and downwelling in the north-east corner. The meridional overturning is thus enhanced, but drives to depth surface waters that are not as cold as the ones in the deep convection regions.

Rayleigh friction with a no-normal-flow boundary condition (a vorticity closure is used whose primary effect is to reduce vertical velocities along the boundaries by allowing horizontal recirculation) induces a more efficient thermohaline circulation with better agreement between convection regions and areas of downward velocities, colder deep water, much weaker meridional overturning and Veronis effect, but higher poleward heat transport. However, this parameterization lacks physical justifications and is not as satisfying as the Laplacian closure in terms of interior geostrophic and Sverdrup balance. Nevertheless, it is an interesting alternative to implement it along with the no-normal-flow boundary conditions, since free-slip and no-slip boundary conditions are shown here to lead to very similar circulations, regardless of the momentum dissipation scheme. The parameterization of the Reynolds stresses and associated dynamical boundary conditions are shown to have a large influence on the steady-state tracer field and the thermohaline circulation in our experiments. The role of dynamical boundary conditions is more important than the interior momentum dissipation in coarse-resolution thermohaline circulation models.

4.4 Heat Transport in an Idealized Coupled Climate Model

An idealized coupled ocean-atmosphere model was utilized to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system (Fanning and Weaver, 1997b). A series of experiments ranging from 4o to 0.25o resolution, with appropriate horizontal viscosities and diffusivities in each case were performed. The coupled atmosphere-ocean model results contradict earlier studies which showed that the heat transport associated with time varying circulations counteracts increases in the time mean so that the total remained unchanged as resolution was increased. Even though the total oceanic heat transport had not converged, the net planetary heat transport had essentially converged owing to the strong constraint of energy balance at the top of the atmosphere. Consequently, the atmospheric heat transport was reduced to offset the increasing oceanic heat transport.

To interpret these results, the oceanic heat transport was 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 occurred in the steady currents. In particular the baroclinic gyre transport increased 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 were 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 was more strongly constrained in the restoring experiments, results were similar to those in the coupled model. Again, the total heat transport 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.

4.5 A Curvilinear Coordinate OGCM and Coupled Model

Michael Eby, a Research Associate working with me, has recently developed a transformation which allows the GFDL MOM2 ocean model to use a curvilinear coordinate system. This will be enormously beneficial to the community as it will alleviate problems at the poles due to converging merdians. In addition, it will allow for better representations of coastlines and passages. He has also recently completed the conversion of our energy-moisture balance model to curvilinear coordinates. We anticipate using the curvilinear version of our coupled model for future research that includes a detailed resolution of the Arctic Ocean.

5. 1996-1997 CICS Global Oceans Papers Arising

1. Fanning, A.F. and A.J. Weaver, 1997a: On the role of flux adjustments in an idealized coupled model. Climate Dynamics, in press.

2. Fanning, A.F. and A.J. Weaver, 1997b: A horizontal resolution and parameter sensitivity study of heat transport in an idealized coupled climate model, Journal of Climate, submitted.

3. Huck, T., A.J. Weaver and A. Colin de Verdière, 1997: The effect of momentum dissipation parameterizations in coarse-resolution thermohaline circulation models. Journal of Physical Oceanography, submitted.

4. Hughes, T.M.C. and A.J. Weaver, 1996: Sea surface temperature - evaporation feedback and the ocean's thermohaline circulation. Journal of Physical Oceanography, 26, 644-654.

5. Myers, P.G. and A.J. Weaver, 1996: On the circulation of the North Pacific Ocean: Climatology, seasonal cycle and interpentadal variability, Progress in Oceanography, 38, 1-49.

6. Myers, P.G., A.F. Fanning and A.J. Weaver, 1996: JEBAR, bottom pressure torque and Gulf Stream separation. Journal of Physical Oceanography, 26, 671-683.

7. Weaver, A.J. and T.M.C Hughes, 1996: On the incompatibility of ocean and atmosphere models and the need for flux adjustments. Climate Dynamics, 12, 141-170.

8. Weaver, A.J. and M. Eby, 1997: On the numerical implementation of advection schemes for use in conjunction with various mixing parameterizations in the GFDL ocean model. Journal of Physical Oceanography, 27, 369-377.

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