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)

Principal Investigator: Andrew Weaver

Progress Report:

April 1, 1996

1. Principal Investigator

Andrew Weaver

2. Institution

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

3. Details of Projects

The purpose of the #11 CICS -- Global Oceans grant is to allow Dr. A. Weaver to participate in the continued development of the Canadian Climate Centre (CCC) coupled atmosphere-ocean model and to act as the Scientific Leader of the Ocean Modelling Division of the CCC. In order to accomplish these tasks research grant money is requested to provide secretarial support (Wanda Lewis), teaching relief (Rolf Lueck), Computer Programming Support (Michael Eby) and minor operating expenses. These three people are all continuing their appointments.

A copy of my progress reports are available on the world wide web at:

This progress report highlights the work conducted during the 1995-1996 fiscal year. Some reference to earlier funded CICS Global Oceans research is also made.

3.1 Oceanic poleward heat transport as a function of OGCM resolution

One of the most important roles of the ocean in climate is its transport of heat from low to high latitudes. A fundamental, yet unanswered, question regarding the ocean's role in climate is whether or not eddies are important in transporting heat and salt poleward. Numerous studies (e.g., Cox, 1985; Bryan, 1986; Boening and Budich, 1992; Drijfhout, 1994) have suggested that eddies do not play a significant role and that the North Atlantic heat/salt transport is dominated by the meridional overturning transport.

All of the above works suffer from a major shortcoming since all the experiments were conducted using ocean-only models. In the limit of specified fluxes of heat and freshwater, the oceanic heat and salt transports are necessarily predetermined at equilibrium since the divergence of the transport gives the zonally-averaged flux. Thus, whether or not eddies are resolved will not change the total heat or salt transport at equilibrium. All of the above studies also used restoring boundary conditions on temperature and salinity. While these boundary conditions do not impose an exact constraint on the oceanic poleward heat and salt transports at equilibrium, they do largely determine the thermocline structure and hence may clamp the ability of the ocean models to freely regulate its heat and salt transport.

Five experiments have been conducted with a single hemisphere (60deg. x 60deg.) Energy-Moisture Balance Model (EMBM) coupled to an OGCM. The coupled model is driven by zonally-uniform wind stress and solar insolation forcing, with horizontal resolution ranging from 4deg. x 4deg. to 0.5deg. x 0.5deg.. Poleward heat transport is shown to significantly increase from coarse to finer resolution. Our coupled atmosphere-ocean model results contradict earlier studies mentioned above which showed that the time-variant (eddy) component of poleward heat transport counteracts increases in the time mean flow. This is perhaps related to the relatively short integration times utilized by these previous works. An additional mechanism may be the inclusion of salinity in our analysis. Previous works utilized buoyancy forcing alone so that eddies were aligned along isopycnals and hence no net heat transport occurs by their presence. In the present work, isotherms and isopycnals no longer coincide and a net heat transport can be expected if eddies propagate across isopycnals. Even though the net oceanic heat transport has not converged, the net planetary heat transport has 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. Currently we are extending the resolution studies to 1/4deg. x 1/4deg.

Of particular importance in this study is that spontaneous decadal variability is found to exist in the 0.5deg. x 0.5deg. resolution case (in both coupled and uncoupled models). We find the variability is strongly linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity from 200 m2/s to 500 m2/s is enough to destroy the variability, while decreasing the diffusivity from 500 m2/s to 200 m2/s (in the 1deg. x 1deg. case) is capable of inducing the variability. This component of the global oceans project has therefore got close links with the Climate Variability component of CICS.

3.2 Flux adjustments and their influence in coupled models

During the last two years, a global ocean model was developed in collaboration with Warren Lee and others in the CCC for coupling to the CCC AGCM. Two versions of this model now exist: the first version is a high resolution model (1.8deg. x 1.8deg. x 29 levels) which has now been coupled to the CCC AGCM to investigate the climatic response to increasing atmospheric greenhouse gases and aerosols. The second version of the model is of slightly coarser resolution (3.6deg. x 1.8deg. x 19 levels) and is currently being used to understand the structure, stability and variability of the global ocean thermohaline circulation.

The surface heat and freshwater fluxes from equilibrium OGCM and AGCM climates have been examined in order to determine the minimum flux adjustment required to prevent climate drift upon coupling (Weaver and Hughes, 1996). It was shown that a dramatic climate drift of the coupled system is inevitable unless ocean meridional heat and freshwater (salt) transports are used as constraints for tuning the AGCM present-day climatology. It was further shown that the magnitude of the mismatch between OGCM and AGCM fluxes is not as important for climate drift as the difference in OGCM and implied AGCM meridional heat and freshwater (salt) transports. Hence a Minimum Flux Adjustment was proposed, which is zonally-uniform in each basin and of small magnitude compared to present flux adjustments. This minimum flux adjustment acts only to correct the AGCM implied oceanic meridional transports of heat and freshwater (salt).

We are also investigating the role of flux adjustments on interdecadal climate variability. The numerical simulations of Delworth et al. (1993), using the GFDL coupled model revealed interdecadal variability of the thermohaline circulation in the North Atlantic. It is not clear to what extent the variability in that study is preconditioned by the heat and salt flux adjustment fields required to prevent climate drift in the coupled model. It is also unclear whether or not this variability is linked to coupled ocean-atmosphere dynamics or to ocean dynamics alone. In order to do elucidate this, the GFDL coupled model has been adapted to our local IBM cluster. The oceanic part of this model is now being run under fixed-flux boundary conditions, made up of atmospheric fluxes (diagnosed from the atmospheric model at equilibrium) and the flux adjustment terms. If similar variability as in the fully coupled experiments is found, we can conclude that the variability is due to internal ocean dynamics alone.

3.3 Finite element modelling

A diagnostic, finite element, barotropic ocean model has been developed and used to simulate the mean circulation in the North Atlantic (Myers and Weaver, 1995). With the inclusion of the joint effect of baroclinicity and relief (JEBAR), the Gulf Stream is found to separate at the correct latitude off Cape Hatteras. Results suggest that the JEBAR term in three key regions (offshore of the separation point in the path of the main jet, along the slope region of the North Atlantic Bight and in the central Irminger Sea) is crucial in determining the separation point. The transport driven by the bottom pressure torque component of JEBAR, dominates the solution, except in the subpolar gyre, and is also responsible for the separation of the Gulf Stream. Excluding high latitudes (in the deep water formation regions) density variations in the upper 1000m of the water column govern the generation of the necessary bottom pressure torque in the model. Examination of results from the WOCE - Community Modelling Effort (CME) indicates that the bottom pressure torque component of JEBAR is underestimated by almost an order of magnitude, when compared to the diagnostic results. The reason for this is unclear, but may be associated with the diffuse nature of the CME model thermocline as suggested by the diagnostic model's sensitivity to the density field above 1000m.

The finite element model was then used to study the circulation of the North Pacific Ocean (Myers and Weaver, 1996). With the inclusion of the JEBAR term, the model produced a very realistic picture of the circulation. All major currents were reproduced with the calculated transports agreeing well with the observations. The effect of using different wind stress climatologies was also examined. Due to the dominance of the JEBAR term in the solution, the resulting circulations were all similar. Analysis of the seasonal cycle in the model supports Sakamoto and Yamagata (1995) in that JEBAR rectification can explain the decreased amplitude of the seasonal cycle and the out of phase relationship between observations and the predictions of flat-bottomed Sverdrup theory. Finally, density fields from 1955-1959 and 1970-1974 were used to examine aspects of interpentadal variability in the North Pacific Ocean.

3.4 CFCs and global ocean models

Robitaille and Weaver (1995) examined three sub-grid scale mixing parameterizations (lateral/vertical; isopycnal/diapycnal; Gent and McWilliams, 1990 -- GM) using a global ocean model in an attempt to determine which yielded the best ocean climate. Observations and model CFC-11 distributions, in both the North and South Atlantic, were used in the model validation (see attached figure for the South Atlantic results). While the isopycnal/diapycnal mixing scheme did improve the deep ocean potential temperature and salinity distributions, when compared to results from the traditional lateral/vertical mixing scheme, the CFC-11 distribution was significantly worse due to too much mixing in the southern ocean. The GM parameterization, on the other hand, significantly improved the deep ocean potential temperature, salinity and CFC-11 distributions when compared to both of the other schemes. The main improvement came from a reduction of CFC uptake in the southern ocean where the "bolus" transport canceled the mean advection of tracers and hence caused the Deacon Cell to disappear. These results suggest that the asymmetric response found in CO2 increase experiments, whereby the climate over the Southern Ocean does not warm as much as in the northern hemisphere, may be an artifact of the particular sub-grid scale mixing schemes used.

Due to the reduction of vertical mixing when the GM scheme was incorporated, numerical problems associated with vertical grid Peclet violations were found to occur. A flux-corrected transport (FCT) scheme (Gerdes et. al 1991) was therefore implemented into the GFDL OGCM and the consequences of using this advection scheme to eliminate these numerical problems are being investigated. Several integrations comparing mixing and advection schemes, in a simple model, demonstrate that it may be necessary to use a more sophisticated advection scheme (like FCT) when using isopycnal mixing parameterizations.

3.5 The Canadian Climate High Resolution OGCM

Collaboration with Warren Lee and other researchers in the Canadian Climate Centre has continued. At present, a simplified version of the CCC OGCM has been developed for the purpose of undertaking test simulations. In addition, a new ocean spin-up has been completed and this ocean model has been coupled to the CCC AGCM II to undertake climate change studies. These ocean models have now been frozen and we are still writing up a manuscript discussing the climatology and sensitivity analysis of the global model. It was jointly decided that Warren Lee would take the lead on preparing a first draft of the article (and hence act as first author) and that A. Weaver would finish the manuscript, in consultation with G. Boer and G. Flato.

In addition, the FCT scheme discussed above has been passed to W. Lee for implementation into the CCC Ocean model. Our intial tests with this advection scheme are very promising and we suspect that it will alleviate the grid-noise which is found to occur when the GM scheme was incorporated into our global model.

3.5 References

Boening, C.W. and R.C. Budich, 1992: Eddy dynamics in a primitive equation model: Sensitivity to horizontal resolution and friction. J. Phys. Oceanogr., 22, 361-381.

Bryan, K., 1986: Poleward buoyancy transport in the ocean and mesoscale eddies. J. Phys. Oceanogr., 16, 927-933.

Cox, M.D., 1985: An eddy resolving numerical model of the ventilated thermocline, J. Phys. Oceanogr., 15, 1312-1324.

Delworth, T., S. Manabe and R.S. Stouffer, 1993: Interdecadal variations of the thermohaline circulation in a coupled ocean-atmosphere model. J. Climate, 6, 1993-2011.

Drijfhout, S.S., 1994: Heat transport by mesoscale eddies in an ocean circulation model. J. Phys. Oceanogr., 24, 353-369.

Gent, P.R. and J.C. McWilliams, 1990: Isopycnal Mixing in Ocean Circulation Models, J. Phys. Oceanogr., 20, 150-155.

Gerdes, R., C. Koeberle and J. Willebrandt, 1991: The influence of numerical advection schemes on the results of ocean general circulation models. Clim Dynamics 5, 211-226.

Myers, P.G., A.F. Fanning and A.J. Weaver, 1996: JEBAR, bottom pressure torque and Gulf Stream separation. J. Phys. Oceanogr., 26, 671-683.

Myers, P.G., and A.J. Weaver, 1996: On the circulation of the North Pacific Ocean: Climatology, seasonal cycle and interpentadal variability, Prog. Oceanogr., in press.

Robitaille, D.Y. and A.J. Weaver, 1995: Validation of sub-grid scale mixing schemes using CFCs in a global ocean model. Geophys. Res. Let., 22, 2917-2920.

Sakamoto, T. and T. Yamagata, 1995: Seasonal transport variations of the wind-driven oceaqn circulation in a two-layer planetray geostrophic model with a continental slope. J. Mar. Res., submitted.

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.

4. Budget request for the 1996-97 fiscal year:

The budget request is the same as for last year. Note that an end of year financial statement will be provided to you by the Accounting Department of the University of Victoria under a separate cover.

1) Continued partial support for a secretary (Ms. Wanda Lewis) $10,000
2) Continued sessional teaching replacement (Dr. Rolf Lueck) $15,000
3) Partial support for one scientific computing Research Associate (Michael Eby ) $30,000
4) Operating and Media Costs $10,000
Total: $65,000

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