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 #3-49730)
In this progress report I will only discuss work that was directly funded by the #11 CICS-Global Oceans grant. A copy of this progress report and the original grant proposal are available on the world wide web at:
In addition, Dr. Rolf Lueck was appointed to teach two courses in the School of Earth and Ocean Sciences (Physical Oceanography and Time Series Analysis) in order to reduce Dr. A. Weaver's teaching load.
Since the #11 CICS-Global Oceans grant was awarded, I have conducted research in a number of areas. I have been heavily involved in the IPCC Second Scientific Assessment and am a lead author of two Chapters (Gates et al., 1996; Grassl et al., 1996). The IPCC process is designed to provide policy makers a treatise on our present understanding of climate change and climate variability.
I have worked collaboratively with Warren Lee in the CCC to develop a high-resolution global ocean model which has now been coupled to the CCC AGCM for the purpose of undertaking climate change/variability forecasts. I have a strong collaboration with the CCC and am the Scientific Leader of their Ocean Modelling effort. We now have a fully coupled atmosphere/ocean/ice model available and we are presently examining ways of reducing the flux correction which must be incorporated.
The climatology of the global ocean model under annual mean forcing (observed winds, restoring to Levitus surface temperature and salinity) was assessed as the basis for a comparison with the equilibrium circulation under diagnosed heat, freshwater and momentum fluxes from the CCC 2nd generation AGCM. The pronounced drift of this equilibrium away from the observed global ocean circulation led to the suggestion of a new method of flux correction for use in coupled atmosphere-ocean GCMs.
In a paper submitted to Climate Dynamics in January (Weaver and Hughes, 1995), we showed that the magnitude of the mismatch between OGCM and AGCM fluxes is not as important for climate drift as the difference between OGCM and implied AGCM heat and freshwater transports. Hence a "Minimum Flux Correction" was proposed which is zonally-uniform in each basin and of small magnitude compared to present flux corrections. This minimum flux correction acts only to correct the AGCM implied oceanic transports of heat and freshwater. A slight extension was also proposed to overcome the drift in the surface waters when the minimum flux correction is used. Finally, we showed that the current methods used to determine flux corrections are all essentially equivalent leading to correction fields which are significantly larger than both AGCM and climatological fields over large regions.
Inspired by the success of the Minimum Flux Correction mentioned above, a separate experiment is underway. In this experiment we force the global ocean model with the newly-derived observed heat and freshwater fluxes over the ocean compiled by Da Silva et al. (1994). This fluxes have been tuned to constrain the zonally-integrated transports of both heat and freshwater toward observed estimates. The purpose of this project is to investigate new ways of spinning up OGCMs prior to coupling them with AGCMs.
This global ocean model has now been given to Prof. Bill Gough at the University of Toronto, to initiate a collaboration on the influence of spurious cross-isopycnal mixing due to lateral diffusivity in regions of sloping isopycnals (the "Veronis effect").
Mr. D. Robitaille, a PhD student working under my supervision, has been using Freon as a tracer to validate the global ocean model. In the first study (Robitaille and Weaver, 1995), three sub-grid scale mixing parameterizations (lateral/ vertical; isopycnal/ diapycnal; Gent and McWilliams, 1990) were used in an attempt to determine which yields the best ocean climate. Observations and model Freon 11 distributions in both the North and South Atlantic were used in the model validation. While the isopycnal/diapycnal mixing scheme does improve the deep ocean potential temperature and salinity distributions, when compared to results from the traditional lateral/vertical mixing scheme, the Freon 11 distribution is significantly worse due to too much mixing in the southern ocean. The Gent and McWilliams (1990) parameterization, on the other hand, significantly improves the deep ocean potential temperature, salinity and Freon 11 distributions when compared to both of the other schemes. The main improvement comes from a reduction of Freon uptake in the southern ocean where the "bolus" transport cancels the mean advection of tracers and hence causes 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 mixing schemes used.
Our analysis of the three different mixing schemes mentioned above has led to the development of several new versions of the global ocean model for coupling to the CCC atmospheric model. We are continuing our research into ways of improving this ocean model in anticipation of the development of a second generation CCC coupled AOGCM.
Gates, L., A. Henderson-Sellers, G. Boer, C. Folland, A. Kitoh, B. McAvaney, F. Semazzi, N. Smith, A.J. Weaver and Q.-C. Zeng, 1996: Climate models-validation. Chapter 5 of the 1995 Second IPCC Scientific Assessment. Ed. Sir J. Houghton, in press.
Gent, P. R. and J.C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, 150-155.
Grassl, H., F. Giorgi, A. Kattenberg, G.A. Meehl, J.F.B. Mitchell, R.J. Stouffer, T. Tokioka, and A.J. Weaver, 1996: Climate models-Projections of future climate. Chapter 6 of the 1995 Second IPCC Scientific Assessment. Ed. Sir J. Houghton, in press.
Robitaille, D.Y. and A.J. Weaver, 1995: Validation of sub-grid scale mixing schemes using CFCs in a global ocean model. Geophysical Research Letters, submitted.
Weaver, A.J., and T.M.C Hughes, 1995: Flux corrections in coupled ocean-atmosphere models. Climate Dynamics, submitted.
1) Continued partial support for a secretary (Ms. Wanda Lewis) $10,000 2) Continued sessional teaching replacement (Dr. Rolf Lueck) $15,000 ________________________________________________________________________ Total: $25,000