Some Issues on the Role of the Ocean in Climate and Global Change



by

Andrew J. Weaver


School of Earth and Ocean Sciences
University of Victoria PO Box 1700
Victoria, B.C., V8W 2Y2, Canada
Tel: (250) 472-4001; Fax: (250) 472-4004
http://wikyonos.seos.uvic.ca



The ocean's role in climate is manifested in its ability to regulate climate variability through its high heat capacity, its own rich internal dynamics and its ability to transport heat poleward. In this study four issues are addressed concerning our present understanding of the ocean and its influence on potential climatic changes associated with increasing atmospheric greenhouse gases. In the first, it is demonstrated through a series of coupled atmosphere-ocean-ice model experiments, with horizontal resolution ranging from 4o by 4o to 1/4o by 1/4o, that oceanic heat transport does not converge, even though the net planetary heat transport does converge owing to the strong constraint of energy balance at the top of the atmosphere (Fanning and Weaver, 1996b). As a consequence, the atmospheric heat transport is reduced to offset the increased oceanic heat transport. This has important implications for climate modelling since our present generation of global coupled atmosphere-ocean-ice models, used to investigate changes due to anthropogenic increases in greenhouse gases, have ocean models with resolution of 1o by 1o or greater.

In this same study it is further demonstrated that decadal variability spontaneously occurs as the resolution increases beyond 1o by 1o. The variability is not due to the higher resolution itself but rather to the reduced value of the horizontal diffusivity allowable when the resolution is decreased. As the resolution is increased still further, baroclinic instability within the western boundary current adds a more stochastic component to the solution such that the decadal variability is less regular and more chaotic. These results further point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer decadal scale variability in models which require less parameterized diffusion. It is suggested that if coupled models wish to capture the full spectrum of low (and higher) frequency internal variability the horizontal resolution should be increased substantially.

The second issue that is raised concerns the use of flux adjustments in present generation coupled atmosphere-ocean climate models. It is shown that a dramatic climate drift of the unadjusted coupled system is inevitable unless ocean meridional heat and freshwater (salt) transports are used as constraints for tuning Atmospheric General Circulation Model (AGCM) present-day climatologies. It is further shown that the magnitude of the local mismatch between Ocean General Circulation Model (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 (Weaver and Hughes, 1996).

Experiments have been conducted with a version of a locally developed coupled Energy-Moisture Balance Atmospheric Model/ Thermodynamic Ice Model/ OGCM (EMBM-TIM-OGCM -- Fanning and Weaver, 1996a) to examine the effects of flux adjustments on the transient and equilibrium response of the coupled climate system to increasing greenhouse gases. Initial results suggest that the use of flux adjustments has only a slight effect on the global mean temperature over the first 75 years (when the radiative forcing is acting), with results diverging after that point. It is shown that this divergence is reduced by demanding that, once the equilibrium ocean and atmosphere climatologies are coupled together (with flux adjustments), the full coupled model is run to a new equilibrium prior to the addition of radiative forcing due to anthropogenic greenhouse gases. Basin-scale diagnostics such as the meridional overturning do, however, diverge between flux adjusted and unadjusted models throughout the transient integration as the perturbation away from the present day-climate grows.

In the third issue, results are shown from a version of our global coupled EMBM-TIM-OGCM (without flux adjustments) in which the equilibrium climate exhibits persistent decadal variability (period 26 years) centered in the North Atlantic with century timescale variability occurring in the Southern Ocean. Initial analysis suggests that a mechanism involving sea-ice/thermohaline circulation inteaction exists, similar to that of Yang and Neelin (1995). An issue which needs to be addressed is to what extent this variability changes as the mean climate state itself changes (i.e., in warmer or colder mean climates). In addition, to what extent is the decadal-interdecadal variability found in existing coupled models (e.g. Delworth at al., 1993) which employ flux adjustments 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.

Finally, through a comparison of model-derived and observational sections of CFC-11 it is demonstrated that the choice of a sub-grid scale mixing parameterization (lateral/vertical; isopycnal/diapycnal; Gent and McWilliams, 1990) has a profound effect on the equilibrium climatology. In particular, the Gent and McWilliams (1990) parameterization significantly improves the CFC-11 distributions when compared to both of the other schemes. The main improvement comes from a reduction of CFC 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 overestimated when conventional mixing schemes are used, although more sophisticated tests with coupled atmosphere ocean models are needed to confirm this.


References

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

Fanning, A.F., and A.J. Weaver, 1996a: An atmospheric energy-moisture balance model: Climatology, interpentadal climate change, and coupling to an OGCM. J. Geophys. Res., 101, 15,111-15,128.

Fanning, A.F., and A.J. Weaver, 1996b: A horizontal resolution and parameter sensitivity study of heat transport in an idealized coupled climate model. J. Climate, submitted.

Gent, P.R. and J.C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, 150-155.

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.

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.

Yang, J and J.D. Neelin, 1993: Sea-ice interaction with the thermohaline circulation. Geophys. Res. Let., 20, 217-220.