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
(#7 CICS-Variability)

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. Progress

The #7 CICS -- Variability Grant is used to undertake research on climate variability on the seasonal-centennial timescale. Research grant funding is to provide full support for one PhD student (T. Huck), partial support for two research associates (S. Zhang, S. Valcke), partial support for a PhD student (A. Fanning) and minor operating expenses.

A copy of my progress reports and the original grant proposal are available on the world wide web at:

http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress.html
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress2.html
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress3.html
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress4.html
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Progress5.html
http://wikyonos.seos.uvic.ca/projects/CCC-Variability-Grant.html

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

3.1 Development of a coupled Energy-Moisture Balance Model (EMBM) -Ocean General Circulation Model (OGCM) - Thermodynamic Ice Model (TIM)

In process studies of the ocean's role in climate, mixed boundary conditions are often employed in which the salt flux is fixed, while a restoring condition on temperature is maintained. It is widely recognized that such an approach has serious deficiencies in its representation of the atmosphere. Hughes and Weaver (1996), as a first step towards developing a better boundary condition for salinity, accounted for the physically-observed dependence of evaporation on SST. They showed that the stability and variability properties of the thermohaline circulation were largely unchanged when this new boundary condition was incorporated.

Using the fact that the timescale of atmospheric variability is generally short compared to the decadal-interdecadal timescales of interest here, an EMBM has been developed (Fanning and Weaver, 1996) for use in studying the ocean's role in climate change and variability. Since the timescale of variability of the cryosphere lies between that for the atmosphere and ocean, a simple thermodynamic ice model (Semtner, 1976) (which includes heat insulation and brine rejection) was also incorporated. This EMBM relates all components of the atmospheric system to only two prognostic variables: surface air temperature and specific humidity. In this manner, the model includes the primary hydrological and thermodynamic processes within the climate system, yet utilizes only modest computer resources, making it useful for coupled ocean-atmosphere studies on climate timescales.

Initial testing under fixed climatological sea surface temperature (SST) and surface wind conditions yields surface air temperatures, specific humidities and surface fluxes which are comparable to direct estimates. Precipitation compares less favorably with observations. As an extension to the climatological forcing case, a simple perturbation experiment in which the 1955-59 pentad was compared to the 1970-74 pentad by driving the model under the respective SST fields has also been conducted. The model exhibits a global air temperature decrease in the latter pentad of 0.27deg.C (comparable to direct estimates) with cooling in the northern hemisphere, and warming in the southern hemisphere. Such large scale cooling in the atmospheric model is driven by local changes in the prescribed SST fields, subsequently smoothed by atmospheric diffusion of heat.

In another project the EMBM has been coupled to the realistic geometry global OGCM described by Weaver and Hughes (1996). Precipitation over land was returned to the oceans through a series of river drainage basins, and we allowed for the water vapor-planetary longwave feedback. Although no flux adjustments are employed, this coupled climate model faithfully represents deep water formation in the North Atlantic and Southern Ocean, with upwelling throughout the Pacific and Indian oceans. Water mass characteristics in the vertical compare very favorably with direct (see Fanning and Weaver, 1996).

3.2 Simulation of the Younger Dryas event

The use of models to simulate past climatic events is an important avenue of investigation if one is to have confidence in their application to future climatic changes. To this end we have undertaken a simulation of the Younger Dryas event (hereafter YD). The realistic geometry, global, coupled OGCM-EMBM is currently being used to investigate the transition between the last glaciation and the present Holocene. During the transition, an abrupt return to glacial climatic conditions, known as the YD occurred. The YD cold episode was particularly pronounced in regions bordering the North Atlantic, and is evidenced in northern European and maritime Canadian lakes and bogs; North Atlantic marine sediments; and northwestern European and central Canadian glacial moraines. Although strongest in the northern Atlantic region, further evidence indicates the impacts of the YD were felt throughout the globe.

While the exact cause of the YD is still unknown, a general consensus has emerged that it was linked to an oceanographic phenomena. The amplification of the YD signal in the northeastern North Atlantic suggests a primary role for the thermohaline circulation, particularly North Atlantic deepwater (NADW) production. The question naturally arises as to what source could supply the necessary excess freshwater needed to reduce NADW formation. The obvious sources are the polar ice caps and the Laurentide and Fennoscandian ice sheets (LIS and FIS, respectively). The traditional viewpoint is that the YD was triggered by the diversion of meltwater (due to the retreating LIS) from the Gulf of Mexico to the St. Lawrence (Broecker et al., 1988). However, the fact that deep water is usually formed in local high-latitude regions of small extent suggests that not only the amount, but also the location of meltwater introduced is crucial for interrupting the North Atlantic Conveyor.

We are currently conducting experiments to reinvestigate the climatic implications of the geographical and temporal change in the runoff from the LIS and FIS, utilizing estimated meltwater and precipitation runoff from drainage basins in and around the North Atlantic, before, during, and after the YD (Teller, 1990). While Maier-Reimer and Mikolajewicz (1989), using an ocean-only model, found they were capable of shutting down NADW production within 200 years from the time they deflected 347 km3/yr from the Gulf of Mexico into the St. Lawrence valley (roughly half that estimated to have occurred). Our results suggest the traditional meltwater diversion theory is incapable of inducing a shutdown of NADW. If, however, we apply the runoff estimates previous to the YD, the conveyor is pushed to the brink, allowing the diversion of LIS waters to completely halt NADW production. In an additional experiment we will consider the role of the FIS meltwater on the YD climate. These results will be written up for publication shortly.

3.3 Decadal variability in OGCMs with various subgrid-scale boundary layer parameterizations

During the last decade, the tuning of large-scale ocean models towards observations has been achieved by adjusting tracer mixing processes and surface boundary conditions. However, few alternatives to the traditional Laplacian closure of Reynolds stress have been implemented in OGCMs. The influence of the momentum dissipation parameterization and dynamical boundary conditions is not likely to be negligible at coarse-resolution and is worth being precisely evaluated. Therefore, an ocean model has been developed for a coarse-resolution, box-geometry, mid-latitude beta-plane, based on the planetary geostrophic equations and allowing for different choices of momentum dissipation (linear, harmonic, biharmonic or none) and associated boundary conditions (no-slip, free-slip or vorticity closure). These models were first compared to the GFDL OGCM with the same geometry and forcing to validate the planetary geostrophic dynamics. Results from this analysis will be written up shortly.

Of particular importance is the effect that different momentum dissipation parameterizations have on the internal decadal-interdecadal variability found in ocean models. Through the use of these efficient ocean models we have found that atmospheric forcing plays the leading role in generating decadal variability in ocean models: flux boundary conditions are the most likely to allow variability as no damping applies to surface anomalies, although the spatial distribution is important. A parameter sensitivity study of the oscillatory behaviour has also been carried out. Results suggest that the horizontal tracer diffusivity has a critical damping effect, while increasing the vertical diffusivity strongly enhances the oscillations. The parameterization or even inclusion of convection is found not to be necessary in sustaining the decadal variability, although it is necessary to remove static instabilities. As pointed out by Winton (1996), the variation of the Coriolis parameter with latitude is not necessary, so saying that Rossby wave propagation is not important for the oscillations. Greatbatch and Peterson (1995) proposed an explanation in terms of Kelvin waves propagating around the basin. This mechanism was investigated by moving the boundaries or by forcing an f-plane model with a symmetric (about the meridional centre of the basin) heat flux. None of these major changes remove the oscillatory behavior; therefore we conclude that Kelvin wave propagation is not important for the oscillation. As the variability is mainly observed in the region of separation of the western boundary current, we are now looking at 2-layer and 2-dimensional models to investigate an advective mechanism, as originally proposed by Weaver and Sarachik (1991).

3.4 Climate Variability as a function of mean climatic state

An interesting phenomenon which I observed in Weaver (1993) was that for different equilibria obtained under normal, 2xCO2, 4xCO2 and 8xCO2 forcing in coupled GCMs (and in the uncoupled CCC AGCM), the total planetary heat transport was fairly constant (in a global warming or cooling scenario there was net heat loss or gain by the planetary system but at equilibrium, the radiation balance at the top of the atmosphere was similar). This phenomenon was exploited in the coupled atmosphere-ocean box model developed by Tang and Weaver (1995). The results of this simple model suggest, as did the uncoupled ocean experiments of Weaver and Hughes (1994), that as the earth warms, the hydrological cycle intensifies and hence the thermohaline circulation becomes more variable.

Over the next two years I hope to continue improving our understanding of the mechanisms of decadal-interdecadal climate variability through the development of increasingly more sophisticated coupled models. The coupled EMBM-TIM-OGCM represents the simplest form of our coupled modelling studies. We shall continue to use it to explore simple thermodynamic feedbacks and gain insight into what results we might expect and which experiments we should undertake with the more complicated GFDL and CCC coupled models. In addition, the GFDL coupled model will be used to investigate questions concerning the existence of variability in the coupled climate system and how it varies as the mean climatic state changes (i.e, does the decadal-interdecadal climate variability found in the coupled model change as CO2 is increased in the atmosphere. Since the GFDL coupled model is far more computationally efficient than the CCC coupled model, it is hoped that the insight we gain from it will allow us to better streamline the future experiments that will be performed with the more sophisticated CCC coupled model.

3.5 References

Broecker, W.S., M. Andree, W. Wolfli, H. Oeschger, G. Bonani, J. Kennett and D. Peteet, 1988: The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event. Paleoceanogr., 3, 1-19.

Fanning, A.F. and A.J. Weaver, 1996: An atmospheric energy moisture-balance model: climatology, interpentadal climate change and coupling to an OGCM. J. Geophys. Res., in press.

Greatbatch, R.J. and K.A. Peterson, 1996: Interdecadal variability and oceanic thermohaline adjustment. J. Phys. Oceanogr., in press.

Hughes, T.M.C. and A.J. Weaver, 1996: Sea surface temperature - evaporation feedback and the ocean's thermohaline circulation. J. Phys. Oceanogr,. 26, 644-654.

Maier-Reimer, E. and U. Mikolajewicz, 1989: Experiments with an OGCM on the cause of the Younger Dryas, Max Planck Institute fur Meteorologie, Report #39, 13 pp.

Semtner, A.J., 1976: A model for the thermodynamic growth of sea ice in numerical investigations of climate. J. Phys. Oceanogr., 6, 379-389.

Tang, B. and A.J. Weaver, 1995: Climate stability as deduced from an idealized coupled atmosphere-ocean model. Clim. Dyn., 11, 141-150.

Teller, J.T., 1990: Meltwater and precipitation runoff to the North Atlantic, Arctic, and Gulf of Mexico from the Laurentide Ice sheet and adjacent regions during the Younger Dryas, Paleoceanogr., 5, 897-905.

Weaver, A.J., 1993: The oceans and global warming. Nature, 364, 192-193.

Weaver, A.J. and T.M.C. Hughes, 1994: Rapid interglacial climate fluctuations driven by North Atlantic ocean circulation. Nature, 367, 447-450.

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

Weaver, A.J., and E.S. Sarachik, 1991: Evidence for decadal variability in an ocean general circulation model: an advective mechanism. Atmos.-Ocean, 29, 197-231.

Winton, M., 1996: The role of horizontal boundaries in parameter sensitivity and decadal-scale variability of coarse-resolution ocean general circulation models. J. Phys. Oceanogr., 26, 2.

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

The budget request is the same as for last year with the addition of $30,000 toward the purchase of an upgrade for my IBM 590 computer (as originally budgeted for in the initial Variability group proposal). 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) Partial support for PhD student A. Fanning $8,000
2) Full support for PhD student T. Huck $15,000
3) Partial support for two Research Associates
(Sheng Zhang and Sophie Valcke)
$35,000
4) Operating costs $2,000
5) Publication charges $5,000
6) Upgrade for Computer $30,000
Total $95,000

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