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: January 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-Grant.html


3.1 Decadal Variability in a Coupled OGCM/EMBM

In the last progress report I noted that A. Fanning had developed and utilized an atmospheric energy-moisture balance model (EMBM, see Fanning and Weaver, 1995) coupled to an ocean general circulation model (the GFDL-MOM model, Pacanowski et al., 1993) in a series of experiments conducted in a single hemisphere (60deg. x 60deg.) basin, driven by zonally-uniform wind stress and solar insolation forcing. The study examined the coupled system's sensitivity to resolution and oceanic parameters. We had completed four experiments ranging from 4deg. x 4deg. resolution to 0.5deg. x 0.5deg. resolution, with appropriate horizontal viscosities, and diffusivities in each case (Bryan, 1991). We have extended these integrations to 0.25deg. x 0.25deg. and compared the results to those obtained using an ocean model under restoring surface boundary. Poleward heat transport is shown to significantly increase from coarse to finer resolution and the net planetary heat transport, and atmospheric heat transport, appear to converge as resolution is increased to 0.25deg. x 0.25deg.

Of particular importance is that spontaneous decadal variability (period ~13 years) is found to exist in the 0.5 x 0.5 and higher resolution cases (in both the coupled and uncoupled models), with poleward heat transport changing by up to one third of the total oceanic heat transport over one oscillation in the thermohaline circulation. The oscillation is best described as an advective-convective mechanism, linked to the turning on and shutting off of convection in the northwest corner of the model domain. 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. In addition, we have found that the oscillation is not linked to the existence of nonlinear terms in the momentum equations. The results of this research are still being written up for publication.

References:

Bryan, K., 1991: Poleward heat transport in the ocean. A review of a hierarchy of models of increasing resolution. Tellus, 43, 104-115.

Fanning, A.F. and A.J. Weaver, 1995: An atmospheric energy moisture-balance model for use in climate studies. J. Geophys. Res., submitted.

Pacanowski, R., K. Dixon and A. Rosati, 1993: The GFDL Modular Ocean Model Users Guide. GFDL Ocean Group Technical Report #2, 46pp.

3.2 Decadal Variability in OGCMs with Various Subgrid-Scale Boundary Layer Dissipation Parameterizations

As reported earlier, T. Huck, a visiting PhD student from France, has developed a hierarchy of simplified thermohaline circulation models in order to study the effect of the momentum dissipation parameterizations on the large-scale ocean features. The main model is based on the Planetary Geostrophic equations, in a coarse resolution Cartesian beta-plane ocean; the choice of momentum dissipation includes the traditional Laplacian viscosity, biharmonic dissipation, and linear Rayleigh friction with different options to solve for the non-hydrostatic boundary layers (Salmon, 1986). In addition, the GFDL-MOM code has been utilized with the same geometry to provide a reference.

A first set of experiments has been done under an atmospheric forcing limited to restoring boundary conditions for the surface layer temperatures. This leads to an equilibrium state after some 3000 years. Planetary Geostrophic dynamics prove to yield a satisfying framework for the mid-latitude basin studied here, as the results with horizontal Laplacian viscosity compare very well with the GFDL-MOM model case under the same conditions. Results also indicate that the vertical momentum dissipation has a very limited influence on the equilibrium temperatures and velocities. The Laplacian viscosity at coarse-resolution produces unreasonably strong vertical velocities, especially along the boundaries. Around the thermocline depth, these spurious boundary vertical transports are comparable to the total interior upwelling. A better agreement between downwelling vertical velocities and convection is found with linear friction, either using a vorticity closure for the tangential velocities along the lateral walls (Winton, 1993), or relaxing the hydrostatic approximation via a vertical friction (linear with the vertical velocity [Salmon, 1986]). In these cases, the vertical velocity fields are much smoother, not so strongly perturbed near the boundaries: the deep water is slightly colder (0.1deg. C), and the polar heat transport 8% larger, although the meridional overturning streamfunction is much weaker, dropping from 15 to 9 Sverdrups. This is not a consequence of the 'no normal flow' boundary conditions, as the use of free-slip boundary conditions with the Laplacian or biharmonic viscosity does not resolve these problems. This comparison is now being written up for publication.

Decadal oscillations have been found in these thermally-only-driven experiments, under restoring boundary conditions for temperature with long restoring time scales, or more readily with constant heat flux. A wide range of tests have shown that convection is not necessary to the oscillation's mechanism, but that a critical damping factor is the horizontal eddy-diffusivity. This variability also occurs on an f-plane, where their amplitude grows with the Coriolis parameter. The geographical distribution of the surface heat flux is of primary importance. During the next six months T. Huck will write a second manuscript discussing the mechanism for the oscillation in these simple models.

References:

Salmon, R., 1986: A simplified linear ocean circulation theory. J. Mar. Res., 44, 695-711.

Winton, M., 1993: Numerical Investigations of Steady and Oscillating Thermohaline Circulations. PhD thesis, University of Washington. 155 p.

3.3 Variability as a Function of Mean Climatic State and Decadal Variability in a Coupled Ocean-Atmosphere Model

Late last year we acquired the GFDL coupled climate model for use on my local work station cluster. We now have this model up and running are using it to investigate questions concerning the existence of climate 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. The GFDL model is far more computationally efficient than the CCC coupled model. It is hoped that the insight we gain from the computationally efficient GFDL coupled model will allow us to better streamline the experiments we perform in the future with the more sophisticated CCC coupled model.

4. Budget request for the 1995-96 fiscal year:

The budget request remains unchanged from the original request. Tertia Hughes who was originally funded off this has been replaced by two research associates (Sheng Zhang and Sophie Valcke). They are partially funded by this project.

The budget request remains unchanged from the initial proposal:

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
Total $65,000

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