Climate Modelling Group
School of Earth and Ocean Sciences


Canadian Climate Research Network - Variability

Progress Report: October 1, 1995

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Principal Investigator:
Dr. Andrew J. Weaver
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, British Columbia
CANADA V8W 2Y2

tel: (250) 472-4001
fax: (250) 472-4004
e-mail: weaver@ocean.seos.uvic.ca
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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)

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) and one research associate (T. Hughes), 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-Grant.html
The projects described in 3.1-3.3 below were presented at the Canadian CLIVAR meeting held at the University of Victoria on October 20, 1995.

3.1 Decadal Variability in a Coupled OGCM/EMBM

A. Fanning, a PhD student, has 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 (60 degree x 60 degree) basin, driven by zonally uniform wind stress and solar insolation forcing. The study examines the coupled system's sensitivity to resolution and oceanic parameters. We have already 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). Poleward heat transport is shown to significantly increase from coarse to finer resolution, although the coupled atmosphere-ocean model results confirm that the time-variant (eddy) component of poleward heat transport counteracts increases in the time mean flow as suggested by Bryan (1986), yielding indistinguishable changes in heat transport from the moderately coarse (1deg.) to higher (0.5deg.) resolution. The net planetary heat transport, and atmospheric heat transport, also appear to converge as resolution is increased. To interpret these results, the heat transport has been decomposed into its baroclinic overturning (related to the meridional overturning and Ekman transports), barotropic gyre (that in the horizontal plane) and baroclinic gyre (the remainder) components. To further assess the results, we have repeated these same experiments under restoring boundary conditions (to apparent temperatures and salinities diagnosed from the 4deg. x 4deg. equilibrium state following Haney, 1971) to elucidate the differences between heat transport in the coupled versus uncoupled model. Currently we are extending the resolution studies (coupled and uncoupled models) to 1/4deg. x 1/4deg. as well as 1/5deg. x 1/5deg. resolution.

Of particular importance is that spontaneous decadal variability (period ~13 years) is found to exist in the 0.5 x 0.5 resolution case (in both the coupled and uncoupled model), 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. The results of this research are currently being written up for publication.

References:

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

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.

Haney, R.L., 1971: Surface thermal boundary condition for ocean circulation models. J. Phys. Oceanogr., 1, 241-248.

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

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 indicate also that the vertical momentum dissipation has a very limited influence on the equilibrium temperatures and velocities. The Laplacian viscosity at coarse-resolution produces unexpectedly 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 will be reported in a paper to be submitted by the end of the year.

The second objective of these models concerns decadal oscillations and their driving mechanisms. Oscillations have occurred 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. The heat flux fields deduced from restoring experiments never lead to oscillations, as opposed to longitudinally uniform fluxes (Greatbatch and Zhang, 1995). Research is in progress to clarify the driving mechanism by simplifying the oscillation to its necessary elements and comparing its behaviour according to the momentum dissipation and boundary conditions.

References:

Greatbatch, R.J., and S. Zhang, 1995: An interdecadal oscillation in an idealized ocean basin forced by constant heat flux. J. Climate, 8, 81-91.

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.

3.1 Variability as a Function of Mean Climatic State

Tertia Hughes recently spent one week at GFDL in Princeton acquiring the GFDL coupled climate model for use on my local work station cluster. We now have this model up and running and will use 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.

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


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) Full support for Research Associate T. Hughes $35,000
4) Operating costs $2,000
5) Publication charges $5,000
Total $65,000


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