Canadian Climate Research Network - Variability
Progress Report: October 1, 1995
- 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: firstname.lastname@example.org
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
A copy of my progress reports and the original grant proposal are available on
the world wide web at:
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.
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
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.
Greatbatch, R.J., and S. Zhang, 1995: An interdecadal oscillation in an
idealized ocean basin forced by constant heat flux. J. Climate,
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:
||Partial support for PhD student A. Fanning
||Full support for PhD student T. Huck
||Full support for Research Associate T. Hughes
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