Below I summarize the progress on the research funded through the NSERC/WOCE Collaborative Special Project Grant. I have also enclosed a list of all publications (since 1992) which were supported through these funds. This progress report and the original WOCE proposal may be viewed on the world wide web at:
4.1 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 (60deg. x
60deg.) 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. 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.
In another project, A. Fanning has coupled the atmospheric energy moisture
balance model to the realistic geometry global ocean general circulation model
described by (Weaver and Hughes, 1995). To allow for the effects of ice in the
polar regions, the coupled model contains a simple thermodynamic ice model
(Semtner, 1976) which includes heat insulation, brine rejection, as well as the
ice-albedo feedback effect. Precipitation over land is returned to the oceans
through a series of river drainage basins as in Weaver and Hughes (1995), and
we allow for the water vapor-planetary longwave feedback by employing the
parameterization of Thomson and Warren (1982). The ocean model begins from the
near equilibrium state of Weaver and Hughes (1995), while the atmospheric model
begins from zero heat and moisture content. We have now integrated the coupled
model to equilibrium. Our equilibrium climate, without flux adjustments, is
very reasonable (see Fanning and Weaver, 1995).
Finally, Trevor Murdock has recently began his MSc. He will be investigating
the effects on ocean circulation and global climate of the closing of ocean
gateways (the isthmus of Panama and Drake passage) with this same coupled
energy-moisture balance model. Mr. Murdock is currently taking courses but has
become familiar with the coupled model.
4.2 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.
4.3 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. Sophie Valcke (an NSERC
postdoctoral fellow) and Sheng Zhang (a research associate) will work on
various aspects of this project.
4.4 Global ocean modelling
Development of the global ocean model continued during this period,
with an investigation of the seasonal cycle, and its relevance to the
annually-averaged water mass properties and overturning circulation. In a first
phase, a seasonal cycle was added only to the temperature and salinity
restoring boundary conditions, and the North Atlantic overturning was found to
increase by a few Sverdrups. A seasonal cycle on the winds was then also added,
changing the annual mean overturning again by a small amount. Preliminary
investigations suggested that variability of the Mediterranean outflow
temperature and salinity was an important influence, however parallel
experiments in an alternate geometry with no Mediterranean Sea showed that
other factors could also be contributing. In particular, large changes in
convective patterns near the western boundaries of the oceans under seasonal
winds gave rise to concern whether the on/off character of the standard
convective parameterization was appropriate for representing seasonal
excursions of the mixed layer. Annual and seasonal experiments with the
isopycnal and Gent-McWilliams formulations (which can replace convection in
regions of steeply-sloping isopycnals) were therefore commenced, but have not
yet been fully analyzed due to an unusual numerical problem which is currently
being investigated by both Daniel Robitaille and Michael Eby in idealized box
models.
Another aspect of the global modelling project has been to assess the
equilibrium circulation produced under observed heat and freshwater fluxes and
surface wind stresses from the recent da Silva et al. (1994)
climatology. Some preliminary treatment was needed to make the fluxes suitable
for forcing an ocean model, since the open water bias of the observations
caused heat losses in the Arctic to be overestimated. However, even after
adjusting the fluxes to be consistent with the climatological ice cover,
excessive cooling of the northern oceans led to the dominance of northern deep
water formation while southern sinking was essentially suppressed. Resolution
is thought to be a major issue here, as suggested by Tziperman and Bryan (1993)
and Schiller (1995). It remains an open research question how to improve
parameterizations in coarse resolution ocean models enough to resolve this
incompatibility. One issue which we are exploring is the suggestion of enhanced
vertical mixing in western boundary currents compared to the quiescent
interior. For this purpose, we have constructed a stripped-down version of the
Bryan-Cox primitive equations model which has no barotropic mode, a linear
equation of state, fixed surface temperatures/ densities, and spatially-varying
vertical diffusivity, and are currently running tests of the sensitivity of the
overturning circulation to inhomogeneous kv in a simple flat-bottomed basin
geometry.
4.5 CFCs and global ocean models
A global version of the GFDL model was run using three different
sub-grid mixing schemes: lateral/vertical, an isopycnal mixing scheme (Redi,
1982; Cox, 1987), and the Gent and McWilliams (1990) parameterization. The
results from these three runs were analyzed by using CFC-11 as a time-dependent
passive tracer, and by comparing with observations. the Gent and McWilliams
parameterization improves the CFC-11 distributions when compared to both of the
other schemes, especially in the southern ocean, where the "bolus" transport
canceled the mean advection of tracers and hence caused 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 an artifact of the particular
sub-grid scale mixing schemes used.
The sensitivity of the Gent and McWilliams parameterization was also evaluated
by introducing an additional stability dependent diapycnal mixing, following
Tandon and Garrett (1995). This additional mixing does not affect the surface
buoyancy flux, but do affect the circulation of passive tracers in the deep
ocean. These results will be analyzed by looking at the role of boundary layer
versus interior mixing.
4.6 Finite element modelling
This four year project involves a systematic procedure for model
development. The last progress report outlined the main accomplishments to
date. Since that time the barotropic finite element model has been tested in
its global version. Time-dependent and nonlinear terms have also been included.
In addition a detailed diagnostic calculation of the annual mean, annual cycle
and interpentadal variability of the North Pacific Ocean circulation has been
completed (Myers and Weaver, 1995).
4.7 References:
Bryan, K., 1986: Poleward buoyancy transport in the ocean and mesoscale
eddies. J. Phys. Oceanogr., 16, 927-933.
Cox, M.D., 1987: Isopycnal Diffusion in a Z-Coordinate Model. Ocean Modelling, 74, 1-5.
da Silva, A.M., C.C. Young and S. Levitus, 1994: Atlas of surface marine data 1994. Volume 1: Algorithms and procedures. NOAA Atlas NESDIS 6. U.S. Government Printing Office, Washington, D.C.
Fanning, A.F. and A.J. Weaver, 1995: An atmospheric energy moisture-balance model for use in climate studies. J. Geophys. Res., submitted.
Gent, P. R. and J. C. McWilliams, 1990: Isopycnal Mixing in Ocean Circulation Models, J. Phys. Oceanogr., 20, 150-155.
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.
Haney, R.L., 1971: Surface thermal boundary condition for ocean circulation models. J. Phys. Oceanogr., 1, 241-248.
Myers, P.G., and A.J. Weaver, 1995: On the circulation of the North Pacific Ocean: Climatology, seasonal cycle and interpentadal variability, Prog. Oceanogr., submitted.
Pacanowski, R., K. Dixon and A. Rosati, 1993: The GFDL Modular Ocean Model Users Guide. GFDL Ocean Group Technical Report #2, 46pp.
Redi, M.H., 1982: Oceanic Isopycnal Mixing by Coordinate Rotation J. Phys. Oceanogr, 12, 1154-1158. Salmon, R., 1986: A simplified linear ocean circulation theory. J. Mar. Res., 44, 695-711.
Schiller, A., 1995: The mean circulation of the Atlantic Ocean north of 30 S determined with the adjoint method applied to an ocean general circulation model. J. Mar. Res., 53, 453-497.
Semtner, A.J., 1976: A model for the thermodynamic growth of sea ice in numerical investigations of climate. J. Phys. Oceanogr., 6, 379-389.
Tandon, A., and C. Garrett, 1995: On as Recent Parameterization of Mesoscale Eddies. J. Phys. Oceanogr, submitted.
Thompson, S.L., and S.G. Warren, 1982: Parameterization of outgoing infrared radiation derived from detailed radiative calculations. J. Atmos. Sci., 39, 2667-2680.
Tziperman, E. and K. Bryan, 1993: Estimating global air-sea fluxes from surface properties and from climatological flux data using an oceanic general circulation model. J. Geophys. Res., 98, 22629-22644.
Weaver, A.J., and T.M.C. Hughes, 1995: On the incompatibility of ocean and atmosphere models and the need for flux adjustments. Clim. Dyn., in press.
Winton, M., 1993: Numerical Investigations of Steady and Oscillating Thermohaline Circulations. PhD thesis, University of Washington. 155 p.