Climate Research Network
Collaborative Research Agreement at the University of
on Behalf of the Canadian Institute for Climate Studies and
(#11 CICS-Global Oceans)
October 1, 1996
1. Principal Investigator
- Andrew Weaver
- School of Earth & Ocean Sciences
- University of Victoria
- PO Box 1700
- Victoria, BC, V8W 2Y2
3. Details of Projects
The purpose of the #11 CICS -- Global Oceans grant is to allow Dr. A.
to participate in the continued development of the Canadian Climate
(CCC) coupled atmosphere-ocean model and to act as the Scientific Leader
Ocean Modelling Division of the CCC. In addition, it allows him to
research into developing improved global ocean models for the purpose of
coupling them with the CCC AGCM.
A copy of previous progress reports are available on the world wide web
This progress report highlights the work conducted during the 1996
Some reference to earlier funded CICS Global Oceans research is also
3.1 Oceanic poleward heat transport as a function of OGCM
The idealized climate model (consisting of an energy-moisture
atmosphere, thermodynamic ice, and an ocean general circulation model,
hereafter referred to as the EMBM-TIM-OGCM -- see Fanning and Weaver,
previously developed by A. Fanning (a PhD. student) has been utilized to
the influence of horizontal resolution and parameterized eddy processes
poleward heat transport in the climate system. The results have recently
submitted for publication in Journal of Climate (Fanning and Weaver,
Model results suggest that as resolution is varied from 4o to
oceanic heat transport steadily increases. Owing to the strong constraint
imposed by the radiation balance at the top of the atmosphere, the
(ocean plus atmosphere) heat transport changes little throughout our
experiments. As a consequence, the atmospheric heat transport generally
decreases to offset the increasing oceanic transport.
The increase in oceanic heat transport as resolution increases is in
to previous ocean-only model studies (e.g., Cox, 1985; Bryan, 1987;
and Budich, 1992; Drijfhout, 1994). This result is also evidenced in a
series of ocean-only experiments where forcing is diagnosed from our
coupled model's equilibrium state (e.g. Haney, 1971; Han, 1984). Although
transport is generally higher in the coupled model, both models behave
similarly, with the primary increases occurring in the baroclinic gyre
component of the oceanic heat transport.
The conspicuous absence of an eddy transport compensation mechanism is
contrast to previous ocean-only model studies. Boning and Budich (1992)
eddy length-scales ranging from 50 to 175 km in their 1/6o model
highest resolution case studied here (0.25o) is adequate to resolve
these features, and spectral analysis of the basin mean kinetic energy
reveals variability (above 95% significance) in the range weeks to a
time scales are consistent with those found by Cox (1985,1987).
To investigate this contradiction further, an additional set of
experiments (more closely approximating the earlier studies) were
particular we wished to test whether an inclusion of salinity forcing
hence a breakdown of the non-acceleration theorem -- eg. McDougall, 1984;
1985; Bryan, 1991; Drijfhout, 1994) could explain the differences in our
results. Results suggest this is not the case, however. Restoring to
temperature alone (as in previous studies) results in higher heat
than the thermal/haline case (due to haline effects on the baroclinic
overturning transport). The latter two experiments are consistent with
previous cases, again increases in the baroclinic gyre transport result
increasing oceanic heat transport.
The thermocline adjustment time scale due to a perturbation (e.g.
switching resolution) should be that for a first mode baroclinic Rossby
cross the basin. Owing to the generally short integration time of these
(generally 10 years or less at highest resolution) it is not clear
time-variant compensation noted is eddy generated or rather an aliased
wave signal (see Cox, 1985,1987). The poleward oceanic heat transport can
scaled as TO ~ V delta(T) where delta(T) is the contrast
between an average thermocline temperature and an average deep water
temperature, and V is an average northward transport in the thermocline
southward transport below). Although the thermocline may undergo
a baroclinic Rossby wave time scale, the surface to deep water contrast
by an advective spin up time scale (order of hundreds of years).
earlier studies involving rather short integration times are not
remove the transients at deep levels (on long advective time scales), or
full equilibration of the meridional overturning circulation.
Although the identification of an eddy compensation mechanism found in
previous studies may be due to the rather short integration times
additional factors exist which may explain the differences we note. Cox,
(1985); Boning and Budich, (1992); and Drijfhout, (1994) each employed an
idealized continental shelf along the western boundary with a promitory
approximately 35oN. Sufficient nonlinearity, along with inertial
could give rise to enhanced eddy activity. Additionally, previous studies
utilized biharmonic closure schemes at highest resolution. Here we chose
do so since a change in closure ultimately alters the 'control' of the
Spontaneous decadal-intradecadal scale variability is found to exist in
higher resolution experiments. The intradecadal scale variability (period
years) is linked to the nonlinear advection terms in the momentum
This variability is similar to that noted by Cox (1985,1987) who found a
year variation in his model. Such variability (period 3 years) was also
by Boning and Budich (1992). Spontaneous decadal scale variability is
found in our present study and its existence is intimately linked to the
of the horizontal diffusivity we employ. Increasing the diffusivity in
resolution cases (below 0.5o) is enough to destroy the variability,
decreasing the diffusivity in our moderately coarse resolution cases
1o) is enough to induce the variability.
The decadal oscillation we describe is a thermally driven
oscillation, characterized by the turning on and shutting off of
activity in the northwestern corner of the model domain (cf. Weaver et
1994; Greatbatch and Zhang, 1995). The fact that decadal scale
exists in an idealized coupled ocean-atmosphere model (which does not
flux adjustments) is an intriguing result. While our model is highly
the question naturally arises: is the variability found in more complete
coupled models (e.g. Delworth et al., 1994) a feature of the coupled
determined by the flux adjustment employed as suggested by Weaver et al.
(1994), and Greatbatch and Zhang (1995). These results point to the
of higher resolution in the ocean component of coupled models, revealing
existence of richer decadal-intradecadal scale variability in models
require less parameterized diffusion.
3.2 Flux adjustments and their influence in coupled models
In another project, A. Fanning is currently investigating the
flux adjustments on the transient and long-term behavior of induced
change experiments. A version of the EMBM-TIM-OGCM has been configured
four-basin, two-hemisphere, sector geometry model which includes a
Mediterranean, Arctic, Pacific and Atlantic basin, joined at the southern
extent by a cyclic circumpolar ocean. This model has been spun up to near
equilibrium, and the resulting surface temperature and salinity fields
then used to spinup an ocean only model (using a restoring timescale of
days). At equilibrium, the resulting differences between the atmospheric
(in equilibrium with the surface SST's) and those implied by the
boundary conditions yields a flux adjustment such that the atmospheric
the coupled model and the oceanic state of the ocean-only model are
We therefore couple these states to yield a flux adjusted model, this
is formally equivalent to one of the standard procedures used in coupling
atmospheric model in equilibrium with fixed SST's to an ocean model
restoring to SST and SSS (e.g., Weaver and Hughes, 1996). The flux
non-flux adjusted model are then subjected to a 4 W/m2
increasing over 75 years) net heating perturbation.
Although still preliminary, results suggest that the transient behaviour
the first 75 years) of each model is similar, with results diverging
point. Additional experiments to test the sensitivity of the flux
model's initial conditions are still being performed, and these results
reported on at a later date.
We are also investigating the role of flux adjustments on interdecadal
variability. The numerical simulations of Delworth et al. (1994), using
GFDL coupled model revealed interdecadal variability of the thermohaline
circulation in the North Atlantic. It is not clear to what extent the
variability in that study is preconditioned by the heat and salt flux
adjustment fields required to prevent climate drift in the coupled model.
also unclear whether or not this variability is linked to coupled
ocean-atmosphere dynamics or to ocean dynamics alone. In order to do
this, the GFDL coupled model has been adapted to our local IBM cluster.
oceanic part of this model is now being run under fixed-flux boundary
conditions, made up of atmospheric fluxes (diagnosed from the atmospheric
at equilibrium) and the flux adjustment terms. If similar variability as
fully coupled experiments is found, we can conclude that the variability
to internal ocean dynamics alone.
3.3 Finite element modelling
Dr. Paul Myers, partially funded through the CICS Global Oceans
received his PhD and has moved to undertake postdoctoral research at the
University of Edinburgh in Scotland. He was working on the development of
global finite element model with specific applications to the circulation
the North Pacific and North Atlantic Oceans. The North Atlantic work was
reported in earlier Progress Reports. Here I only summarize the results
Pacific work which has appeared as Myers and Weaver (1996).
A finite element diagnostic model was used to study the circulation of
North Pacific Ocean. With the inclusion of the JEBAR term, the model
realistic picture of the circulation. All major currents were reproduced
the calculated transports agreeing well with observations. The three
dimensional velocity structure was diagnosed from the thermal wind
assuming a reference velocity at the bottom. This bottom reference
calculated from the Ekman, thermohaline and total transport (from the
element model) velocities. The diagnosed velocity fields were then
with a number of observational sections.
The effect of using different wind stress climatologies was also
to the dominance of the JEBAR term in the solution, the resulting
were all similar. Analysis of the seasonal cycle in the model supported
suggestion of Sakamoto and Yamagata (1995) that JEBAR rectification can
the decreased amplitude of the seasonal cycle and the out of phase
between observations and the predictions of flat-bottomed Sverdrup
Finally, density fields from 1955-1959 and 1970-1974 were used to
aspects of interpentadal variability in the North Pacific Ocean.
3.4 On the role of various subgrid-scale boundary layer
coarse resolution ocean models
Amongst the numerous sub-grid-scale parameterizations necessary in an
general circulation model, the influence of the momentum dissipation
dynamical boundary conditions has been relatively ignored compared to
mixing. However, the ability of the ocean to transport heat poleward may
very sensitive to such closures, since they are the only way the
circulation can depart from geostrophy and thus produce noticeable
velocities that feed the overturning. A thermohaline circulation model
developed for a Cartesian coordinate flat-bottomed beta-plane, based on
planetary geostrophic equations, in order to compare different
parameterizations of the momentum dissipation (Laplacian, biharmonic,
and none) and associated boundary conditions (no-slip, free-slip and
no-normal-flow). It is used at coarse-resolution for a mid-latitude basin
restoring boundary conditions for the surface density and no wind-stress.
Comparison with the GFDL MOM code confirms the negligible effects of
viscosity and total derivatives in the momentum equations. The surface
temperature fields and poleward heat transports are quite similar for the
steady-states obtained using the different viscosity schemes. However,
discrepancies in the bottom water properties and the velocity field show
order one effect of these closures on the mass transports. The
Laplacian friction produces a more satisfying interior circulation, in
agreement with geostrophy and Sverdrup balance, but generates excessively
vertical transports along the lateral boundaries (especially upwelling in
western boundary current - the Veronis effect - and downwelling in the
north-east corner). The meridional overturning is thus enhanced but
depth surface waters that are not as cold as the ones in the deep
Rayleigh friction with a no-normal-flow boundary condition (a vorticity
closure is used whose primary effect is to reduce vertical velocities
boundaries by allowing horizontal recirculation) induces a more efficient
thermohaline circulation with better agreement between convection regions
areas of downwelling, colder deep water, much weaker meridional
Veronis effect, but higher poleward heat transport. However, this
parameterization lacks physical justifications and is not as satisfying
Laplacian closure in terms of interior geostrophic and Sverdrup balance.
analysis of the correlations between the large scale diagnostics of these
models points out the Veronis effect as the major contributor to warm
water, diffuse thermocline, large overturning but weak poleward heat
in agreement with Böning et al. (1995). The role of dynamical
conditions is more important than the interior momentum dissipation in
this short-cut of the thermohaline loop.
This research has either been submitted (Huck et al., 1996a) or will be
submitted shortly (Huck et al., 1996b, c) for publication.
3.5 Flux Corrected Transport Algorithms and Sub-grid-scale Mixing in
Finally Weaver and Eby (1996) have implemented a flux-corrected
transport advection algorithm (Gerdes et al., 1991) into the GFDL MOM2
compared it with traditional second order centred difference advection
This technology has been passed to the CCC and may be implemented in the
generation of global ocean models.
The results from ocean model experiments conducted with isopycnal and
isopycnal thickness diffusion parameterizations for subgrid scale mixing
associated with mesoscale eddies were examined from a numerical
was shown that when the mixing tensor is rotated, so that mixing is
along isopycnals, numerical problems may occur and non-monotonic
which violate the second law of thermodynamics may arise when standard
difference advection algorithms are used. These numerical problems can be
reduced or eliminated if sufficient explicit (unphysical) background
diffusion is added to the mixing scheme. A more appropriate solution is
of more sophisticated numerical advection algorithms, such as the
flux-corrected transport algorithm. This choice of advection scheme adds
additional mixing only where it is needed to preserve monotonicty and so
retains the physically-desirable aspects of the isopycnal and isopycnal
thickness diffusion parameterizations, while removing the undesirable
noise. The price for this improvement is a computational increase.
Böning, C.W. and R.C. Budich, 1992: Eddy dynamics in a
equation model: Sensitivity to horizontal resolution and friction. J.
Oceanogr., 22, 361-381.
Bryan, K., 1987: Poleward buoyancy transport in the ocean and mesoscale
J. Phys. Oceanogr., 16, 927-933.
Bryan, K., 1991: Poleward heat transport in the ocean. A review of a
of models of increasing resolution. Tellus, 43, 104-115.
Cox, M.D., 1985: An eddy resolving numerical model of the ventilated
thermocline, J. Phys. Oceanogr., 15, 1312-1324.
Cox, M.D., 1987: An eddy resolving numerical model of the ventilated
thermocline: Time dependence, J. Phys. Oceanogr., 17,
Delworth, T., S. Manabe and R.S. Stouffer, 1994: Interdecadal variations
thermohaline circulation in a coupled ocean-atmosphere model. J.
Climate, 6, 1993-2011.
Drijfhout, S.S., 1994: Heat transport by mesoscale eddies in an ocean
circulation model. J. Phys. Oceanogr., 24, 353-369.
Fanning, A.F., and A.J. Weaver, 1996a: An atmospheric energy-moisture
model: Climatology, interpentadal climate change, and coupling to an
J. Geophys. Res., 101, 15,111-15,128.
Fanning, A.F., and A.J. Weaver, 1996b: A horizontal resolution and
sensitivity study of heat transport in an idealized coupled climate model
Gerdes, R., C. Koeberle and J. Willebrandt, 1991: The influence of
advection schemes on the results of ocean general circulation models.
Dynamics 5, 211-226.
Greatbatch, R.J., and S.Z. Zhang, 1995: An interdecadal oscillation in an
idealized ocean forced by constant heat flux. J. Climate,
Han, Y.J., 1984: A numerical world ocean general circulation model, Part
baroclinic experiment, Dyn. Atmos. Oceans 8, 141-172.
Haney, R.L., 1971: Surface thermal boundary condition for ocean
models. J. Phys. Oceanogr., 1, 241-248.
Huck, T., A. J. Weaver, and A. Colin de Verdiere, 1996a: The effect of
dissipation parameterizations in coarse-resolution thermohaline
models. J. Phys. Oceanogr., submitted.
Huck, T., A. Colin de Verdiere, and A. J. Weaver, 1996b: The effect of
dissipation parameterizations in coarse-resolution thermohaline
models: geostrophy, Sverdrup balance and Veronis effect. in preparation.
Huck, T., A. J. Weaver, and A. Colin de Verdiere, 1996c: Decadal
simplified models of the thermohaline circulation. in preparation.
McDougall, T.J., 1984: The relative roles of diapycnal and isopycnal
subsurface water massconversion. J. Phys. Oceanogr., 14,
Myers, P.G., and A.J. Weaver, 1996: On the circulation of the North
Ocean: Climatology, seasonal cycle and interpentadal variability,
Oceanogr., in press.
Sakamoto, T. and T. Yamagata, 1995: Seasonal transport variations of the
wind-driven ocean circulation in a two-layer planetray geostrophic model
continental slope. J. Mar. Res., submitted.
Weaver, A.J. and M. Eby, 1996: On the numerical implementation of
schemes for use in conjunction with various mixing schemes in the GFDL
model. J. Phys. Oceanogr., in press.
Weaver, A.J. and T.M.C Hughes, 1996: On the incompatibility of ocean and
atmosphere models and the need for flux adjustments. Climate
Dynamics, 12, 141-170.
Weaver, A.J., S.M. Aura, and P.G. Myers, 1994: Interdecadal variability
idealized model of the North Atlantic, J. Geophys. Res.,
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