TECHNICAL PROGRESS REPORT
to Scripps Institution of Oceanography
for NOAA OFFICE OF GLOBAL PROGRAMS
Subcontractor: University of Victoria
SEMI-ANNUAL REPORT PERIOD: April 1, 1996 through October 31, 1996
Agency: National Oceanic and Atmospheric Administration,
Project Title: The Lamont/Scripps Consortium for Climate Research -
of Climate Change.
NOAA Award No: NA47GP0188
Principal Investigator: Andrew Weaver
Project Period: May 1, 1994 through April 30, 1997
Budget Period: April 1, 1996 through October 31, 1996
Performance Report Completed: October 8, 1996
Below I summarize the progress on the research funded through the NOAA
Lamont/Scripps Consortium for Climate Research.
1 Decadal Variability in a coupled Energy-Moisture Balance Model
-Ocean General Circulation Model (OGCM) - Thermodynamic Ice Model
Ed Wiebe, an MSc. student, is currently utilizing a version of
coupled EMBM-TIM-OGCM. The model employs the same horizontal resolution,
geometry, and forcing as that described by Fanning and Weaver (1996a)
it is now coupled to the GFDL MOM2 model (Pacanowski, 1995). The only
appreciable difference between the models is the implementation of the
corrected tracer algorithm (Gerdes et al., 1991; Weaver and Eby, 1996)
explicit convection scheme of Rahmstorf (see Pacanowski, 1995). Of
importance is the generation of spontaneous decadal-scale variability
26 years) centered in the North Atlantic. Oscillations in the meridional
overturning streamfunction span about 10 Sv in magnitude with
temperature anomalies of almost 5oC.
We are currently analysing the mechanism for the decadal oscillation,
continuing the model integration time to ascertain whether centennial
variability (centred in the Southern Ocean) is a robust feature of the
climate system, or merely a transient phenomena.
In addition to this analysis Ed Wiebe has also made extensive
a collection of existing IDL routines used to visualize the output from
coupled model. Extensions include the capability to calculate mean fields
plot anomalies. In addition, movies of time-dependent phenomena can be
and saved in a compact form for later viewing. This software is
adaptable and may be of use to other researchers in the field of ocean
2 Simulation of the Younger Dryas event
This version of the EMBM-TIM-OGCM has been used to investigate the
between the last glaciation and the present Holocene (the Younger Dryas -
hereafter the YD). The traditional viewpoint is that the YD was triggered
the diversion of meltwater (due to the retreating Laurentide Ice Sheet)
the Gulf of Mexico to the St. Lawrence (Broecker et al., 1988). In an
to clarify the temporal and geographical roles of meltwater discharges on
triggering the YD, estimates for volumes of runoff from the Laurentide
sheet (Teller, 1990) were applied for the 1500 year period encompassing
cold episode. Model results indicate the traditional Laurentide meltwater
diversion theory is insufficient to induce a YD climate signature,
preconditioning by the pre-YD meltwater discharge in conjunction with the
The model predicted YD climate shift is global in nature, and is
linked to North Atlantic deepwater (NADW) formation. The global
circulation provides an interhemispheric teleconnection with the Southern
Ocean, while changes in the atmospheric heat transport (reacting to a
redistribution of oceanic heat transport) provides a mechanism for
teleconnection. Changes in model surface air temperature generally agree
the pattern and magnitude of YD temperature change deduced from
reconstructions based on existing paleothermometers.
Our results indicate that supplying both pre and post-YD meltwater
results in a total collapse of the North Atlantic conveyor. In the
additional model feedbacks, this state appears to be relatively stable,
equivalent to the Southern Sinking state identified by Manabe and
(1988). Reestablishment, if it were to occur, would appear to be a
process as in previous ocean-only model studies under a polar halocline
catastrophe (e.g., Marotzke, 1989; Weaver and Sarachik, 1991). If instead
allow for the effects of this climate state to feedback onto the surface
reestablishment occurs on a faster time scale. This is due to an
surface salinification through latent heat loss and Ekman transport of
salinity anomaly out of the region of deepwater formation. This result is
consistent with previous studies of freshwater perturbations on the North
Atlantic Conveyor (e.g., Schiller et al., 1996; Mikolajewicz, 1996),
unlike these studies the model settles into a new equilibrium state with
reduced NADW formation as in Rahmstorf (1994). We also note that unlike
same studies, the time scale for reestablishment of NADW, and hence
of the YD signal is an advective spinup time scale (order 1000 years) as
opposed to decadal-century. The reason for this discrepancy is unclear,
be associated with the used of fixed salt flux fields employed by those
While we have not explicitly addressed the question of the role of the
Fennoscandian Ice Sheet's demise on the YD, our results suggest at most
would have prolonged the YD episode. This raises a final point, is the
advective spinup time scale found here representative of the time scale
YD termination? Considering the d18O record at Summit
al., 1993), the bulk of warming signaling the transition from the YD to
Holocene occurred over a 200 year period (although full termination took
longer). So, it appears that further treatment of model feedbacks (e.g.,
effects) and perhaps radiative forcing (due to increasing levels of CO2
needed to investigate the Younger Dryas termination further. These
recently been submitted for publication (Fanning and Weaver, 1996c).
3 Paleoclimatic response of the closure of the Isthmus of
The role of meridional heat transport by the oceans is one of the most
important current research topics in climate, and many difficulties exist
measuring it (Bryden, 1993). There exist, however, natural "experiments"
earth's history, whereby changes in ocean gateways have caused changes to
ocean heat transport (as well as other important changes). There exist
increasing amounts of data on observations of paleoclimate. Ocean models
been run in the past (Mikolajewicz, Maier-Reimer, et al., 1990, 1993) to
investigate paleocirculation, but a fully coupled climate model is
investigation of important climate questions such as the changing roles
ocean and atmospheric heat transport under different gateway cases, and
possible links to glaciation and initiation of glacial cycles
The effects on ocean circulation and world climate of changes in ocean
gateways is being investigated. In particular, attention is being
the role of meridional heat transport by the ocean with different
of the Isthmus of Panama and Drake Passage gateways, the two most recent
changes in gateways (about 3 and 30 Million years ago, respectively), and
relation to glacial events in the paleoclimatic record.
The experiments are being performed using the coupled EMBM-OGCM-TIM.
model is ideal for this project as it does not require flux adjustments,
imposing as little modern day observational bias as possible.
Research is ongoing, including running the coupled model under the open
Isthmus of Panama configuration. Results thus far agree with previous
only) model studies (Mikolajewicz, Maier-Reimer, et al., 1990, 1993).
data obtainable from the coupled model, which is hoped to provide insight
the changed heat transport and relation to climate and glaciation, has
analysed. Initials results have been published in Murdock et al. (1996).
Drake Passage runs are almost complete, and analysis of their results
begin upon completion of the Isthus of Panama analysis.
4 Decadal Variability in the GFDL Coupled Model
The numerical simulations of Delworth et al. (1993), using the fully
ocean-atmosphere model developed by the Geophysical Fluid Dynamics
(GFDL) in Princeton, NJ, showed interdecadal variability of the
circulation in the North Atlantic. However, it is still unclear if this
variability is a coupled ocean-atmosphere or an ocean-only phenomenon.
In order to clarify this problem, the GFDL coupled model, previously run
Cray at GFDL, has been adapted to our IBM machines. The oceanic part of
model has been spun-up by Dr. S. Valcke (an NSERC postdoctoral fellow) to
equilibrium in the same configuration as in the run of Delworth et al.
This ocean model is now being run under fixed-flux boundary conditions,
fluxes being the sum of the atmospheric fluxes (diagnosed from the
model alone at equilibrium) and the flux adjustment terms (artificial
used in coupled models to correct the fluxes going into the ocean in
remove systematic climate drifts). If the same decadal variability as in
fully coupled experiments is observed, or if it appears when a stochastic
forcing is added to the fixed-flux boundary conditions, we will conclude
the variability is due to the internal ocean dynamics.
Dr. S. Zhang is also working with the GFDL model. He has spent a good
time trying to make the atmospheric component of the model run more
on our local workstation cluster. Dr. Zhang is attempting to obtain a
of the GFDL coupled model which does not require flux adjustments. He has
identified a number of problems and is currently seeking methods to
them. These problems include: a) an ocean model surface flux which is
significantly weaker than that produced by the atmospheric model. This is
responsible for a large part of the flux adjustment; b) a very strong
salinity adjustment which is related to the melting of ice; c)
strong restoring in the ocean with same strength in temperature and
during the oceanic spin up. If all of the problems are solved, then the
component of the coupled model will probably not be the source of climate
In the atmospheric component of the coupled model Dr. Zhang has reduced
frequency of synoptic eddies while retaining all the model physics. He
implemented a number of other acceleration techniques in an attempt to
the atmospheric model up. Specifically, this is achieved by using the
time step in the dynamical code and using a much longer time step for all
processes. This effectively assumes that the response time of the
much shorter than the ocean, and that only the mean of synoptic system is
important for the energy balance and that its variance only generates
least on timescales longer than a decade.
5 Decadal variability in OGCMs with various subgrid-scale boundary
Thierry Huck, a PhD student, is using locally-developed planetary
geostrophic ocean models under various sub-grid-scale boundary layer
parameterizations to study the mechanism of decadal variability found in
only models (Greatbatch and Zhang 1995). Under flux boundary condition on
surface density, a parameter sensitivity analysis has been carried out.
horizontal tracer diffusivity has a critical damping effect, while the
diffusivity (which determines the strength of the thermohaline
enhances the oscillatory behaviour. The inclusion of a parameterization
convection (excluding the effect of vertical velocities) and the
are found not to be necessary in sustaining the variability, and so
role of Rossby waves in the mechanism (Winton 1996). The influence of the
lateral boundaries along which convection takes place (weakening the
stratification so that Kelvin waves may propagate with decadal time
Greatbatch and Peterson, 1996) has been rejected in two ways: 1) By
northward the polar boundary (by several tens of degrees), with no
forcing in the extended area (a buffer zone where the stratification
strong). In this case the oscillatory behaviour is weakly modified. 2)
symmetric forcing in an f-plane model (that is twice as wide as in the
experiments), so that a 'tropical' thermocline is present along both
boundaries of the basin. In this case the oscillation is more profoundly
affected but even stronger. None of these major changes weaken the
which is maximum in the region of Gulf Stream separation and its eastward
Comparisons between the primitive equation GFDL-MOM code and the
geostrophic models at various horizontal resolution suggest that the
resolution, the more likely the occurrence of decadal variability (even
changing the horizontal diffusivity along with the resolution). In
below 100 km resolution, the non-linear terms and the time-derivative in
horizontal momentum equations play a driving role in producing the
In the vertical, a discretization as crude as two levels can sustain the
Since the oscillations have a strong signature in the zonally-averaged
a two-dimensional mechanism is being investigated. This would be
with the findings of decadal variability within the idealized coupled
ocean-atmosphere of Sarvanan and McWilliams (1995). With non-steady
forcing in a zonally-averaged ocean model, we expect the ocean to
decadal time-scales associated with the overturning period.
We are presently writing up this work in Huck et al. (1996c).
6 Climate Variability as a function of mean climatic state
Over the next two years I hope to continue improving our understanding
mechanisms of decadal-interdecadal climate variability through the
of increasingly more sophisticated coupled models. The coupled
represents the simplest form of our coupled modelling studies. We shall
continue to use it to explore simple thermodynamic feedbacks and gain
into what results we might expect and which experiments we should
with the more complicated GFDL and CCC coupled models. In addition, the
and EMBM-OGCM-TIM coupled model will be used to investigate questions
concerning the existence of variability in the coupled climate system and
it varies as the mean climatic state changes (i.e, does the
decadal-interdecadal climate variability found in the coupled model
CO2 is increased in the atmosphere. Since the GFDL coupled model is far
computationally efficient than the CCC coupled model, it is hoped that
insight we gain from it will allow us to better streamline the future
experiments that will be performed with the more sophisticated CCC
7 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.
8 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.
9 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.
10 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.
11 Flux Corrected Transport Algorithms and Sub-grid-scale Mixing in an
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.
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