Global Ocean Modelling
CICS - #11 --Global Oceans
A proposal submitted to the Canadian National Climate Research
Committee for renewal of the #11 CICS Global Oceans project
for a further three year term
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, B.C., V8W 2Y2
The Canadian Institute for Climate Studies (CICS) Global Oceans
is now in its third year and I am reapplying to the Canadian National
Research Committee for a renewal of this project for a further three
CICS funds were initially used under a joint agreement between the
of Victoria, CICS and the Atmospheric Environment Service (AES) to
with additional research time and support to assist the Canadian Centre
Climate Modelling and Analysis (CCCma) in developing global ocean general
circulation models (OGCMs) for the purpose of coupling them to the CCCma
atmospheric general circulation model (AGCM). The initial phase of this
is now complete in that a fully-coupled AOGCM has been developed by the
This coupled model has now been used to undertake climate change
The building and improving of the coupled model is a continuous process
is for this reason that I seek continued support for the project. While
has been developed (in collaboration with Warren Lee at the CCCma) we are
seeking methods for improving this model through the incorporation of
numerics and subgrid scale parameterizations.
Michael Eby is currently employed as a full time programmer/Research
in my lab to develop improved global ocean models for the purpose of
undertaking climate change experiments. We are coupling these new OGCMs
our locally-developed atmospheric energy-moisture balance model (Fanning
Weaver, 1996) and, via technology transfer to the CCCma, to the CCCma
arrived at the University of Victoria following the departure of Dr. T.
to Princeton University to work with Dr. J. Sarmiento. His duties include
development of new numerical schemes to handle advection, the
new subgrid scale mixing schemes, and the undertaking of numerous
studies to various model parameterizations (see e.g., Weaver and Eby,
The knowledge gained from these experiments is transferred to the CCCma
modelling groups through discussions at our weekly coupled modelling
Unlike other network proposals which fund teams of researchers across
this proposal has only one principal investigator. The CCCma moved to
to take advantage of local expertise in ocean modelling. In particular, I
been heavily involved in the development of the new coupled model through
the participation in weekly coupled model meetings and the development,
and experimenting with various global ocean models and simple coupled
The purpose of this proposal is therefore to take advantage of the unique
opportunity afforded by my proximity to the CCCma modelling group. The
does not have extensive ocean modelling expertise within their division
OGCM is a crucial component of the coupled model. By maintaining strong
expertise in ocean modelling down the hall from the CCCma we will be able
continue our fruitful collaboration into the development of the next
of ocean models.
The science plan for the research that I propose to do in support of
modelling activities is outlined in the attached proposal which was
submitted to NSERC in the form of a strategic grant. In my NSERC
grant I stated that I would seek support from the CICS (via the global
project) for Michael Eby. In addition, I am applying for partial support
computer systems manager (Daniel Robitaille) and some small operating
SCIENCE PLAN SUBMITTED TO NSERC AS A STRATEGIC PROJECT
This proposal is being written to support new research linking
expanding upon my NOAA Scripps-Lamont Consortium on the Ocean's Role in
funding and my Canadian Institute for Climate Studies (CICS) Global
Variability grants. In addition, this research will be interrelated with
activities being conducted in the Canadian Centre for Climate Modelling
Analysis (CCCma) at the University of Victoria. Over the last few years,
developed a large research group of about 15 students, research
support personnel and it is hoped that this level of research activity
maintained by the strategic, CICS and NOAA grants, especially in light of
NSERC's recent decision not to fund the Canadian WOCE effort (although my
individual proposal received extremely favourable reviews -- available
Over the next five years I hope to continue improving our
of the mechanisms of decadal-interdecadal climate variability through the
development of increasingly more sophisticated coupled models. A coupled
Energy-Moisture Balance Model (EMBM)/Thermodynamic Ice Model (TIM)/Ocean
General Circulation Model (OGCM) will represent the simplest form of our
coupled modelling studies. We shall use it to explore simple
feedbacks and gain insight into what results we might expect and which
experiments we should undertake with the more complicated GFDL and CCCma
coupled models. In addition, the GFDL coupled model will be used to
questions concerning the existence of variability in the coupled climate
and how 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?). The GFDL coupled model is less
sophisticated and therefore less computationally costly than the CCCma
model and it is hoped that the insight we gain from it will allow us to
streamline the future experiments that will be performed with the more
sophisticated CCCma coupled model.
The specific objectives of this proposal are to:
1 understand processes of decadal-interdecadal variability using a
OGCM-EMBM-TIM. The OGCM will have variable resolution ranging from 4o
x 4o to 1/4o x 1/4o.
2 understand the role of eddies in the poleward transport of heat and
3 use CFC-11 as a tracer to validate the climatology of OGCMs as well
their sub-grid scale mixing parameterizations.
4 develop simple models to understand the processes involved in
decadal-interdecadal climate variability.
5 investigate the effects of sub-grid-scale OGCM parameterizations on
decadal-interdecadal climate variability.
6 use a global OGCM and coupled OGCM-EMBM-TIM to examine
associated with processes in the North Atlantic.
7 use the GFDL coupled model and the coupled EMBM-OGCM-TIM to
processes of decadal-interdecadal variability in the coupled climate
its dependence on the mean climatic state.
8 undertake a simulation of the Younger Dryas event using the coupled
EMBM-OGCM-TIM. In addition the climatic effects of opening and closing
gateways will be examined.
This section has been partitioned into a number of subsections,
which deals with a particular subcomponent of the project. Throughout the
section scientific questions are posed and a brief description of how
questions will be addressed is also presented. More details of the
to be used can be found in section 4.
3.1 Decadal-interdecadal variability
There is substantial evidence for decadal climate variability in
air-sea-ice climate system. Recent hypotheses suggest that decadal
in the Pacific may be either linked to changes in the El Niño/La
Niña signal in the equatorial Pacific (Trenberth and Hurrell,
to midlatitude air-sea instabilities (Latif and Barnett, 1994). It is not
whether similar mechanisms exist in the Atlantic or whether there is a
relationship between the Pacific and Atlantic modes of variability. Below
recent evidence is presented to show that decadal variability can exist
uncoupled ocean models in basins where deep water formation occurs.
Under mixed boundary conditions self-sustained internal variability on
decadal-interdecadal timescale can exist in ocean models (e.g., Weaver
Sarachik, 1991; Weaver et al., 1991). Often this variability is linked to
turning on and shutting off of high latitude convection and the
generation and removal of east-west steric height gradients which cause
thermohaline circulation to intensify and weaken over a decadal
Horizontal advection sets the oscillation timescale. Decadal internal
variability still persists or may even be excited when a stochastic
is added to the freshwater forcing (Weaver et al., 1993; Weisse et al.,
It has also recently been shown that decadal internal oceanic variability
also exist in ocean models driven only by thermal forcing (Weaver et al.,
Greatbatch and Zhang, 1995; Winton, 1996).
The existence of such model results makes it difficult to interpret
effects of decadal variability in current coupled climate models which
flux-adjustments (see Weaver and Hughes, 1996). This follows since if the
adjustment is large in magnitude, one might expect that the oceanic
is determined by this structure, with the higher frequency air-sea flux
variability providing a stochastic forcing which simply excites it.
In this proposal advances to our understanding of decadal-interdecadal
variability will be achieved through the use of coupled models of varying
complexity which do not employ flux adjustments. This will expand upon
early uncoupled OGCM results. The use of the EMBM, developed by Fanning
Weaver (1996), will allow for simple thermodynamic feedbacks. In
GFDL coupled model will be used to investigate the dependence of
decadal-interdecadal climate variability on the mean climatic state.
Furthermore, the use of OGCMs of varying resolution will allow for both
analysis of the effects of horizontal boundary layers (as discussed in
1996) and the role of eddies in decadal-interdecadal climate
3.2 Centennial-timescale variability
A fundamental period for oceanic model variability also occurs on
overturning timescale (Mikolajewicz and Maier-Reimer, 1990; Winton and
Sarachik, 1993; Weaver et al., 1993). The presence of a positive salinity
anomaly in the low latitude surface regions tends to slow the meridional
overturning slightly, since thermal effects tend to accelerate the
circulation and haline effects act to brake it. The weakened thermohaline
circulation is then more affected by the specified flux on salinity which
to intensify the positive anomaly at low latitudes and induce a negative
anomaly at high latitudes. When the low latitude salinity anomaly reaches
high latitudes, convection and an intensified thermohaline circulation
The whole process begins anew when the saline anomaly resurfaces at low
latitudes. Thus the oscillation has a slow phase, which is associated
saline anomaly being at low surface latitudes, and a rapid phase in which
salinity anomaly is at high latitudes or in the deep ocean.
Whether or not such centennial timescale variability exists in the
climate system, and the extent to which it is modified by allowing for
feedbacks within the coupled system, is still an open question.
3.3 Millennial-timescale variability
While variability on this timescale is beyond the central theme
proposal, we will be able to address an open and important question
the potential existence of the so-called flushes (discussed
These catastrophic climate swings have previously only been observed in
uncoupled OGCMs. If they are found to survive in coupled models then
mechanism serves as a potential explanation for variability found in
perhaps future, climates.
Ocean GCMs forced using mixed boundary conditions in which high latitude
freshening is strong are often susceptible to polar halocline
Associated with the polar halocline catastrophe is a collapsed
circulation state which is not stable since low latitude diffusion acts
the deep waters warm and saline with horizontal diffusion acting to
these waters laterally. Eventually, at high latitudes the deep waters
sufficiently warm so that the water column becomes statically unstable
rapid convection sets in. The result is a flush (Marotzke 1989;
and Sarachik, 1991; Wright and Stocker, 1991) in which a violent
occurs whereby the ocean loses all the heat it had taken thousands of
store in a matter of a few decades. At the end of the flush, high
freshening eventually suppresses convection and the thermohaline
once more collapses. The collapse/flush sequence repeats itself with the
timescale between flushing events being set by diffusion.
The existence of flushes is linked to both the relative importance of
freshwater flux over thermal forcing, and the strength of the wind
compared to the high latitude freshening. As the stratification is
homogeneous in near-surface layers at high latitudes, the Ekman-driven
overturning cells contribute very little to the meridional heat and salt
transports. This situation changes once the polar halocline catastrophe
occurred as the surface layer becomes very fresh and the equatorward
transport of fresh water is compensated for by a poleward return
more saline water, resulting in a net poleward salt transport. Moreover,
poleward salt transport due to the horizontal subtropical gyre increases
substantially during the collapsed phase of the thermohaline
High-latitude surface freshening may be counteracted by the wind-driven
transport to make the high-latitude surface waters sufficiently saline,
deep convection resumes and the thermohaline circulation reestablishes
If the surface freshening cannot be compensated for by the wind-driven
transport, the thermohaline circulation remains in the collapsed state
flush sets in. When a stochastic term is added to the mean freshwater
forcing field the frequency of flushing events increases while their
decreases, with increasing magnitude of the stochastic term. This follows
the probability of a sufficiently large evaporation anomaly increases so
ocean need not warm as much before a flush occurs.
As mentioned above, these flushes have never been found in
climate models. This may be due to the fact that feedbacks in the coupled
system prevent their occurrence. On the other hand, no coupled model has
been run long enough to quantitatively analyze this. Indeed, the coupled
OGCM-AGCM of Manabe and Stouffer (1994) reveals that under 4xCO2 forcing,
thermohaline circulation collapsed and remained collapsed for 500 years.
situation with no ventilation of the deep waters cannot exist
indefinitely as a
stable equilibrium and so deep water must eventually form. Whether deep
will form through a flush or via a more gentle re-establishment is unknow n.
Through the use of our hierarchy of coupled models we will be able to
3.4 Surface boundary conditions
Much research has been conducted to develop improvements
so-called restoring boundary conditions and mixed boundary conditions
have often been used by the ocean modelling community. In restoring
conditions, surface temperatures and salinities in an ocean model are
with a specified timescale, to some climatological values whereas under
boundary conditions, a specified flux on salinity is used in conjunction
the restoring condition on temperature.
Sea surface temperature anomalies, regardless of their scale, are
damped under restoring boundary conditions. In reality, the damping time
depend on the scale of the anomaly since latent and sensible heat loss
inefficient mechanisms for the removal of large-scale sea surface
anomalies. This follows since heat lost over one part of the ocean must
advected by the atmospheric winds away from the anomaly for it to be
effectively removed. Therefore, as pointed out by Bretherton (1982), in
limit of a global scale anomaly, radiational damping (long timescale) is
only mechanism for its removal. These ideas have been utilized in the
development of a number of improved parameterizations of the thermal
boundary condition (e.g., Zhang et al., 1993; Mikolajewicz and
1994; Rahmstorf and Willebrand, 1995).
Recent developments have also taken place in the improvement of the
condition on salinity in ocean models. Hughes and Weaver (1996) developed
salinity boundary condition which incorporates the dependence of
sea surface temperature. The internal variability of the thermohaline
circulation documented in sections 3.1-3.3 was found to still exist under
new boundary condition although it was modified slightly.
A cautionary note should be added here regarding the variability found
OGCMs under mixed boundary conditions. It is evident from the new surface
boundary condition parameterizations discussed in this subsection that
variability may be either damped or modified when more realistic surface
boundary conditions are used. One should therefore view their results
caution until they are verified with fully-coupled ocean-ice-atmosphere
Through the coupling of the OGCM to both an EMBM and a simplified
AGCM we will be able to quantitatively examine whether or not feedbacks
coupled system dampen, enhance or excite such variability.
3.5 Sub-grid-scale parameterizations
Recent advances have been made in the parameterization of
scale mixing associated with mesoscale eddies (Gent and McWilliams, 1990
hereafter GM; Danabasoglu et al. 1994) in coarse resolution OGCMs.
et al. (1994) have illustrated promising initial results from experiments
their parameterization for mesoscale eddy-induced mixing. When their
parameterization was incorporated into a global ocean model the
became sharper, the deep ocean became colder, the meridional transport of
and salt became greater and the overturning in the North Atlantic
meridionally. All of these features are improvements on the results
global models using traditional horizontal/vertical mixing schemes.
Furthermore, in the Southern Ocean, the Deacon Cell vanished through an
eddy-induced cancellation of the mean flow advection of tracers. The
a reduction in the heat and salt transport by the ocean across the
Robitaille and Weaver (1995) examined three sub-grid scale mixing
parameterizations (lateral/vertical; isopycnal/diapycnal; GM) using a
ocean model in an attempt to determine which yielded the best ocean
Observations and model CFC-11 distributions, in both the North and South
Atlantic, were used in the model validation. While the isopycnal/
mixing scheme did improve the deep ocean potential temperature and
distributions, when compared to results from the traditional
mixing scheme, the CFC-11 distribution was significantly worse due to too
mixing in the southern ocean. The GM parameterization, on the other hand,
significantly improved the deep ocean potential temperature, salinity and
CFC-11 distributions when compared to both of the other schemes. The main
improvement came from a reduction of CFC-11 uptake in the southern ocean
the "bolus" transport canceled the mean advection of tracers and hence
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
artifact of the particular sub-grid scale mixing schemes used.
These initial results have inspired us to continue to use CFC-11 as a
for validating the climatology of our ocean models and in particular
sub-grid scale parameterizations. We will examine the effects of
dependent diapycnal mixing which will be added to the GM
addition, the role of boundary layer versus interior mixing will be
through the inclusion of spatially varying diapycnal mixing. Furthermore,
surface lateral mixing will be added as a parameterization of diabatic
processes in the surface mixed layer arising from the presence of surface
fluxes and small scale processes acting in series with the mesoscale
3.6 Eddy heat and salt transport
One of the most fundamental, yet unanswered, questions regarding
ocean's role in climate is whether or not eddies are important in
heat and salt poleward. Numerous studies (e.g., Bryan, 1991; Böning
Budich, 1992; Drijfhout, 1994) have suggested that eddies do not play a
significant role in the transport of heat and salt poleward. They suggest
the heat and salt transport is dominated by the meridional overturning
transport in the North Atlantic.
All of the above works suffer from a major shortcoming since all the
experiments were conducted using ocean-only models. In the limit of
fluxes of heat and freshwater, the oceanic heat and salt transports are
necessarily predetermined at equilibrium since the divergence of the
gives the zonally-averaged flux. Thus, whether or not eddies are resolved
not change the total heat or salt transport at equilibrium. All of the
studies also used restoring boundary conditions on temperature and
While these boundary conditions do not impose an exact constraint on the
oceanic poleward heat and salt transports at equilibrium, they do largely
determine the thermocline structure and hence may clamp the ability of
ocean models to freely regulate their heat and salt transport. In
will include salinity in our analysis. Previous works utilized buoyancy
alone so that eddies are aligned along isopycnals and hence no net heat
transport occurs by their presence. In the present proposal, isotherms
isopycnals will no longer coincide and a net heat transport should be
if eddies propagate across isopycnals.
To quantitatively address this problem we shall use an idealized OGCM of
North Atlantic with resolution ranging from 4o x 4o to 1/4o x
1/4o coupled to an EMBM. The only constraint on the coupled system will then
incoming solar radiation at the top of the atmosphere. As the ocean model
resolution increases, the coupled system will be allowed to adjust,
the specified constraint on the incoming solar radiation. By partitioning
heat transport into time mean and time dependent terms we will be able to
whether or not transient eddies are indeed important for the transport of
and salt poleward. In addition, by examining the time-mean component we
able to quantify the importance of the barotropic versus overturning
3.7 Global teleconnections
The North Atlantic Ocean is fundamentally linked with the rest of
world's ocean via communication through the Antarctic Circumpolar Current
the Southern Ocean. The extent to which processes occurring in the North
Atlantic affect the circulation in other oceans and vice versa is still
A fundamental question regarding the North Atlantic and its relationship
the rest of the world's oceans concerns the reason why the North Atlantic
deep water yet the Pacific does not. Warren (1983) singles out the more
stratification of the North Pacific (where surface waters are on average
psu and deep waters 34.6-34.7 psu compared to 34.9 and 34.9-35.0 psu in
North Atlantic) as the explanation and identifies a number of causes for
For example, there is nearly twice as much evaporation over the North
as over the North Pacific; the water introduced into the North Atlantic
lower latitudes is more saline than its counterpart in the North Pacific,
the residence time of this water in the region of net precipitation at
latitudes is shorter. However, as Warren (1983) concedes, none of these
is independent of the already existing thermohaline circulation in the
Atlantic. The salinities of the surface and bottom water masses are
because one is being actively converted into the other; the higher
is related to higher sea surface temperatures, which are due in part to
greater northward advection of warm subtropical water by the Gulf Stream
the Gulf Stream itself is partly thermohaline-driven. Finally, the
contribution to the western boundary current and the active conversion of
surface to deep waters accounts for the shorter residence time in the
Many geographical clues also exist regarding the asymmetry of the
circulation in the two oceans. The first and most obvious one is that the
Atlantic extends farther north than does the Pacific, and has a deeper
connection with the Arctic. Furthermore, the Pacific is twice as wide as
Atlantic. In connection with the Broecker et al. (1990) argument that the
interbasin atmospheric water vapour transport drives the conveyor, the
Isthmus of Panama at low latitudes allows westward freshwater export via
trade winds (Weyl, 1968), while the Rocky Mountains block an opposite
higher latitudes. Schmitt et al. (1989) have proposed that the narrower
of the Atlantic compared to the Pacific would cause a greater fraction of
area to be susceptible to the incursions of cold dry continental air that
favour evaporation. Within the ocean, the salty Mediterranean outflow
in preconditioning intermediate water flowing into the Norwegian Sea to
deep convection, as does the exchange with South Indian waters off the
Good Hope. Finally, Reid (1961) has hypothesized that the poleward
South America compared to South Africa might impede the transport of
out of the Pacific by the ACC.
The global coupled EMBM-TIM-OGCM will be used to try and unravel which
different mechanisms is the most crucial in determining the observed
for deep water formation in the Atlantic instead of the Pacific.
As discussed in section 3.1, numerical models have revealed
decadal-interdecadal variability of the North Atlantic thermohaline
circulation. It is not clear, however, to what extent this variability
the rest of the world oceans. In addition, it is not clear how changes in
basins would affect the existence of North Atlantic decadal-interdecadal
variability. For example, Toggweiler and Samuels (1993, 1995) suggest
southern ocean winds may regulate the rate of North Atlantic Deep Water
formation. Hughes and Weaver (1994), however, suggest that the winds over
Southern Ocean are only one of a number of ways of regulating NADW
In order to examine the interrelationships between the North Atlantic and
global oceans, the global coupled EMBM-TIM-OGCM will once more be used in
various configurations as outlined in section 4.2.
Furthermore, we wish to address the debate as to where the return flow
occurs. Coarse resolution models (e.g. Hirst and Godfrey, 1993) suggest
most of the return flow happens in the cold water route through Drake
Eddy resolving models, which essentially prescribe the deep temperature
salinity structure of the ocean, suggest that most of the return flow
in the warm water route via eddy generation in the Agulhas Current
Chervin, 1992). Observations suggest that some combination of the two
appropriate (Broecker 1991). In this project we shall apply perturbations
passive and active) to the North Atlantic thermohaline circulation to
the global equilibrium response, the transient adjustment phase as well
paths through which the perturbations travel.
In this section more details are provided as to the methodology
will be used to address the questions posed in section 3.
4.1 The EMBM-TIM
Recently Augustus Fanning, a PhD student working under my
developed a diffusive heat transport energy balance model (EBM), that has
tested in both simplified and global domains. The EBM is loosely based
models of Budyko (1969), Sellers (1969), and North (1975). We have
these models to allow coupling with the GFDL OGCM (Pacanowski et al.,
allowing latent, sensible and radiative heat transfers between the ocean
atmosphere. In an effort to completely couple the ocean-atmosphere
moisture balance equation has also been added to the EBM so that
fluxes can be predicted for the ocean model.
The resultant EMBM has been run in a global 2o x 2o domain with
sea surface temperatures (Fanning and Weaver, 1996). Under climatological
oceanic conditions, the surface air temperatures, specific humidities and
surface fluxes are comparable to direct estimates. As an extension to the
climatological forcing case, a simple perturbation experiment was
which the 1955-59 pentad was compared to the 1970-74 pentad by driving
model under the respective sea surface temperatures. The model exhibits
as well as basin-mean temperature changes in the latter pentad comparable
direct estimates (Jones, 1988).
A version of the fully coupled ocean-atmosphere model (EMBM coupled to
GFDL-MOM) has been run in a single-hemisphere (60o x 60o) basin,
by zonally uniform wind stress and solar insolation forcing. A series of
several experiments of varying horizontal resolution (ranging from 4o
4o to 1/4o x 1/4o) and viscosity will be conducted to assess the
effect on the components of the net poleward heat transport. We will
the relative contributions of the mean and time variant components of the
transport. These include the effects of the barotropic gyre transport (in
horizontal plane), the meridional overturning transport (in the zonal
the baroclinic gyre transport, as well as the eddy and diffusive heat
It is not clear whether or not the coupled atmosphere-ocean system can
actually allow multiple equilibrium states when full coupling of both the
and freshwater are allowed. This follows since a strong constraint is
the heat transport of the coupled system through the incoming solar
at the top of the atmosphere.
Ocean-only studies under mixed boundary conditions (Bryan, 1986;
Willebrand, 1991; Hughes and Weaver, 1994) do reveal the existence of
equilibria. However, in these studies the freshwater flux forcing is
and the atmosphere is allowed to have infinite heat capacity. The coupled
two-dimensional zonally-averaged ocean model-EBM of Stocker et al. (1992)
revealed multiple equilibria but once more the freshwater flux forcing
essentially specified. While the fully coupled AGCM-OGCM of Manabe and
(1988) displayed multiple equilibria of the North Atlantic overturning,
adjustments an order of magnitude larger than climatological mean forcing
fields had to be used.
As discussed by Weaver and Hughes (1996), the use of flux adjustments
strong constraint on the implied oceanic heat and salt transports.
multiple equilibria studies using flux adjustments do not allow the ocean
freely adjust. Our fully coupled OGCM-TIM-EMBM will avoid the need for
adjustments and will allow free coupling of both heat and freshwater
the ocean and the atmosphere. Once we have analyzed the results from the
two-hemisphere studies we will extend our OGCM to two-basin geometry (as
Hughes and Weaver, 1994).
Initial results from the single-hemisphere coupled OGCM-TIM-EMBM have
spontaneous decadal variability in the coupled system. These results will
analyzed during this study and the effects of increasing horizontal
(i.e., the role of eddies) will also be studied. Initial benchmarks with
IBM workstation cluster have shown that these experiments are feasible
In addition, by integrating the coarse resolution version of the coupled
for many thousands of years, and by applying freshwater perturbations (as
Bryan, 1986) to the high latitude salinity budget or by applying
forcing to the system (as in Weaver et al., 1993), we will be able to
investigate the processes of centennial and millennial timescale
discussed in sections 3.2 and 3.3, respectively.
4.2 The global OGCM
A global ocean model was recently developed in collaboration with
Warren Lee and others for coupling of the CCCma AGCM. Two versions of
model now exist: The first version is a high resolution model (1.8o x
1.8o x 29 levels) and has now been coupled to the CCCma AGCM to
the climatic response to increasing atmospheric greenhouse gases and
The second version of the model is at slightly coarser resolution
1.8o x 19 levels) and is currently being used to understand the
stability and variability of the global ocean thermohaline circulation.
description of this coarse resolution model may be found in Weaver and
In order to address the scientific questions posed in section 3,
will use the coarse resolution version of this model in both coupled
and uncoupled forms. We will investigate the response of the global
perturbations in the North Atlantic by introducing passive tracers in the
Atlantic and examining the paths of the perturbations. A subset of these
experiments (i.e., those that yield interesting scientific results) will
repeated using the higher resolution version of the GCM. Furthermore, the
knowledge gained from the more idealized-geometry experiments discussed
last section will be used to investigate mechanisms and processes
North Atlantic decadal-interdecadal variability and their global
As discussed above, the EMBM has been extended to a global domain
Weaver, 1996). This model will be coupled to the global OGCM to address
question as to why the Atlantic forms deep water instead of the Pacific.
will use the global OGCM in various configurations to investigate the
importance of the different mechanisms outlined in section 3.7. For
basin geometries will be modified; the hydrological cycle will be
various regions; the winds will be varied. Hughes and Weaver (1994)
that the North Atlantic overturning is proportional to the
steric height gradient from the tip of South America to the latitude of
water formation. They further suggest that changes in the aforementioned
external parameters simply act to change this depth-integrated steric
gradient and hence the rate of NADW formation. This hypothesis will be
reexamined in the coupled system through the analysis of the steric
4.3 Ocean model validation using CFC-11
As discussed in section 3.5 we will continue to use CFC-11 to
the climatology and sub-grid scale parameterizations of both our global
North Atlantic models. The theoretical analysis leading to the
the GM scheme will be developed in collaboration with Drs. Amit Tandon
Chris Garrett. In particular, we shall advance this sub-grid scale
parameterization through the inclusion of the processes discussed in the
paragraph of section 3.5.
At the surface of our ocean models, the flux of CFC-11 across the
interface is expressed as
where lambda is the gas transfer velocity (taken from Liss and
1986), modified for use with CFC-11 (as in Wanninkhof, 1992); Cesw is the
concentration of CFC-11 at equilibrium for a given water temperature and
salinity (taken from Warner and Weiss, 1985); C is the concentration of
at the top level of the model.
In ice-covered regions, the gas transfer velocity for the CFC-11 is
using the formula:
where F is the fraction of sea-ice cover in tenths (0 = no ice; 10 =
ice cover). So far we have only considered annual mean forcing so that an
annual mean ice cover climatology was obtained from the monthly ice
concentrations of Gloersen et al (1992) and used to determine the
gas transfer velocity.
The initial validation process of our global OGCM was conducted by
and Weaver (1995). Inspired by the success of this analysis we will
climatologies of the OGCMs using each of the different sub-grid scale
parameterizations outlined in section 3.5. By quantitatively assessing
ability of the OGCMs to simulate present CFC-11 distributions the
weakness of each parameterization will be realized.
Due to the reduction of vertical mixing when the GM scheme was
numerical problems associated with vertical grid Peclet violations were
to occur. A flux-corrected transport (FCT) scheme (Gerdes et. al 1991)
therefore implemented into the GFDL OGCM and the consequences of using
advection scheme to eliminate these numerical problems will be
Several integrations comparing mixing and advection schemes, in a simple
demonstrate that it may be necessary to use a more sophisticated
scheme (like FCT) when using isopycnal mixing parameterizations.
4.4 The dynamical coupled AGCM-TIM-OGCM
Late last year we acquired the GFDL coupled climate model for use
local work station cluster. Drs. S. Valcke, S. Zhang and I, now have this
up and running and are using it (in close collaboration with Ron Stouffer
Suki Manabe) to investigate questions concerning the existence of climate
variability in the coupled climate system and how it varies as the mean
climatic state changes (i.e, does the decadal-interdecadal climate
found in the coupled model change as CO2 is increased in the
are also investigating the role of flux adjustments on interdecadal
variability. The numerical simulations of Delworth et al. (1993), 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 oceanic part of this model will be run under fixed-flux
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. In addition, we propose to test the
Weaver and Hughes (1996) to see whether or not we can reduce the
adjustments by simply adjusting the zonal mean fluxes of heat and salt
hence the implied oceanic heat and salt transports).
The experiments proposed above are analogous and complementary to those
will be performed with the CCCma coupled model. Since the GFDL coupled
simpler and hence more computationally efficient than the CCCma coupled
it is hoped that the insight we gain from it will allow us to better
the future experiments performed with the more sophisticated CCCma
4.5 Decadal variability in OGCMs with various subgrid-scale
During the last decade, the tuning of large-scale ocean models
observations has been achieved by adjusting tracer mixing processes and
boundary conditions. However, few alternatives to the traditional
closure of Reynolds stress have been implemented in OGCMs. The influence
momentum dissipation parameterization and dynamical boundary conditions
likely to be negligible at coarse-resolution and is worth being precisely
evaluated. Therefore, an ocean model has been developed for a
coarse-resolution, box-geometry, mid-latitude beta-plane, based on the
planetary geostrophic equations and allowing for different choices of
dissipation (linear, harmonic, biharmonic or none) and associated
conditions (no-slip, free-slip or vorticity closure). These models were
compared to the GFDL OGCM with the same geometry and forcing to validate
planetary geostrophic dynamics. Results from this analysis will be
We shall investigate the effect that different momentum dissipation
parameterizations have on the internal decadal-interdecadal variability
in ocean models. Through the use of these efficient ocean models we have
that atmospheric forcing plays the leading role in generating decadal
variability in ocean models: flux boundary conditions are the most likely
allow variability as no damping applies to surface anomalies, although
spatial distribution is important. A parameter sensitivity study of the
oscillatory behaviour has also been carried out. Initial results suggest
the horizontal tracer diffusivity has a critical damping effect, while
increasing the vertical diffusivity strongly enhances the oscillations.
parameterization or even inclusion of convection is found not to be
in sustaining the decadal variability, although it is necessary to remove
static instabilities. As pointed out by Winton (1996), the variation of
Coriolis parameter with latitude is also not necessary implying that
wave propagation is not important for the oscillations. Greatbatch and
(1995) proposed an explanation in terms of Kelvin waves propagating
basin. This mechanism was investigated by moving the boundaries or by
an f-plane model with a symmetric (about the meridional centre of the
heat flux. None of these major changes remove the oscillatory behavior;
therefore we conclude that Kelvin wave propagation is not important for
oscillation. As the variability is mainly observed in the region of
of the western boundary current, we are now looking at 2-layer and
2-dimensional models to investigate an advective mechanism, as originally
proposed by Weaver and Sarachik (1991). We believe that we will also be
develop a very simple one spatial dimension (meridional), non-linear
differential equation to explain the essential characteristics of the
variability found in coarse-resolution OGCMs.
4.6 Simulation of the Younger Dryas event
The use of models to simulate past climatic events is an
avenue of investigation if one is to have confidence in their application
future climatic changes. To this end we shall undertake a simulation of
Younger Dryas event (hereafter YD). The realistic geometry, global,
OGCM-EMBM will be used to investigate the transition between the last
glaciation and the present Holocene. During the transition, an abrupt
glacial climatic conditions, known as the YD occurred. The YD cold
particularly pronounced in regions bordering the North Atlantic, and is
evidenced in northern European and maritime Canadian lakes and bogs;
Atlantic marine sediments; and northwestern European and central Canadian
glacial moraines. Although strongest in the northern Atlantic region,
evidence indicates the impacts of the YD were felt throughout the
While the exact cause of the YD is still unknown, a general consensus
emerged that it was linked to an oceanographic phenomena. The
the YD signal in the northeastern North Atlantic suggests a primary role
the THC, particularly NADW production. The question naturally arises as
source could supply the necessary excess freshwater needed to reduce NADW
formation. The obvious sources are the polar ice caps and the Laurentide
Fennoscandian ice sheets (LIS and FIS, respectively). The traditional
is that the YD was triggered by the diversion of meltwater (due to the
retreating LIS) from the Gulf of Mexico to the St. Lawrence (Broecker et
1988). However, the fact that deep water is usually formed in local
high-latitude regions of small extent suggests that not only the amount,
also the location of meltwater introduced is crucial for interrupting the
We shall conduct experiments to reinvestigate the climatic implications
geographical and temporal change in the runoff from the LIS and FIS,
estimated meltwater and precipitation runoff from drainage basins in and
the North Atlantic, before, during, and after the YD (Teller, 1990).
Maier-Reimer and Mikolajewicz (1989), using an ocean-only model, found
were capable of shutting down NADW production within 200 years from the
they deflected 347 km3/yr from the Gulf of Mexico into the St.
Lawrence valley (roughly half that estimated to have occurred), our
results suggest the traditional meltwater diversion theory is incapable
inducing a shutdown of NADW. If, however, we apply the runoff estimates
previous to the YD, the conveyor is pushed to the brink, allowing the
of LIS waters to completely halt NADW production. In an additional
we will consider the role of the FIS meltwater on the YD climate.
As an extension of this paleoclimatic experiment we will also conduct a
sequence of experiments with the coupled model to investigate the
of opening and closing oceanic gateways such as the Isthmus of Panama and
Passage (about 3.5 and 30 Million years ago, respectively). In
attention will be focussed on the changes in meridional oceanic heat
and the relation to glacial events in the paleoclimatic record.
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BUDGET REQUESTThe following CICS Global Oceans budget request is for each of
fiscal years 1997-98, 1998-99 and 1999-2000
|1) Full support for Programmer (Michael Eby)
Salary 42,000$ p.a. + 10% benefits
|2) Partial maintenance support for my workstation cluster
|3) Operating costs for Weaver and Eby
|4) Travel for Eby
|5) Additional hard disk space (9GB) in each year
|6) Partial support for systems operator (Daniel Robitaille)
Justification for Proposed CICS Budget
1) Salaries and benefits
a) Graduate Students
All graduate students working on this project (Fanning, Murdock,
Poussart, Wiebe) will be supported off other grants.
b) Postdoctoral Fellows
Dr. Sophie Valcke (working on this project) will be supported off
c) Technical/Professional Assistants
Partial support ($7,000) for a computer systems manager/programmer
(D. Robitaille) who is in charge of maintaining my cluster of 13 workstations
their peripheral devices. The remainder of his support will come from my
d) Other (Research Associates)
I am also seeking full support for a Michael Eby who will conduct
ocean modelling experiments and research in support of CCCma coupled
efforts. I no longer have the time to undertake extensive model runs
so rely on a competent programmer/research associate to undertake them on
behalf. Dr. Sheng Zhang, also working on this project, will be funded off
a) Purchase or Rental
I recently acquired a new IBM rack mounted disk system which
to 72 GB of slotted disk storage. I currently only have 18GB allocated in
machine and would purchase an additional 9GB each year to put into the
system as the computer output becomes more voluminous.
b) Operation and maintenance costs
Partial support for the maintenance contract for my IBM
which was slightly over $12,000 for the 1995-96 fiscal year. I have found
maintenance contract to be imperative as I have a lot of
Every now and again a component breaks down and it is too expensive to
them. At $1000 per day, on site repairs are not affordable without a
3) Materials and Supplies
Toner cartridges, computer paper, magnetic tapes, computer
manuals, drafting costs, video cassettes for computer movies,
charges, communication charges (telephone, fax, courier), publication
Travel for Mike Eby to attend national CMOS meeting or workshops
ocean and coupled modelling.
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