Climate Modelling Group
School of Earth and Ocean Sciences

NSERC World Ocean Circulation Experiment Collaborative Special Project

[Image] ___________________________________________________________________
Principal Investigator:
Dr. Andrew J. Weaver
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, British Columbia

tel: (250) 472-4001
fax: (250) 472-4004


One of the primary aims of the International World Ocean Circulation Experiment (WOCE) is the development and testing of ocean models for the purpose of prediction of potential climate change and variability. I wish to begin by quoting directly from the most recent WOCE Newsletter (No. 12, February, 1992):
"The development of models for the prediction of climate change is the primary goal of WOCE. The SSG (Scientific Steering Group) emphasizes that models need good data sets to test them and that a close interaction between modellers and observations needs to be maintained. Noting the importance of coupled models for climate prediction, the SSG encourages national and international institutes to strengthen ocean and coupled ocean-atmosphere modelling and the comparison of models with observations. This needs to be in addition to the present climate modelling effort. The SSG emphasizes the need for significant investment in people and computer power well before the end of the WOCE intensive field period, and that data assimilation techniques need to be further developed before the preparation of WOCE data sets.

The SSG recognizes that resource problems vary from country to country (people, money, facilities), but emphasizes the importance of modelling as a central element of WOCE. Without substantial advances in modelling, WOCE will not meet its major objectives."

My proposed research plan is aimed directly at this challenge. I propose to not only develop new models for the purpose of large scale ocean/climate prediction but also to use existing models. I further propose to undertake a comparison of these different models with the purpose of understanding their individual shortcomings/assets. One of the goals of this project is to obtain an analysis of the stability and variability properties of the global ocean's thermohaline circulation.

With regards to the coupling problem, I shall be investigating the role of simple atmospheric feedbacks on the stability and variability properties of the ocean's thermohaline circulation. Specifically, in collaboration with D. Harvey at the University of Toronto, I propose to couple an energy balance atmospheric climate model (see Harvey, 1992) to the Bryan-Cox Ocean General Circulation Model (OGCM). Below I shall also discuss other efforts which I shall undertake with my research team as a step towards eventually developing a a global ocean/ice model for the purpose of coupling to an atmospheric GCM. The fully coupled atmosphere-ocean-ice general circulation model will then be used for climate change/prediction simulations. This later project will involve collaboration with Dr. G. Boer and N. McFarlane at the Canadian Climate Centre towards the end of this round of WOCE.

For the sake of brevity I have kept the introduction short although I am also enclosing a copy of a recent review article which I wrote (Weaver and Hughes, 1992) regarding the current state of the art of large scale ocean modelling, the ocean's thermohaline circulation, and its link to climate.

Summary of Research Projects

Below I begin by briefly summarizing the 10 research projects which I propose to undertake during the next round of WOCE. I have numbered each of these projects and have referenced these numbers in the milestone section on page 2 of this application. Figure 1 lists the researchers/collaborators involved in each project and also how each individual component of the research program fits into the final goal (the box at the bottom of Fig. 1). Research into a few of these projects has recently begun and hence I am only requesting partial support for them to aid their completion.

1) - Conduct realistic geometry/topography experiments to examine the stability and variability properties of the thermohaline circulation in the North Atlantic. The goal of this project is to look at the timescales and nature of internal variability, if any, in the North Atlantic.

2) - Study the stability, variability and equilibria of the global thermohaline circulation in idealized geometry. The will allow for a thorough understanding of the role of competing forcings (wind stress, freshwater flux and heat flux) without being burdened by the complexity of a global ocean model.

3) - Examine the structure, stability and variability properties of the global thermohaline circulation by introducing realistic global geometry and topography. An understanding of the global thermohaline circulation and the importance of sub-surface topography will be obtained.

4) - To take the multiple equilibria found in project 2) and determine the relationship between the north-south pressure gradient and the east-west pressure gradient in different basins with and without deep water formation. A similar analysis will be done on the global OGCM fields associated with project 3). Such an analysis will be a useful diagnostic for users of the zonally-averaged model developed by Wright and Stocker (1991).

5) - Implement semi-Lagrangian advection schemes into the Bryan-Cox OGCM. This research will be supported from an AES contract for the first year of the next round of WOCE. Upon implementation of the new advective schemes, comparisons will be done in both idealized and global basins between the Bryan-Cox OGCM with and without the new schemes. In years 2 and 3 the adjoint of this code will be developed.

6) - Develop a finite element, semi-Lagrangian OGCM. Upon development of the code, comparisons will be done with the Bryan-Cox OGCM with and without the semi-Lagrangian advection schemes. The purpose of this project is to introduce more sophisticated numerical techniques (than the standard centred differences) into an ocean model.

7) - Couple an energy balance climate model to the Bryan-Cox OGCM. We shall begin by considering simple idealized basins and then move on to global geometry. The purpose of this project is to investigate the role of simple atmospheric feedbacks on the stability and variability properties of the thermohaline circulation.

8) - To force the Bryan-Cox OGCM with heat and freshwater fluxes and windstress obtained from the 2nd and 3rd generation Canadian Climate Centre (CCC) atmospheric GCM's present day and 2xCO2 equilibrium climates. The main question which we wish to address is whether or not the CCC atmospheric model fields will drive an ocean model to a reasonable climatology. This will provide important information on potential problems which may arise when the CCC atmospheric model is eventually coupled to an ocean model.

9) - To develop simple ice models for coupling to the aforementioned GCMs. The development of an ice model is fundamental if one wishes to eventually couple an ocean model to a fully prognostic atmospheric GCM.

10) - Develop simple analytical box models to understand the stability and variability properties of the thermohaline circulation obtained in GCM experiments.

In the section below I provide a detailed discussion regarding the methodology, rationale and importance of these individual projects. Their order is not meant to be illustrative of their individual priority/importance.

Detailed Discussion of the Research Projects

1) - North Atlantic Thermohaline Circulation and the Role of Arctic Freshwater Flux Forcing

The analysis of Weaver et al. (1991, 1992) showed that strong freshwater flux (P-E) forcing, with a high latitude local net evaporation maximum, was conducive to the existence of internal thermohaline circulation variability. In particular, we pointed out that the P-E field over the North Atlantic resembled the idealized field which we used. To quantitatively look at the properties of the internal variability of the North Atlantic thermohaline circulation S. Aura (an MSc student) has recently analysed the results from a number experiments conducted using realistic geometry experiments and the Bryan-Cox OGCM. The model was driven by Hellerman and Rosenstein (1983) winds, Levitus (1982) sea surface temperature, and Schmitt et al. (1989) P-E fields. We further specified various freshwater transports through the northern boundary to parameterize the inflow of Arctic freshwater.

In all our runs, when there was no freshwater input directly into the Labrador Sea internal, self-sustained 22 year period variability existed throughout the entire integration. The poleward heat transport at 39 degrees N associated with the oscillation varied from a maximum of 0.8 petawatts, when the thermohaline circulation was most intense, to a minimum of 0.5 petawatts. The variability was most pronounced in the western North Atlantic, especially in the region of the Labrador Sea. The eastern North Atlantic also underwent variability which was out of phase with the west. The oscillations could, however, be suppressed by adding a slight transport of freshwater directly into the northern regions of the Labrador Sea which caused to the system to evolve into a stable equilibrium. The variability was relatively insensitive to the amount of imposed freshwater flux into the East Greenland Current. These results are, however, very recent so that a complete analysis of them has yet to be done. Whether or not this sort of variability is observed in the real ocean is not known, although, the Greenland ice core data of Hibler and Johnsen (1979) does exhibit 20 year timescale variability, adding an element of credibility to the model results.

The role of bottom topography on the stability and variability of the thermohaline circulation has also not been treated adequately. The above uncoupled ocean GCM simulations used a flat bottom topography. Moore and Reason (1992) suggest that some of the internal variability of the thermohaline circulation might be sensitive to the inclusion of bottom topography although it is not clear how well this topography is resolved in their 12 level model. We are currently including realistic bottom topography in the 3 degrees x 3 degrees North Atlantic experiments discussed above (with 20, 40, 80 and 160 vertical levels) to investigate its role quantitatively. Our initial results suggest that while topography acts to reduce the variability slightly, it still exists especially at high vertical resolution or when a stochastic component is added to the freshwater flux forcing field. The main role of topography appears to be linked to how deep convection can penetrate. With shallow topography, convection occurs to shallower depths so that the subsequently set up east-west pressure gradients are reduced compared to integrations with no topography. I expect that this project will be completed early in the first year of my WOCE research program and hence only request partial support to aid in its completion.

All of the long integrations which were discussed in this section were done using non-eddy resolving ocean models without an ice component. To extend this research project I shall examine whether or not the conclusions which have been drawn from the coarse resolution simulations carry over to high-resolution, eddy-resolving models. This analysis will begin using idealized geometry GCM experiments of the North Atlantic (as in Weaver and Sarachik, 1991a,b) and then move on to the realistic geometry/topography model discussed above. I expect to take on a new MSc or PhD student to look at this project. Similarly, I hope to take on a Postdoctoral Fellow/Research Associate to develop a simple ice model to couple to the ocean model in order to investigate possible feedbacks in the ice-ocean system (see project 9). Furthermore, the mechanism for the generation of the 22 year variability discussed above no longer appears to be the same as the "loop oscillation" described by Weaver and Sarachik (1991), Weaver et al. (1991), and Winton and Sarachik (1992). I will be taking on a PhD student (S. Zhang) in September who will try and develop simple box models to explain this new type of thermohaline variability (see project 10).

2) - Multiple Equilibria of the Global Ocean Thermohaline Circulation: A Process Study

Marotzke and Willebrand (1991) expanded upon early work by Bryan (1986a,b) by examining the potential existence of multiple equilibria in an idealized flat-bottomed OGCM consisting of two symmetric basins, representing the Atlantic and Pacific Oceans, coupled via a southern ocean with specified transport and prognostic baroclinic interchange of water masses between basins. They found that four equilibria were possible under their symmetric (with respect to the equator and each basin) surface forcing fields. These were the Northern Sinking equilibrium, with deep water forming in both the North Atlantic and North Pacific; the Southern Sinking equilibrium, with deep water formation only in the southern ocean; the Conveyor Belt equilibrium in which deep water formed only in the North Atlantic with upwelling in the North Pacific; the Inverse Conveyor Belt equilibrium in which deep water formed only in the North Pacific, with upwelling in the North Atlantic.

Hughes and Weaver (1992) have recently extended the two-basin analysis of Marotzke and Willebrand (1991) to allow for asymmetric basin widths/extents, asymmetric forcing (as in Wells and Mead, 1990), and by adding a deep sill across the now fully prognostic Antarctic Circumpolar Current. Their model was spun up using restoring boundary conditions which were derived from zonal averages of the Levitus (1982) data set for each basin separately. The present day Conveyor Belt equilibrium under mixed boundary conditions, with order 9 Sv of deep water sinking to intermediate depths in the North Atlantic and overriding AABW from the south, was obtained under a diagnosed hydrological cycle with net evaporation over the North Atlantic and net precipitation over the North Pacific.

Under these diagnosed mixed boundary conditions, steady states with no North Atlantic Deep Water (NADW) sinking and deep NADW sinking (with overturning in excess of 20 Sv) were also found; however North Pacific Deep Water formation was never found. The northward extension of the North Atlantic rendered the Northern Sinking and Inverse Conveyor Belt equilibria unattainable even under symmetric temperature forcing in each basin and a hydrological cycle with 1.5 times as much precipitation over the North Atlantic as evaporation in the previous experiment (and almost zero net precipitation in the North Pacific). This preference of the model for Conveyor Belt type circulations was found to be partly set up by the wind stress forcing. In addition, Hughes and Weaver (1992) found multiple versions of the Southern Sinking equilibrium in which deep water formed either only on the inside of the ACC, or also at the polar boundary.

It should be noted that if, as an additional forcing, anomalous amounts of freshwater were taken out of the North Pacific and placed in the North Atlantic (a strong reversal of the present day hydrological cycle) an Inverse Conveyor Belt equilibrium could arise. However, once the external reversal of the hydrological cycle was removed, so that the model was now only forced by the original mixed boundary conditions, the Inverse Conveyor Belt equilibrium became unstable and a transition to either the Conveyor Belt or Southern Sinking equilibria ensued.

The analysis of this work is by no means complete and we are currently investigating the role of different forcing mechanisms (freshwater flux vs heat flux vs wind stress) in determining the stability and variability properties of the global thermohaline circulation. By keeping our geometry simple we are therefore able to focus directly on the important processes involved in driving the global ocean circulation. We expect this project to be completed during the first year of the next round of WOCE.

3) - Multiple Equilibria of the Global Ocean Thermohaline Circulation: A Global Ocean Model

As a progression in my model hierarchy I hope to apply the knowledge already obtained and that which will be gained from the results of projects 1 and 2 above, to look at the stability and variability properties of the global thermohaline circulation. In this global Bryan-Cox OGCM I shall include realistic geometry and topography and force the model using realistic P-E, heat flux and wind fields. The purpose of these numerical experiments is obtain an understanding of the global thermohaline circulation and examine the importance of sub-surface topography (e.g., Mid-Atlantic Ridge). Numerous experiments will be conducted (with and without winds, with and without seasonal cycle, for example), to look at the competing effects of the dominant forcing mechanisms. Furthermore, the P-E forcing fields (and hence the hydrological cycle) will be perturbed to look at the transition between equilibria within the system.

To assist me with this research I propose to hire a Research Associate. I have already had communications with M. England, M. Winton, D. Holland, and F. Yin regarding this project. They have all shown interest in coming to the University of Victoria. M. England would be the ideal candidate as he has already developed a global Bryan-Cox OGCM in which he took a great deal of care to reproduce the present day water masses of the global ocean (see England, 1992a,b). His initial experiments were aimed at tuning the model to represent the present day climatology and he has not done any of the experiments which I propose above to focus specifically on the stability and variability properties of the global thermohaline circulation.

As the Bryan-Cox OGCM is currently the most widely used and tested ocean model. We will use it as a control to which we will compare the results of the semi-Lagrangian GCM (project 5) and finite element GCM (project 6). Dr. R. Greatbatch and his Research Associate S. Zhang at Memorial University will also be developing a global version of their planetary geostrophic model (Zhang et al. 1992a,b). Furthermore, Josef Cherniawsky at the Institute of Ocean Science is starting to develop a global version of the Oberhuber (1990) isopycnal coordinate model. We will collaboratively undertake a comparison of the climatologies obtained from all these models to identify their individual assets/shortcomings. As an important first stage, comparisons will be done in idealized flat-bottomed geometries as in the work of Weaver and Sarachik, (1991a,b) and Zhang et al. (1992a).

4) - The Relationship between the North-South and East-West Pressure Gradients in a Coarse Resolution OGCM

Wright and Stocker (1991) recently developed a sophisticated zonally-averaged ocean model as a major extension to that of Marotzke et al. (1988) for the purpose of undertaking long-term climate simulations. To obtain a closure for their system, Wright and Stocker (1991) parameterized the east-west pressure gradient, (pE - pW)/(rho * D), in terms of the north-south density gradient. Here D = lambdaE - lambdaW is the width of the basin and rho is a reference density. This condition guaranteed that u, v = 0 at the lateral boundaries and also removed the Fickian diffusion assumption of Marotzke et al. (1988).

The parameterization used by Wright and Stocker (1991) is:

(1) equation unavailable
where lambdaE and lambdaW are the longitudes of the eastern and western boundaries, respectively, f is the latitude and the overbar denotes a zonal average. In order to determine an appropriate choice for the closure parameter epsilon, I conducted a single hemispheric basin simulation using a fully three-dimensional ocean GCM. The integration to equilibrium used exactly the same configuration, parameters and forcing as the 33 level model discussed by Weaver and Sarachik (1990), with wind-stress taken to be identically zero everywhere. To utilize the results of this simulation Wright and Stocker (1991) replaced (1) by the weaker condition,
(2) equation unavailable
obtained by taking a z derivative of (1) and using the hydrostatic relation. This was further integrated from the surface downward to obtain
(3) equation unavailable
or equivalently
(4) equation unavailable
where pS is the surface pressure and pES, pWS are the surface pressures at the eastern and western boundaries, respectively.

Wright and Stocker (1991) took the aforementioned ocean GCM output and plotted the left hand side of (4) as a function of the right hand side, for 27 different latitudes and for two depths. They concluded that for the particular GCM simulation which they analysed, a constant value of epsilon = 0.3 represented a good fit except above 500 m where epsilon decayed towards 0 at z=0, consistent with the use of zonally uniform restoring boundary conditions in the OGCM, which essentially demanded rhoE - rhoW about equal to 0 at z=0.

The model of Wright and Stocker (1991) was extended by Stocker and Wright (1991) and Wright and Stocker (1992) to allow transport between various zonally-averaged oceanic basins. Stocker and Wright (1991) connected a zonally-averaged Atlantic Ocean basin to a zonally-averaged Pacific Ocean basin via a zonally-averaged Southern Ocean, in which they allowed for the lack of meridional barriers by choosing epsilon = 0.0001 in some experiments.

Wright and Stocker (1992) extended the model still further by incorporating an Indian Ocean basin, parameterizing the effects of meridional Ekman transport, and allowing for transport through the Indonesian Archipelago and the Bering Strait. They showed that when fluxes from marginal seas such as the Mediterranean and the Red Seas were incorporated into the model, it gave a fairly realistic representation of zonally-averaged water mass characteristics in each of the three ocean basins. They further undertook a detailed scaling analysis and parameter sensitivity study of their model (see Wright and Stocker, 1992, for more details).

Although Stocker and Wright (1991a) point out that the closure parameter epsilon used in their model should vary inversely with the basin width, there is no apparent reason why it should have the same value in basins both with deep water formation and without. Furthermore, it is not clear to what extent the parameterization is valid if wind-stress forcing is included or if one is near a region of deep convection. I propose to quantitatively investigate this by analysing the fields obtained in projects 2 and 3 above. Such an analysis will provide useful information to those users of the zonally-averaged model. T. Hughes, a PhD student, will be undertaking this research.

5) - Implementation of Semi-Lagrangian Advection Schemes into the Bryan-Cox OGCM

As already stated above, the most widely used OGCM is the Bryan-Cox model developed at the Geophysical Fluid Dynamics Laboratory several years ago. This model is based on standard finite difference numerical techniques (centred difference in space and leap-frog in time, with the diffusion terms lagged by a timestep for stability reasons). There are several problems which one encounters when one tries to use this model to undertake global coarse resolution studies. The first major problem is that one often has to use the acceleration techniques of Bryan (1984) in which the baroclinic velocity, barotropic vorticity and tracer equations are integrated using different timesteps (all limited by the CFL criterion). The second major problem occurs due to the convergence of the meridians near the poles and the strong CFL constraint this implies. A way around the first problem is to eliminate the nonlinear terms in the momentum equations. One is then limited by the CFL condition applied to the advection of tracers. A way around the latter problem is to use Fourier filtering at high latitudes but this crude technique often causes more problems than it solves.

I have recently received a contract from the Atmospheric Environment Service to hire Dr. S. Das as Research Associate in the School of Earth and Ocean Sciences. He will start working on replacing the numerical advection algorithms in the Bryan-Cox GCM by semi-Lagrangian algorithms (following Robert, 1981; Bates and McDonald, 1982; Ritchie, 1991; Coté and Staniforth, 1991,). This will entail using the GFDL MOM (Modular Ocean Model) version of the code (Pacanowski et al., 1991) which has been written in a much more directly accessible way than the Cox (1984) version. Dr. R. Bermejo, an expert on semi-Lagrangian numerical techniques will visit us at the University of Victoria to discuss progress in both this project and project 6.

The contract for Dr. Das expires as of June 30, 1994. During the first year of the contract he will develop the schemes and test them against the old version of the Bryan-Cox model. I am herein requesting funding for Dr. S. Das for only the last two years of the WOCE proposal. Dr. Das did his PhD work on data assimilation using the adjoint method. The adjoint of the Bryan-Cox model was developed by Thacker, Long and Hwang and further enhanced by Marotzke (1992). I am proposing that Dr. Das develop the adjoint of his semi-Lagrangian advection algorithms for implementation into the Marotzke (1992) version of the Thacker, Long and Hwang code. This will involve collaborative research with J. Marotzke at MIT. Furthermore, once the semi-Lagrangian advection algorithms have been developed and tested we will develop a global ocean model using them for direct testing with the model developed in project 3.

6) - Development of a Finite Element, Semi-Lagrangian OGCM

The ocean models which are presently used for climate predictions are largely based on traditional finite-difference techniques. Due to the nature of the differencing procedure enormous problems are encountered near the poles. Furthermore, land boundaries are not well resolved using rectangular geometry. In addition, if irregular grid spacing is used, in order to focus on boundary current regions, one degree of accuracy is lost.

In this project I am seeking full support for a PhD student (P. Myers) to develop a finite element, semi Lagrangian OGCM. This will be a four year project involving a systematic procedure for model development. He will start with the linearized barotropic vorticity equation. Nonlinear terms will then be included, followed by topography and finally stratification. Upon development of the code, comparisons will be done with the Bryan-Cox OGCM with and without the semi-Lagrangian advection schemes in idealized basin geometry. Eventually the model will be extended to a global domain for comparison with the aforementioned global models but this latter project is not within the scope of P. Myers' PhD thesis.

Dr. Salil Das, who will be implementing the semi-Lagrangian advection algorithms into the MOM version of the GFDL model, will also assist in this project. Furthermore, R. Bermejo who has recently returned to Spain has agreed to come out to Victoria a couple of times to provide some additional guidance.

7) - A Coupled Energy Balance Climate Model/Ocean General Circulation Model

Much of my past research has been focussed on the stability and variability of the thermohaline circulation under mixed boundary conditions. It must be borne in mind that apart from other idealizations, this approach yields serious shortcomings in its conceptual formulation of the atmospheric coupling. In specifying sea surface temperature (SST) and P-E almost independently of the oceanic state, there is a very weak feedback of oceanic heat transport on SST. Moreover, there is no feedback of the SST on the hydrological cycle; e.g., a warm SST anomaly should cause enhanced evaporation which, by invoking a conceptual atmospheric water vapor transport, would be likely to change the overall P-E pattern. It is plausible that these two effects counteract each other. If the thermohaline circulation is vigorous, heat transport is large and high-latitude SST should rise. Thereby, the relative influence of the P-E forcing would increase, compared to the thermal forcing (which actually is given by the temperature contrast between high and low latitudes), which would tend to destabilize the state of vigorous meridional overturning. On the other hand, the increased high-latitude SST leads to increased evaporation, and one may conceive that the total atmospheric water vapor transport from low to high latitudes is reduced. This would reduce the high-latitude freshening and thus stabilize the thermohaline circulation.

As a first step towards coupling an OGCM to an AGCM, Danny Harvey and I will couple an energy balance climate model (EBCM) to the Bryan-Cox OGCM. D. Harvey will spend part of his upcoming sabbatical (summer, 1992) at the School of Earth and Ocean Sciences, University of Victoria. We will couple a quasi-2 dimensional EBCM based on Harvey (1992) in which an idealized hydrological cycle is to be parameterized. D. Harvey is presently refining his EBCM to include a hydrological cycle which will be ready for coupling in August 1992. This project will involve coupling a hierarchy of models, starting from an idealized one-hemisphere OGCM, then moving to the two-basin model of project 2 and finally to the fully global model of project 3.

8) - The Global Ocean Response to Forcing from the Canadian Climate Centre Atmospheric GCM

I have recently been in contact with Norm McFarlane at the Canadian Climate Centre in Downsview, Ontario to discuss the possibility of using the 2nd and 3rd generation CCC AGCM surface heat, P-E and windstress data to drive a global OGCM. The purpose of this experiment is to see whether or not these surface fluxes would force an OGCM into a stable climatology which resembles the present day ocean climatology. If this turns out to be true then we would take the doubled CO2 CCC AGCM surface fields to see what the compatible oceanic doubled CO2 response would look like. This sort of analysis is an important step towards developing a fully coupled atmosphere/ocean model as it will give good indications as to where problems in the coupling procedure may arise. To begin with, of course, we will analyse the divergence of the surface P-E and heat flux forcing fields to see what surface oceanic heat and salt transport they will imply at equilibrium.

In April 1992, T. Hughes and I visited AES to discuss this research with N. McFarlane and B. Gough and we picked up the AGCM fields. T. Hughes, a PhD student under my supervision, is starting to develop a global ocean model in order to look at the stability and variability properties of the global thermohaline circulation. The last part of her thesis will be to undertake these CCC AGCM flux/wind forced global ocean experiments.

9) - A Coupled Ocean-Ice Model

As already mentioned, My most recent research on the stability and variability properties of the thermohaline circulation (Weaver and Sarachik, 1991a,b; Weaver et al., 1991, 1992) has been conducted using uncoupled OGCMs. In project 7 I have proposed to couple a simple EBCM to investigate atmosphere-ocean feedbacks on both the local and global scale. This research will also be extended by incorporating a simple ice model into the coupled OGCM/EBCM based on the ice model described in Harvey (1992).

Before undertaking this task I propose to couple the Bryan (1969) ice model to a single-hemisphere OGCM to investigate the first-order role of ocean-ice feedbacks to the stability and variability properties of the thermohaline circulation. This analysis will be extended to two-hemisphere/two basin and global ocean models as the knowledge of the coupled system is increased.

Richard Greatbatch and Sheng Zhang at Memorial University will be undertaking similar experiments using their planetary geostrophic model and a close collaboration between our two groups will ensue. Furthermore, Greg Flato and Greg Holloway at the Institute of Ocean Sciences will be using the Hibler ice model to study the Arctic Ocean. We will also be in close communication as our research develops.

I am applying for full support for a Research Associate who will work on this project as well as project 3.

10) - Analytic Box Model Development

Simple analytic box models have been the subject of much interest over the last few years. They allow for a quantitative analysis of the competing roles of thermal and haline surface forcing in driving the thermohaline circulation without using sophisticated OGCMs (for a review see Weaver and Hughes, 1992). In particular they have shown that:
1) Several distinct equilibria of the ocean's thermohaline circulation are possible under the same boundary conditions.
2) Self-sustained thermohaline oscillations may occur. Recently Zhang (1991) analysed the two-box model of Stommel (1961) and the two box thermohaline oscillator of Welander (1982). He further proposed a conceptual four-box model of the thermohaline circulation in a single hemisphere consisting of an equatorial and a polar Welander vertical box model connected by pipes of negligible volume.
Lingqi Zhang has recently applied to the University of Victoria to do a PhD under my supervision. I am applying for full support for his PhD research in which he will develop simple box models to try and quantitatively understand the decadal/interdecadal variability found in OGCM simulations (especially project 1). He will begin by analysing the conceptual four-box model proposed above.

It appears that the variability found in the OGCMs may be linked to either low-latitude Welander (1982) or Welander (1989) oscillations (project 1) or "loop-oscillations" (Winton and Sarachik, 1992; Weaver et al., 1991) based upon the "Howard-Malkus loop" presented by Welander (1986). A detailed analysis of model output fields and the development of simple analytic box models will shed light on the physical mechanisms causing them.


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