Climate Modelling Group
School of Earth and Ocean Sciences


NOAA Scripps Lamont Consortium on the Ocean's Role in Climate

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Principal Investigator:
Dr. Andrew J. Weaver
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, British Columbia
CANADA V8W 2Y2

tel: (250) 472-4001
fax: (250) 472-4004
e-mail: weaver@ocean.seos.uvic.ca
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Proposal Title: Ocean/Climate Modelling and Prediction on the Decadal Timescale

Total Proposed Cost: $327,000

Budget Period: Three full years from receipt of award (May 1, 1994 - April 30, 1997)

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The development of a quantitative understanding of decadal-century climate variability is in its early stages. The modelling and prediction of decadal-century scale climate variability needs to be refined and expanded through a hierarchy of both coupled and uncoupled ice/ocean/atmosphere models. Model simulations must be carefully analysed and compared to both existing and future observations.

Together with research associates and students, I propose to use existing models and develop new models for the purpose of large scale ocean/climate prediction on the decadal timescale. I further propose to undertake a comparison of these different models with the purpose of understanding their individual shortcomings/assets. One of the major goals of this project is to obtain an analysis of the stability and variability properties of the global ocean's thermohaline circulation.

Further advances in obtaining an understanding of the ocean's role in climate on the decadal timescale can only be obtained using coupled models. Another goal of this proposal is to develop 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 who will move to Victoria this summer.

Statement of Work

Introduction

For the sake of brevity I have kept the introduction short although I refer the reader to a 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. The rationale for the following proposal is also detailed in the Science Plan for the Consortium on the Ocean's Role in Climate.

Summary of Research Projects

Below I begin by briefly summarizing the 5 research projects which I propose to undertake together with my students and research associates. A more detailed discussion of each individual project will follow.
1) - Examine the structure, stability and variability properties of the global thermohaline circulation in a global ocean model. The climatology of the global ocean model will be checked using Freon tracer data.

2) - Develop a finite element, semi-Lagrangian OGCM. Upon development of the code, comparisons will be done with the Bryan-Cox OGCM with and without semi-Lagrangian advection schemes (which we are currently implementing into the GFDL model). Two questions will also be addressed: 1) What is the importance of Bering Strait, and Indonesian throughflow and Mediterranean outflow on the global thermohaline circulation? 2) What is the relative partitioning of the return flow of NADW through either the warm water route or the cold water route?

3) - 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.

4) - Couple both thermodynamic and dynamic ice models to the aforementioned GCMs. The purpose of this project is to investigate ice-ocean and ice-ocean-atmosphere feedbacks on the stability and variability of the global thermohaline circulation.

5) - Use simple zonally-averaged models to understand the stability and variability properties of the thermohaline circulation obtained in GCM experiments.
In the section below I provide a more 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.

More Detailed Discussion of the Research Projects

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

In this project I hope to apply the knowledge already obtained from earlier work, to examine 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 freshwater flux (P-E), heat flux and wind fields. The purpose of these numerical experiments is to obtain an understanding of the global thermohaline circulation and examine the importance of sub-surface topography (e.g., Mid-Atlantic Ridge, Greenland-Iceland sills) on its stability and variability properties. 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.

Before undertaking any of the sensitivity experiments it is important that the global model reproduces the present day climatology successfully. One of the major tests which we will use is the simulation of present ocean Freon distributions. I recently obtained a comprehensive data set (surface boundary condition information oceanic observations will arrive later) from Dr. R. Weiss at Scripps. The surface data will be injected into the ocean model and the predicted and observed Freon distributions will be compared.

As the Bryan-Cox OGCM is currently the most widely used and tested ocean model. We will use it as a control with which we will compare the results of the semi-Lagrangian GCM and finite element GCM (project 2).

2) - 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 and straits are not well resolved. In addition, if irregular grid spacing is used in order to focus on boundary current regions, one degree of accuracy is lost.

This will be a four year project involving a systematic procedure for model development. We 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. The model will be extended to a global domain for comparison with the aforementioned global models.

We shall use this global model to investigate the effects of Bering and Indonesian throughflow as well as Mediterranean outflow on the global thermohaline thermohaline circulation. Furthermore, we wish to address the debate as to where the return flow of NADW occurs. Coarse resolution models (e.g. Hirst and Godfrey, 1993) suggest that most of the return flow happens in the cold water route via the Aghulas Retroflection. Eddy resolving models, which essentially prescribe the deep temperature and salinity structure of the ocean, suggest that most of the return flow happens in the warm water route via eddy generation in the Aghulas Current (Semtner and Chervin, 1988, 1992). Observations suggest that some combination of the two routes is appropriate (Broecker 1991).

3) - 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. This approach yields serious shortcomings in its 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. However, 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.

In order to investigate simple feedbacks in the coupled air-sea system and their role in decadal climate variability, we will couple a simple energy balance model to both global, and more idealized, versions of the Bryan-Cox model. We will parameterize a hydrological cycle in this coupled model following the work of Nakamura et al. (1993).

4) - A Coupled Ocean-Ice Model

Since timescales of variability in the ocean are of the order of months or longer, it is common in numerical weather prediction (NWP) to consider the ocean as a simple mixed layer. Conversely, since the timescale of variability of atmospheric processes is short compared to the decadal timescales of interest here, as a first step a simple energy balance atmosphere model will be coupled to the global ocean models (project 3). Since the timescale for variability of the cryosphere lies between that for the atmosphere and ocean, both dynamic (Flato and Hibler, 1992) and thermodynamic (Semtner, 1976) ice models will be used used to investigate coupled air-sea-ice feedbacks.

Before undertaking this task I propose to couple the Semtner (1976) 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 the global ocean models as the knowledge of the coupled system is increased.

5) - Simple Coupled Zonally-Averaged Models

Here I propose to take the Wright and Stocker (1991) model and coupled it to both a Semtner (1976) thermodynamic ice model and a Sellers (1969) energy balance model (with a parameterized hydrological cycle included) to investigate simple oscillations in the coupled system. This simple coupled model will be used as a tool to try and interpret results from the more complicated global coupled models.

I envision analysing possible mechanisms for interdecadal variability in the ocean/atmosphere system as follows: Stronger thermohaline circulation; more evaporation; more high latitude precipitation; slower thermohaline circulation; and again. Furthermore, in the ice/ocean system we should see interdecadal variability as in Yang and Neelin (1992): Stronger thermohaline circulation; more ice melt; weaker thermohaline circulation; more ice growth; and again. It is not clear how these two independent oscillation would interact in the fully coupled system.

References

Broecker, W.S. 1991. Oceanography, 4: 79-89
Flato, G.M., & Hibler, W.D. III, 1992.J. Phys. Oceanogr. 22:, 626-651
Hirst, A.C. & Godfrey, J.S. 1993. J. Phys. Oceanogr., in press
Nakamura, M., Stone, P.H. & J. Marotzke, 1993. Nature, submitted.
Sellers, W.D., 1969., J. Appl. Meteorol., 8: 392-400
Semtner, A. 1976. J. Phys. Oceanogr., 6: 379-389
Semtner, A. J. & Chervin, R. M. 1988. J. Geophys. Res., 93: 15502-15522
Semtner, A. J. & Chervin, R. M. 1992. J. Geophys. Res., 97: 5493-5550.
Weaver, A.J., & Hughes, T.M.C. 1992. Trends in Physical Oceanography, Research Trends Series, Council of Scientific Research Integration, Trivandrum, India, in press.
Wright, D.G. & Stocker, T.F. 1991. J. Phys. Oceanogr., 21: 1713-1724
Yang, J.,& Neelin, J.D., 1993. Geophys. Res. Let., submitted.

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