Climate Modelling Group
School of Earth and Ocean Sciences


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

Semi-Annual Report: Report Period 5/1/94 through 10/31/94

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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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Agency: National Oceanic and Atmospheric Administration, Office of Global Programs

Project Title: Ocean/Climate Modelling and Prediction on the Decadal Timescale

NOAA Award No: NA47GP0188

Project Period: 5/1/94 through 4/30/97

Budget Period: 5/1/94 through 10/31/94

Performance Report Completed: October 24, 1994

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NOAA Programmatic Requirements: Progress reports shall contain brief (no longer than two pages) information on the following:

1) A comparison of actual accomplishments with the goals and objectives established for the period, the findings of the investigator, or both. Whenever appropriate and the output of programs or projects can be readily quantified, such quantitative data should be related to cost for computation of unit costs.
2) Reasons why established goals were not met, if appropriate.
3) Other pertinent information including, when appropriate, analysis and explanation of cost overruns or high unit costs.
Note: Recipients shall immediately notify Joint Institute for Marine Observations-Administrator 619-534-9668 of developments that have a significant impact on the award supported activities, problems, delays, or adverse conditions which materially impair the ability to meet the objectives of the award.

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Below I summarize the progress on the research funded through the NOAA Scripps-Lamont Consortium on the oceanŐs role in climate. I have enclosed a number of preprints of research which has been funded, either wholly or partially, by my NOAA grant. Two additional research projects relevant to my original NOAA grant were also recently completed. In the first of these (Reynaud et al., 1994) we analysed archived data from the Labrador Sea region of the North Atlantic. Several diagnostic models were used to study the climatological mean summer circulation in the area. We have recently extended this to examine the circulation during different decades over the past century (Reynaud et al., 1994b). In the second of these (Wohlleben and Weaver, 1994) we analysed historical data to propose a mechanism for decadal-interdecadal climate variability in the subpolar North Atlantic.

1) - Global Ocean Modelling

A global ocean model has been developed for coupling to the Canadian Climate Centre Atmospheric GCM (Weaver and Lee, 1994). Furthermore a series of sensitivity experiments have been conducted to tune the present-day climate of the OGCM. Weaver (1994) has recently conducted a series of experiments using this model to suggest the inevitability of using flux corrections in coupled models unless the basic physics (e.g. clouds) in the atmospheric model is better represented. T. Hughes and I have also completed the preliminary analysis of the global thermohaline circulation simulated in a low-resolution version of the Canadian Climate Centre's global ocean model. The model's climatology under both restoring and mixed boundary conditions is acceptable (compared to present-day observations) in terms of the tracer fields and overturning circulation; however, the response of the global "conveyor" to a time-dependent perturbation occurs through a "warm water route" (Gordon, 1986) which is increasingly in disagreement with current thinking.

D. Robitaille and I have also introduced Freon into a hierarchy of models based on the GFDL OGCM: North Atlantic models with horizontal resolution of 1 degree by 1 degree and 2 degree by 2 degree; idealized North-Atlantic and two-hemisphere Atlantic models; a global model. In each of the models, comparison with Freon observations gives an idea of their strengths and weaknesses. The next step is to include an isopycnal scheme (Gent and McWilliams, 1990; Danabasoglu et al., 1994) to test, using Freon as a tracer, its effects. The scheme proposed in these two papers has been introduced into the GFDL model. A comparison is currently underway with the results obtained using constant horizontal mixing in different model configurations.

A simple parameterisation of the sea surface temperature-evaporation feedback has also been developed for use in uncoupled ocean models under mixed boundary conditions. The importance of the feedback was first tested in a couple of simple perturbation experiments, and then applied to three case studies featuring natural internal variability of the thermohaline circulation on decadal, century and millennial timescales. These results will be written up shortly.

Another project currently underway involves the use of apparent temperatures and salinities (following the original work of Haney, 1971) instead of climatological values as restoring surface boundary conditions on temperature and salinity in ocean general circulation models. The usefulness of this method is under investigation.

2) - Finite-Element Modelling

This four year project involves a systematic procedure for model development. We have started with the barotropic vorticity equation which allows for full topography and specified stratification (Myers and Weaver, 1994). More recently we have added time-dependence to the nonlinear model and it has been extended to spherical coordinates and global geometry. Upon development of the three-dimensional code, comparisons will be done with the Bryan-Cox OGCM, initially using idealized geometry and proceeding to the global domain (project 1). The model will eventually be used for climate simulations (P. Myers, a PhD student, is working on this project).

The model has already proved to be extremely useful in diagnosing the transport of the North Atlantic (Myers et al. 1994). In this manuscript we suggest that the reason why 3-D ocean models do not get the Gulf Stream to separate at the correct latitude is due to a poor representation of the density field in the upper ocean. Research is also progressing well into the development of semi-Lagrangian advection algorithms appropriate for ocean models (e.g.. Das and Weaver 1994).

Side by side, the thermocline equations (with Laplacian friction in the boundary layers) in Cartesian co-ordinates are being solved. The semi-Lagrangian scheme is applied to the tracer equations only as the momentum equations will be diagnostic with frictional (Laplacian) boundary layers. The tri-cubic spline and cubic-Lagrange interpolating schemes are already in the working stage. A test scheme using an Eulerian method has also been set up for comparison and is now working for the advection-diffusion equation.

3) - Energy Balance Atmosphere/ Ocean Coupled Modelling

A diffusive heat transport energy balance model (EBM) has been developed and tested in both simplified and global domains. The EBM is loosely based upon the models of Budyko (1969), Sellers (1969), and North (1975). We have extended these models to allow coupling with the GFDL-MOM ocean general circulation model (Pacanowski et al., 1993) by allowing latent, sensible and radiative heat transfers between the ocean and atmosphere. In an effort to completely couple the ocean-atmosphere system, a moisture balance equation has also been added to the EBM so that freshwater fluxes can be predicted for the ocean model. The resultant energy-moisture balance model (EMBM) has been run in a global 2ˇ x 2ˇ domain under climatological forcing (sea surface temperature and surface wind speed fields), as well as for the decadal sea surface temperature fields for the 1950's and 1970's. Model results are quite encouraging, and a manuscript is currently in preparation (Fanning and Weaver, 1994).

A version of the ocean model has been coupled to the EMBM, however, additional testing and model verification are necessary. We intend to reexamine the variability studies of Weaver et al. (1993) with the coupled EMBM/OGCM.

4) - Coupled Ice/Ocean Modelling

So far we have coupled a thermodynamic ice model to the zonally averaged ocean model discussed in section 5. A visiting student from the Netherlands (Mr. Geert Lenderlink) and two new postdocs (Dr. David Holland and Dr. Sheng Zhang) who arrive in 1995 will work on the 3-D modelling project. Nevertheless, a thermodynamic ice model has been coupled to the global ocean model discussed in 1) for the purpose of further coupling to the Canadian Climate Centre Atmospheric GCM.

5) - Simple Coupled Models

Tang and Weaver (1994a) have worked on a simple coupled ocean-atmosphere box model. Incorporating the idea of Weaver (1993), the atmosphere water transport in the model is calculated from the poleward heat flux budget, so that the thermohaline circulation/fresh water interaction can be studied. It was found in the coupled model that the present-day high-latitude sinking thermohaline circulation becomes increasingly unstable as the earth warms, owning to the enhanced poleward fresh water transport.

Dr. Tang has recently extended this coupled box model study to a coupled zonally-averaged model. Being simpler but including the essential physics, the model is being used to study the variability of the coupled system, the role of atmosphere freshwater transport, and mechanisms of climate drift. The ocean component of the coupled model is being used to study the parameter regimes of stability and variability in a two-hemisphere ocean (Tang and Weaver 1994b).

References:

Budyko, M.I., 1969: The effect of solar radiation variations on the climate of the earth. Tellus, 21, 611-619.
Danabasoglu, G, J.C. McWilliams, & P.R. Gent, 1994: The role of mesoscale tracer transports in the global ocean circulation. Science, 264, 1123-1126.
Das, S.K. & A.J. Weaver, 1994: Semi-Lagrangian advection algorithms for ocean circulation models. J. Atmos. Ocean. Tech. submitted.
Fanning, A.F., & A.J. Weaver, 1994: An Atmospheric Energy Moisture Balance Model for Use in Climate Studies. To be sumbmitted to Atmos.-Ocean.
Gordon, A. L., 1986: Interocean exchange of thermocline water. J. Geophys. Res., 91, 5037-5046. Gent, P.R. & J.C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20,150-155.
Haney, R.L., 1971: Surface thermal boundary condition for ocean circulation models. J. Phys. Oceanogr., 1, 241-248.
Myers, P.G. & A.J. Weaver, 1994: A diagnostic barotropic finite element ocean circulation model. J. Atmos. Ocean. Tech., in press.
Myers, P.G., A.F. Fanning & A.J. Weaver, 1994: On the cause of Gulf Stream separation in ocean models. J. Phys. Oceanogr., submitted.
North, G.R., 1975: Theory of energy balance climate models. J. Atmos. Sci., 32, 2033-43.
Pacanowski, R., K. Dixon & A. Rosati, 1993: The GFDL Modular Ocean Model Users Guide. GFDL Ocean Grp Tech. Rep. #2.
Reynaud, T.H., A.J. Weaver, & Greatbatch, R.J. 1994a: Summer mean circulation in the western North Atlantic. J. Geophys. Res., in press.
Reynaud, T.H., A.J. Weaver & R.J. Greatbatch, 1994b: Interdecadal changes in the circulation of the western North Atlantic. To be submitted to J. Geophys. Res.
Sellers, W.D., 1969: A global climatic model based on the energy balance of the earth-atmosphere system. J. App. Met., 8, 392-400.
Tang, B., & A. J. Weaver, 1994a: Climate stability as deduced from an idealized coupled atmosphere-ocean model. Clim. Dyn., submitted.
Tang, B. & A. J. Weaver, 1994b: Stability and variability of the thermohaline circulation in two-hemisphere ocean models. To be submitted to J. Geophys. Res.
Weaver, A.J., 1994: Flux corrections in coupled ocean-atmosphere models. To be submitted to J. Clim.
Weaver, A.J. & W.G. Lee, 1994: A global OGCM for coupling to the Canadian Climate Centre AGCM: climatology and sensitivity analysis. To be submitted to Atmos.-Ocean.
Weaver, A.J., J. Marotzke, P.F. Cummins & E.S. Sarachik, 1993: Stability and variability of the thermohaline circulation. J. Phys. Oceanogr., 23, 39-60.
Wohlleben, T., & A.J. Weaver, 1994: Interdecadal climate variability in the subpolar North Atlantic. Clim. Dyn., submitted.

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