Climate Modelling Group
School of Earth and Ocean Sciences

NSERC CSHD Collaborative Special Project

Paleoclimatic Modelling using Coupled Ocean-Ice-Atmosphere Models (April 17, 1996)

1. Introduction

The motivation and background for the Climate System History and Dynamics Paleoclimate proposal has been given in the Science Plan. Here we focus solely on our component of this collaborative project.

Recently Augustus Fanning, a PhD student working under A. Weaver's supervision, developed a diffusive heat transport energy balance model (EBM), that has been 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 ocean general circulation model (OGCM -- 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 EMBM has been run in a global 2deg. x 2deg. domain with fixed 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 considered in which the 1955-59 pentad was compared to the 1970-74 pentad by driving the model under the respective sea surface temperatures. The model exhibits global, as well as basin-mean temperature changes in the latter pentad comparable to direct estimates (Jones, 1988). These validations of the EMBM against the present climate give us greater confidence in its ability to be applied to past climates.

A global ocean general circulation model (OGCM) has also recently been developed in collaboration with Warren Lee and others for coupling to the Canadian Centre for Climate Modelling and Analysis (CCCMA) atmospheric general circulation model (AGCM). Two versions of this OGCM now exist: the first is a high resolution model (1.8deg. x 1.8deg. x 29 levels) which has now been coupled to the CCC AGCM to investigate the climatic response to increasing atmospheric greenhouse gases and aerosols. The second version of the model is at slightly coarser resolution (3.6deg. x 1.8deg. x 19 levels). It is this latter, computationally-efficient, OGCM that we propose to use in this project, a description of which may be found in Weaver and Hughes (1996).

Fanning and Weaver (1996) also show how the coupling of this EMBM and OGCM, into which a thermodynamic ice model (TIM) has been incorporated, leads to a very reasonable simulation of the present climate. The virtue of this simple atmospheric model is that we do not need to employ flux adjustments to keep the simulation of the present climate stable. Thus, it is relatively easy to apply this model to past climates through a change in the seasonality and magnitude of incoming solar radiation and through other changes in radiative forcing. We have also recently incorporated a simple parameterization which allows for wind-stress feedbacks through the calculation of geostrophic wind anomalies where Rayleigh friction becomes important near the Equator. This coupled model is one of the numerical tools which we shall apply to the projects detailed below.

This project will also include some aspects of the modelling work initiated within CSHD 5. [The latter project will be terminated as a result of Dr. Hyde leaving Dalhousie University. Only aspects of CSHD 5 which we intend to continue within the present project will be discussed here.] Within CSHD 5, the low order climate model of Wright and Stocker has been extended to include a representation of the major land masses as well as seasonal cycles associated with variations in incoming shortwave radiation. These developments are essential prerequisites for the inclusion of an active cryosphere component in the model. An inorganic carbon cycle component of the model has also been developed in collaboration Thomas Stocker of the Climate Research Institute at the University of Bern. This work represents a step towards allowing for the influence of variations in atmospheric CO2 associated with exchanges between the ocean and the atmosphere. Both of these developments are relevant to the examination of climatic variations on the millenial timescale which represents the second major focus of this project.

2. Simulation of the Younger Dryas event and the Transition from the Last Glacial Maximum to the Holocene

The use of models to simulate past climatic events is an important avenue of investigation if one is to have confidence in their application to future climatic changes. To this end the realistic geometry, global, coupled OGCM-EMBM-TIM (mentioned above) will be used to simulate the transition between the last glacial maximum and the present Holocene. During the transition, an abrupt return to glacial climatic conditions, known as the Younger Dryas (YD) occurred. The YD cold episode was particularly pronounced in regions bordering the North Atlantic, and is evidenced in northern European and maritime Canadian lakes and bogs; North Atlantic marine sediments; and northwestern European and central Canadian glacial moraines. Although strongest in the northern Atlantic region, further evidence indicates the impacts of the YD were felt throughout the globe.

While the exact cause of the YD is still unknown, a general consensus has emerged that it was linked to oceanographic phenomena. The amplification of the YD signal in the northeastern North Atlantic suggests a primary role for the thermohaline circulation, particularly North Atlantic Deep Water (NADW) production. The question naturally arises as to what source could supply the necessary excess freshwater needed to reduce NADW formation. The obvious sources are the polar ice caps and the Laurentide and Fennoscandian ice sheets (LIS and FIS, respectively). The traditional viewpoint 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 al., 1988). However, the fact that deep water is usually formed in local high-latitude regions of small extent suggests that not only the amount, but also the location of meltwater introduced is crucial for interrupting the North Atlantic Conveyor. Simulations of changes in ocean circulation during the Younger Dryas event have already been done within CSHD 5 and 8, but the idealized models used in these projects do not permit examination of the influence of meltwater input location. This study is thus a natural next step to be taken within the CSHD project.

We shall conduct experiments to reinvestigate the climatic implications of the geographical and temporal change in the runoff from the LIS and FIS, utilizing estimated meltwater and precipitation runoff from drainage basins in and around the North Atlantic, before, during, and after the YD (Teller, 1990). While Maier-Reimer and Mikolajewicz (1989), using an ocean-only model, found they were capable of shutting down NADW production within 200 years from the time they deflected 347 km3/yr from the Gulf of Mexico into the St. Lawrence valley (roughly half that estimated to have occurred), our initial results suggest the traditional meltwater diversion theory is incapable of inducing a shutdown of NADW in our EMBM. If, however, we apply the runoff estimates previous to the YD, the conveyor is pushed to the brink, allowing the diversion of LIS waters to completely halt NADW production. In an additional experiment we will consider the role of the FIS meltwater on the YD climate.

3. The Effect of Oceanic Gateways on Paleoclimate

As an extension of the above paleoclimatic experiment, we will also conduct a sequence of experiments with the coupled model to investigate the consequences of opening and closing oceanic gateways such as the Isthmus of Panama and Drake Passage (about 3.5 and 30 million years ago, respectively). In particular, attention will be focused on the changes in meridional oceanic heat transport and the relation to glacial events in the paleoclimate record.

4. The Dependence of Paleoclimate Variability on the Mean Climatic State

Late last year we acquired the GFDL coupled climate model for use on A. Weaver's local work station cluster. Drs. S. Valcke, S. Zhang and A. Weaver, now have this model running and are using it (in close collaboration with Ron Stouffer and Suki Manabe) to investigate the existence of climate variability in the coupled climate system and how it varies as the mean climatic state changes. We plan to initially use the coupled EMBM-TIM-OGCM to streamline the experiments that we will perform with the GFDL coupled model as it is simpler and hence more computationally efficient.

We intend to examine the hypotheses of Weaver and Hughes (1994) and Tang and Weaver (1995) that warmer climates may be more variable due to an intensification of the hydrological cycle. In addition, we will investigate whether colder climates are also more unstable through ice-thermohaline circulation instabilities (Broecker et al., 1990) or through diffusive-timescale internal oceanic oscillations (Weaver and Sarachik, 1991). That is, we wish to understand why the Holocene has exhibited a remarkably stable climate which appears to be unparalleled in recent Earth's climatic history.

5. Confirmation of Dansgaard-Oeschger oscillations in low-order climate models

Considering the potential significance of the discovery of Dansgaard-Oeschger-like oscillations in the model of Sakai and Peltier (1995), it is appropriate to determine if similar variability may exist in the less viscid model of Wright and Stocker. If similar variability is found, it will be of interest to determine if the same physical mechanism is responsible for the variability in the two models. As part of this investigation, a generalization of the Wright and Stocker code to explicitely include vertical and meridional diffusion of momentum is being developed. Analysis of model runs will help to clarify the strengths and weaknesses of both modelling approaches and subsequent work will be carried out with an appropriately generalized formulation. It should also be noted that the earlier work of Tang and Weaver (1995) is directly relevant to this topic and comparisons with their results will also be investigated.

6. Improved representations of land-sea and seasonal variations in the extended Wright and Stocker climate model

In order to extend a low order climate model to include an active cryosphere, it must include both realistic seasonal variations and land-sea contrast. The low order climate model of Wright and Stocker has been extended to include these effects within the CSHD 5 project. However, comparison with the Crutcher-Meserve and GEDEX data sets reveals significant discrepancies in both aspects of the model's variability, suggesting inadequacies in the atmospheric component of the model. Indeed, it appears likely that the atmosphere is the weakest component of the Wright and Stocker climate model and improvement is highly desirable. As part of the present project, we will perform detailed diagnostic analysis of model results to determine the major source(s) of these discrepancies; implement appropriate improvements in the atmospheric model, hydrological cycle, and coupling procedure; and verify results against available data.

7. Investigation of the influence of variations in the thermohaline circulation on ice sheet growth and decay

Once the atmospheric component of the climate model has been improved and verified, we will implement an idealized cryosphere component into the model. We will survey the literature to identify an existing model of the cryosphere which is consistent with our overall goal of developing a highly efficient but dynamically consistent model of the global climate system. The modelling approach followed by Weertman (1961, 1976), Pollard (1978) and Oerlemans and co-workers (1980, 1982, 1984) are generally consistent with ours and these and more recent adaptations will be considered. After an appropriate model is identified, implemented, tested and (presumably) modified we will examine the oceanic influence on ice sheet variations. The final aspect of this sub-project will be the incorporation of a carbon cycle component in the model which will allow for the exchange of carbon between the oceanic and atmospheric reservoirs. Initially, our intention is simply to determine the magnitude of the changes induced by including atmospheric carbon dioxide variations in a model which includes the feedbacks associated with both the oceans and the cryosphere.

8. References not in the NSERC Personal Data Forms 100

Broecker, W.S. et al., 1988: The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event. Paleoceanogr., 3, 1-19.

Broecker, W.S. et al., 1990: A salt oscillator in the glacial North Atlantic? -- The concept. Paleoceanogr., 5, 469-477.

Budyko, M.I., 1969: The effect of solar radiation variations on the climate of the earth. Tellus, 21, 611-619.

Jones, P.D., 1988: Hemispheric surface air temperature variations: Recent trends and an update to 1987. J. Climate, 1, 654-660.

Maier-Reimer, E. and U. Mikolajewicz, 1989: Experiments with an OGCM on the cause of the Younger Dryas, Max Planck Institute fur Meteorologie, Report #39, 13 pp.

North, G.R., 1975: Theory of energy balance climate models. J. Atmos. Sci., 32, 2033-2043.

Oerlemans, J., 1980: Model experiments on the 100,000 yr glacial cycle. Nature, 287, 430-432.

Oerlemans, J., 1982: Glacial cycles and ice-sheet modelling. Clim. Change, 4, 353-374.

Oerlemans, J. and C.J. van der Veen, 1984: Ice Sheets and Climate, 217pp., Riedel.

Pacanowski, R. et al., 1993: The GFDL Modular Ocean Model Users Guide, GFDL Ocean Group Technical Report #2, 46pp.

Pollard, D., 1978: An investigation of the astronomical theory of the ice ages using a simple climate-ice sheet model. Nature, 272, 233-235.

Sakai, K. and W.R. Peltier, 1995: A Simple Model of the Atlantic Thermohaline Circulation: Inernal and Forced Variability with Paleoclimatological Implications, J. Geophys. Res., 100, 13,455-13,479.

Sellers, W.D., 1969: A global climatic model based on the energy balance of the earth-atmosphere system. J. Appl. Meteorol., 8, 392-400.

Teller, J.T., 1990: Meltwater and precipitation runoff to the North Atlantic, Arctic, and Gulf of Mexico from the Laurentide Ice sheet and adjacent regions during the Younger Dryas, Paleoceanogr., 5, 897-905.

Justification for Budget Request:

1) Salaries and benefits

a) Graduate Students

Full support for one student (P. Poussart who arrives in September 1996; partial support for A. Fanning and T. Murdock until September, 1996) at the current NSERC rate. This project will involve 3 graduate students (Fanning, Murdock, Poussart). Fanning and Murdock should graduate early into the term of this project. I suspect Murdock will continue on in the PhD program once he receives his MSc. Murdock will be partially supported off my NOAA Grant and Fanning is partially supported by an Atlantic Career Development Award.

d) Postdoctoral Fellows/Research Associates

Full support for S. Zhang (UVic) and D. Brickman (Dal) for 1996-1997 & 1997-1998. Full support for Dr. Sophie Valcke from year 2 onwards. She is currently completing the first year of her NSERC Post doctoral fellowship.

3) Materials and Supplies

Toner cartridges, computer paper, magnetic tapes, computer manuals, drafting costs, video cassettes for computer movies, photocopying charges, communication charges (telephone, fax, courier) etc.

4) Travel

Travel to CHSD annual meetings.

5) Dissemination Costs

Publication and reprint charges. The total ($8,000) is significantly less than we have had to pay out during the 1995-96 fiscal year (~$25,000).
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