Climate Modelling Group
School of Earth and Ocean Sciences

to Scripps Institution of Oceanography

Subcontractor: University of Victoria

SEMI-ANNUAL REPORT PERIOD: November 1, 1995 through March 31, 1996

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

Project Title: The Lamont/Scripps Consortium for Climate Research - Dynamical Modeling of Climate Change.

NOAA Award No: NA47GP0188

Principal Investigator: Andrew Weaver

Project Period: May 1, 1994 through April 30, 1997

Budget Period: November 1, 1995 through March 31, 1996

Performance Report Completed: March 27, 1996

Below I summarize the progress on the research funded through the NOAA Lamont/Scripps Consortium for Climate Research. I have also enclosed a list of all publications which were supported through these funds and a few reprints for articles which have recently appeared.

1 Simulation of the Younger Dryas event

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, as part of the Abrupt Climate Change component of the Consortium Science Plan we have undertaken a simulation of the Younger Dryas event (hereafter YD). A realistic geometry, global, coupled OGCM-energy moisture balance model (EMBM) is currently being used to investigate the transition between the last glaciation and the present Holocene. During the transition, an abrupt return to glacial climatic conditions, known as the 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 an oceanographic phenomena. The amplification of the YD signal in the northeastern North Atlantic suggests a primary role for the THC, particularly North Atlantic deepwater (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.

We are currently conducting 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 results suggest the traditional meltwater diversion theory is incapable of 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 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. These results will be written up for publication shortly.

2 Oceanic poleward heat transport and decadal variability as a function of OGCM resolution

Five experiments have been conducted with a single hemisphere (60deg. x 60deg.) EMBM/OGCM, driven by zonally uniform wind stress and solar insolation forcing, with horizontal resolution ranging from 4deg. x 4deg. to 0.5deg. x 0.5deg.. Poleward heat transport is shown to significantly increase from coarse to finer resolution. Our coupled atmosphere-ocean model results contradict earlier studies which showed that the time-variant (eddy) component of poleward heat transport counteracts increases in the time mean flow. This is perhaps related to the relatively short integration times utilized by these previous works. An additional mechanism may be the inclusion of salinity in our analysis. Previous works utilized buoyancy forcing alone so that eddies were aligned along isopycnals and hence no net heat transport occurs by their presence. In the present work, isotherms and isopycnals no longer coincide and a net heat transport can be expected if eddies propagate across isopycnals. Even though the net oceanic heat transport has not converged, the net planetary heat transport has converged owing to the strong constraint of energy balance at the top of the atmosphere. Consequently, the atmospheric heat transport is reduced to offset the increasing oceanic heat transport. Currently we are extending the resolution studies to 1/4deg. x 1/4deg..

Of particular importance in this study is that spontaneous decadal variability is found to exist in the 0.5deg. x 0.5deg. resolution case (in both coupled and uncoupled models). We find the variability is strongly linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity from 200 m2/s to 500 m2/s is enough to destroy the variability, while decreasing the diffusivity from 500 m2/s to 200 m2/s (in the 1deg. x 1deg. case) is capable of inducing the variability.

3 Flux adjustments and their influence in coupled models

We are also investigating the role of flux adjustments on interdecadal climate variability. The numerical simulations of Delworth et al. (1993), using the GFDL coupled model revealed interdecadal variability of the THC 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. It is also unclear whether or not this variability is linked to coupled ocean-atmosphere dynamics or to ocean dynamics alone. In order to do elucidate this, the GFDL coupled model has been adapted to our local IBM cluster. The oceanic part of this model is now being run under fixed-flux boundary conditions, made up of atmospheric fluxes (diagnosed from the atmospheric model at equilibrium) and the flux adjustment terms. If similar variability as in the fully coupled experiments is found, we can conclude that the variability is due to internal ocean dynamics alone.

4 Decadal variability in OGCMs with various subgrid-scale boundary layer parameterizations

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 momentum dissipation (linear, harmonic, biharmonic or none) and associated boundary conditions (no-slip, free-slip or vorticity closure). These models were first compared to the GFDL OGCM with the same geometry and forcing to validate the planetary geostrophic dynamics. Results from this analysis will be written up shortly. Of particular importance to the NOAA Consortium is the effect that different momentum dissipation parameterizations have on the internal decadal-interdecadal variability found in ocean models. Through the use of these efficient ocean models we have found that atmospheric forcing plays the leading role in generating decadal variability in ocean models: flux boundary conditions are the most likely to allow variability as no damping applies to surface anomalies, although the spatial distribution is important. A parameter sensitivity study of the oscillatory behaviour has also been carried out. Results suggest that the horizontal tracer diffusivity has a critical damping effect, while increasing the vertical diffusivity strongly enhances the oscillations. The parameterization or even inclusion of convection is found not to be necessary in sustaining the decadal variability, although it is necessary to remove static instabilities. As pointed out by Winton (1996), the variation of the Coriolis parameter with latitude is not necessary, so saying that Rossby wave propagation is not important for the oscillations. Greatbatch and Peterson (1995) proposed an explanation in terms of Kelvin waves propagating around the basin. This mechanism was investigated by moving the boundaries or by forcing an f-plane model with a symmetric (about the meridional centre of the basin) heat flux. None of these major changes remove the oscillatory behavior; therefore we conclude that Kelvin wave propagation is not important for the oscillation. As the variability is mainly observed in the region of separation 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)

5 Development of a finite element OGCM

The finite element model described in the previous progress report has been used to study the circulation of the North Pacific Ocean (Myers and Weaver, 1996). With the inclusion of the JEBAR term, the model produced a very realistic picture of the circulation. All major currents were reproduced with the calculated transports agreeing well with the observations. The effect of using different wind stress climatologies was also examined. Due to the dominance of the JEBAR term in the solution, the resulting circulations were all similar. Analysis of the seasonal cycle in the model supports Sakamoto and Yamagata (1995) in that JEBAR rectification can explain the decreased amplitude of the seasonal cycle and the out of phase relationship between observations and the predictions of flat-bottomed Sverdrup theory. Finally, density fields from 1955-1959 and 1970-1974 were used to examine aspects of interpentadal variability in the North Pacific Ocean.

6 Mixing schemes in ocean models

Due to the reduction of vertical mixing when the Gent and McWilliams (1990) scheme is incorporated into the GFDL OGCM, numerical problems associated with vertical grid Peclet violations were found to occur. A flux-corrected transport (FCT) scheme (Gerdes et. al 1991) was therefore implemented into the GFDL OGCM and the consequences of using this advection scheme to eliminate these numerical problems are being investigated. Several integrations comparing mixing and advection schemes, in a simple model, demonstrate that it may be necessary to use a more sophisticated advection scheme (like FCT) when using isopycnal mixing parameterizations.

References not in attached list:

Gent and McWilliams, 1990: Isopycnal Mixing in Ocean Circulation Models, J. Phys. Oceanogr., 20, 150-155.

Broecker, Andree, Wolfli, Oeschger, Bonani, Kennett and Peteet, 1988: The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event. Paleoceanogr., 3, 1-19.

Delworth, Manabe and Stouffer, 1993: Interdecadal variations of the THC in a coupled ocean-atmosphere model. J. Climate, 6, 1993-2011.

Gerdes, Koeberle and Willebrand, 1991: The influence of numerical advection schemes on the results of ocean general circulation models. Clim Dynamics 5, 211-226.

Greatbatch and Peterson, 1996: Interdecadal variability and oceanic thermohaline adjustment. J. Phys. Oceanogr., in press.

Maier-Reimer and Mikolajewicz, 1989: Experiments with an OGCM on the cause of the Younger Dryas, MPI, Report #39, 13 pp.

Sakamoto and Yamagata, 1995: Seasonal transport variations of the wind-driven oceaqn circulation in a two-layer planetray geostrophic model with a continental slope. J. Mar. Res., submitted.

Teller 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.

Weaver and Sarachik, 1991: Evidence for decadal variability in an ocean general circulation model: an advective mechanism. Atmos.-Ocean, 29, 197-231.

Winton 1996: The role of horizontal boundaries in parameter sensitivity and decadal-scale variability of coarse-resolution ocean general circulation models. J. Phys. Oceanogr., 26, 2.

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