In this progress report I will only discuss work that was directly funded by the CICS - Variability grant. A copy of this progress report and the original grant proposal are available on the world wide web at:
T. Hughes (partially funded off the CICS-Variability grant) recently ontained her PhD and has written a manuscript on results from the second chapter of her thesis (Hughes and Weaver, 1995). In this manuscript we explored the role of the sea surface temperature - evaporation feedback for the ocean's thermohaline circulation. Briefly, a positive feedback is found whereby the overturning circulation warms the high latitudes through advection, increasing the latent heat loss and raising the sea surface salinity, which then feeds back on to the overturning. The magnitude of the time-dependent component of the evaporation is quite small however, which tends to support the use of fixed freshwater fluxes as a good first approximation to air-sea interaction. In agreement with this, two examples with internal variability of the thermohaline circulation on decadal and millennial timescales were studied. The variability was not fundamentally altered under the new feedback compared to control runs under "mixed boundary conditions", although both the period and the duration of the variability were shortened in some cases. This research will be presented at the CMOS Annual Congress this spring.
A diffusive heat transport energy balance model (EBM) has been developed by PhD student Augustus Fanning (partially funded off the CICS-Variability grant) 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 (Geophysical Fluid Dynamics Laboratory Modular Ocean 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 2 degree x 2 degree domain with fixed sea surface temperatures (Fanning and Weaver, 1995). 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).
The interpentadal model variability is quite robust. This effect was demonstrated by rerunning the model with parameters representative of several different unrealistic climatologies. The resulting interpentadal difference fields change remarkably little even when the background state has changed dramatically. Such a result appears to add convincing support for the use of flux corrections in coupled ocean-atmosphere modeling studies.
A version of the fully coupled ocean-atmosphere model (EMBM coupled to the GFDL-MOM) has been run in a single-hemisphere (60 degree x 60 degree) basin, driven by zonally uniform wind stress and solar insolation forcing. A series of several experiments of varying horizontal resolution (ranging from 4 degree x 4 degree to 1/4 degree x 1/4 degree) and viscosity have been conducted to assess the effect on the components of the net poleward heat transport. The model integrations are now complete, and we are currently analyzing the relative contributions of the mean and time variant components of the heat transport. These include the effects of the barotropic gyre transport (in the horizontal plane), the meridional overturning transport (in the zonal plane), the baroclinic gyre transport, as well as the eddy and diffusive heat transport components. An article on this work will be written up and submitted to Nature shortly.
In another project, A. Fanning is developing a double-hemisphere model representative of the Atlantic basin. This model incorporates a thermodynamic ice model (Semtner, 1976) (which includes heat insulation as well as brine rejection) into the coupled ocean-atmosphere model. This model will form the basis for a number of future studies. The first concerns whether multiple equilibria exist in an idealized coupled ocean-atmosphere model. Next we will investigate the causes for rapid climate variability during glacial-interglacial transitions, examining the roles of deglacial meltwater forcing as opposed to change in the solar insolation distribution.
Thierry Huck arrived at UVic on January 1, 1995 and will spend 16 months here on a France/Canada scientific exchange program (in lieu of French military service). His salary is fully supported off the CICS-Variability grant. T. Huck was midway through his PhD (under the supervision of Dr. A. Colin de Verdière) at the Université de Brest before he came to Victoria. He is continuing his PhD research here now jointly supervised by Dr. A. Colin de Verdière and me.
In his research T. Huck is investigating the effect of various momentum dissipation parameterizations in thermohaline circulation models using the planetary geostrophic equations. The traditional Laplacian momentum dissipation used presently in most ocean general circulation models is based on a conservation law appropriate for small scale viscous processes. For the large scales considered by ocean general circulation models, it parameterizes the process of barotropic instability better than the process of baroclinic instability. We are trying to determine what the consequences of this choice of momentum parameterization are in terms of the boundary layer structure of the ocean circulation. To this end comparisons will be done with other parameterizations of dissipation (e.g. Rayleigh friction) in both hydrostatic primitive equation models and non-hydrostatic planetary geostrophic models.
A new three-dimensional ocean circulation model has been developed using diagnostic planetary geostrophic dynamics and fully prognostic equations for potential temperature and salinity. Horizontal momentum dissipation is parameterized by linear Rayleigh friction, and different methods are used to solve for the non-hydrostatic boundary layers. The results in idealised geometry are being compared to those obtained using traditional Laplacian dissipation and also using fully prognostic dynamical equations. The comparison is based on identical atmospheric forcing, in order to analyse the effect of only the parameterization change in each run. The structure of the boundary layers and the large-scale circulation will be compared, as well as the thermohaline variability, in order to improve the understanding of the mechanisms involved.
T. Huck and A. Fanning both attended the recent NATO Advanced Study Institute, Les Houches, France, February 13-24 on decadal climate variability.
Over the past few months I have conducted research in a number of other areas. I have been heavily involved in the IPCC Second Scientific Assessment and am a lead author of two Chapters (Gates et al., 1996; Grassl et al., 1996). The IPCC process is designed to provide policy makers a treatise on our present understanding of climate change and climate variability.
At the end of 1994 I got together with Ed Boyle (a geochemist) at MIT to write a short piece for Nature (Boyle and Weaver, 1994) concerning multiple equilibria and variability of the North Atlantic thermohaline circulation and paleoclimatic data from the last glacial maximum.
In a paper submitted to Climate Dynamics in January (Weaver and Hughes, 1995), we showed that the magnitude of the mismatch between ocean general circulation model (OGCM) and atmospheric general circulation model (AGCM) fluxes is not as important for climate drift as the difference between OGCM and implied AGCM heat and freshwater transports. Hence a "Minimum Flux Correction" was proposed which is zonally-uniform in each basin and of small magnitude compared to present flux corrections. This minimum flux correction acts only to correct the AGCM implied oceanic transports of heat and freshwater. A slight extension was also proposed to overcome the drift in the surface waters when the minimum flux correction is used. Finally, we showed that the current methods used to determine flux corrections are all essentially equivalent leading to correction fields which are significantly larger than both AGCM and climatological fields over large regions.
Inspired by the success of the Minimum Flux Correction mentioned above, a separate experiment is underway. In this experiment we force the global ocean model with the newly-derived observed heat and freshwater fluxes over the ocean compiled by Da Silva et al. (1994). This fluxes have been tuned to constrain the zonally-integrated transports of both heat and freshwater toward observed estimates. The purpose of this project is to investigate new ways of spinning up OGCMs prior to coupling them with AGCMs.
Another project which is underway is an investigation of the importance of the seasonal cycle for the global thermohaline circulation. Preliminary results under seasonal sea surface temperatures and salinities show an intensification of the North Atlantic overturning circulation by > 3 Sv, which is accompanied by a cooling of the upper ocean and warming of the deep ocean in all three basins of the world ocean (Atlantic, Indian, Pacific). The Antarctic Circumpolar Current also warms significantly, although these changes appear (upon cursory inspection) to be driven by the Mediterranean. A next step will be to permit the winds to also vary seasonally.
I also worked collaboratively with Warren Lee in the Canadian Climate Centre (CCC) to develop a high-resolution global ocean model which has now been coupled to the CCC AGCM for the purpose of undertaking climate change/variability forecasts. I have a strong collaboration with the CCC and am the Scientific Leader of their Ocean Modelling effort. We now have a fully coupled atmosphere/ocean/ice model available and we are presently examining ways of reducing the flux correction which must be incorporated.
da Silva, A. M., C. C. Young and S. Levitus, 1995: Atlas of Surface Marine Data 1994, Volume 1: Algorithms and Procedures. NOAA Atlas NESDIS 7. In press.
Jones, P.D., 1988: Hemispheric surface air temperature variations: Recent trends and an update to 1987. J. Climate, 1, 654-660.
North, G.R., 1975: Theory of energy balance climate models. J. Atmos. Sci. 32, 2033-2043.
Pacanowski, R., K. Dixon and A. Rosati, 1993: The GFDL Modular Ocean Model Users Guide, GFDL Ocean Group Technical Report #2, 46pp.
Sellers, W.D., 1969: A global climatic model based on the energy balance of the earth-atmosphere system. J. Appl. Meteorol. 8, 392-400.
Semtner, A.J., 1976: A model for the thermodynamic growth of sea ice in numerical investigations of climate. J. Phys. Oceanogr., 6, 379-389.
2) CKST AM1040 Vancouver (1 hour phone-in show)
1) Victoria Times Colonist
2) Vancouver Sun
2) IBM Visions
2.* Boyle, E and A.J. Weaver, 1994: Conveying past climates. Nature, 372, 41-42.
3. Weaver, A.J., 1994: Decadal-millennial internal oceanic variability in coarse resolution ocean general circulation models. In: The Natural Variability of the Climate System on the 10–100 Year Time-Scales, National Academy Press, in press.
4. Tang, B., and A. J. Weaver, 1995a: Climate stability as deduced from an idealized coupled atmosphere-ocean model. Climate Dynamics, in press.
5. Wohlleben, T., and A.J. Weaver, 1995: Interdecadal climate variability in the subpolar North Atlantic. Climate Dynamics, in press.
6. Grassl, H., F. Giorgi, A. Kattenberg, G.A. Meehl, J.F.B. Mitchell, R.J. Stouffer, T. Tokioka, and A.J. Weaver, 1996: Climate models — Projections of future climate. Chapter 6 of the 1995 Second IPCC Scientific Assessment. Ed. Sir J. Houghton, in press.
7. Gates, L., A. Henderson-Sellers, G. Boer, C. Folland, A. Kitoh, B. McAvaney, F. Semazzi, N. Smith, A.J. Weaver and Q.-C. Zeng, 1996: Climate models — validation. Chapter 5 of the 1995 Second IPCC Scientific Assessment. Ed. Sir J. Houghton, in press.
8. Weaver, A.J., and C. Green, 1995: Global climate change/variability: Action or adaptation to increasing greenhouse gases? — Lessons from the past. Science and Public Policy, submitted.
9.* Fanning, A.F., and A.J. Weaver, 1995: An atmospheric energy moisture-balance model for use in climate studies. Journal of Geophysical Research, submitted.
10.* Weaver, A.J., and T.M.C Hughes, 1995: Flux corrections in coupled ocean-atmosphere models. Climate Dynamics, submitted.
11.* Hughes, T.M.C. and A.J. Weaver, 1995: Sea surface temperature — evaporation feedback and the ocean’s thermohaline circulation. Journal of Physical Oceanography, submitted.
13. Tang, B. and A.J. Weaver, 1995b: Stability and variability of the thermohaline circulation in two-hemisphere ocean models. To be submitted to Journal of Geophysical Research.
1) Partial support for PhD student A. Fanning: $8,000 2) Full support for PhD student T. Huck: $15,000 3) Full support for Research Associate T. Hughes: $35,000 4) Operating costs: $2,000 5) Publication charges $5,000 ___________________________________________________________________ Total: $65,000