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Secular Trends and Climate Drift in Coupled Ocean-Atmosphere GCMs

PI: Curt Covey
Institution: Lawrence Livermore National Laboratory / PCMDI
Additional Investigators: Peter J. Gleckler
A long standing problem with coupled ocean-atmosphere GCMs is that when the ocean and atmosphere are linked together, the simulated climate can drift away from observations unless constrained by ad hoc adjustments to air-sea fluxes ("flux adjustments"). In 2001 the 3rd Assessment Report of the IPCC noted that some modern coupled GCMs are able to maintain stable runs without flux adjustments. In the PCMDI's first appraisal of the state of the art of coupled GCMs, we confirmed and elaborated this finding by examining output from the most recent phase of the Coupled Model Intercomparison Project. We found that surface temperature trends are small compared with observed trends, as expected for control run simulations in which external climate forcing factors are held constant. Deep ocean temperature and salinity trends are small enough for model control runs to extend over 1000 years or more, but trends in some regions, most notably the Arctic, are inconsistent among the models and may be problematic.

We plan to repeat this study using control run output from models contributing to the 4th Assessment Report of the IPCC. We require monthly means of surface temperature, 3D ocean temperature and 3D ocean salinity. We may also examine related fields such as ocean currents and air-sea fluxes of water, energy and momentum. Results for the IPCC 4AR models, when compared with the earlier generation of models in the CMIP database, will allow assessment of
model improvements.
  • Covey, C., P. J. Gleckler, T. J. Phillips, and D. C. Bader, 2006: Secular trends and climate drift in coupled ocean-atmosphere general circulation models. J. Geophys. Res., 111, D03107, doi:10.1029/2005JD006009. Abstract. Edit.

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