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gfdl's home page > gfdl on-line bibliography > 1999: The Seasonal Cycle in Coupled Ocean-Atmosphere General Circulation Models, 28 pp
The Seasonal Cycle in Coupled Ocean-Atmosphere General Circulation Models
| Covey, C., A. A-Ouchi, G. J. Boer, G. M. Flato, B. A. Boville, G. A. Meehl, U. Cubasch, E. Roeckner, H. Gordon, E. Guilyardi, L. Terray, X. Jiang, R. Miller, G. Russell, T. C. Johns, H. LeTreut, L. Fairhead, G. Madec, A. Noda, S. B. Power, E. K. Schneider, R. J. Stouffer, and J-S. von Storch, 1999: The Seasonal Cycle in Coupled Ocean-Atmosphere General Circulation Models, PCMDI Report No. 51 - UCRL-JC-133438, Livermore, CA: University of California, Lawrence Livermore National Laboratory, 28 pp with figures. |
| Abstract: We examine the seasonal cycle of near-surface air temperature simulated by 17 modern coupled ocean-atmosphere general circulation models. Nine of the models use ad hoc "flux adjustment" at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux adjusted models separately and examine the behavior of each class. Near-surface air temperatures averaged over all flux-adjusted models fall within 2°K of the observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ~6 K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ~5 K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. |
| We use the temperature difference between July and January as a measure of seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ~30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ~5 K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. |
| The ability of coupled GCMs to simulate a reasonable seasonal cycle has been cited as demonstrating their ability to respond properly to external forcing, which supports their use for prediction of climatic changes such as anthropogenic greenhouse warming. To test this connection, we compare seasonal cycle amplitudes with equilibrium warmings under doubled atmospheric carbon dioxide for the models in our data base. A small positive correlation exists between these two quantities, as expected from a simple conceptual model of the climate system. |