AN INTERCOMPARISON BETWEEN 4 TROPICAL OCEAN MODELS - THERMOCLINE VARIABILITY

A multivariate model testing procedure is used to intercompare several tropical ocean models, using the evolution of the thermocline depth d uring the 1982-1984 FOCAL/SEQUAL experiment in the equatorial Atlantic as observational basis. Four models of increasing complexity are cons idered: Cane's linear multimode model, a nonlinear 2-layer model devel oped at LODYC, the KNMI GCM and the LODYC GCM. Some of the uncertainti es in the atmospheric forcing are taken into account by forcing the mo dels with 3 equally plausible windstress fields whose differences are consistent with the wind measurement and sampling errors, and the drag coefficient indeterminacy. Although the resulting uncertainties in mo del response are substantial, none of the models is fully consistent w ith the observations, i.e., within the error bars. The more complex mo dels represent the thermocline depth variations significantly better t han the simpler ones, in particular when the comparison is done over b road geographical areas. When the whole intercomparison domain (12 deg rees N-12 degrees S) is considered, the LODYC GCM performs better than the KNMI GCM, both for the yearly mean conditions and for the variati ons around the 3-year mean, while the 2 models have comparable perform ance in the equatorial waveguide (3 degrees N-3 degrees S). In view of its simplicity, the linear model shows much skill, in particular for simulating the longterm mean. The 2-layer model poorly represents the long-term mean position of the thermocline, but represents well the va riations around the 3-year mean, outperforming the linear model in the equatorial waveguide. Along meridional sections, model performance ma y be more variable, stressing the need for a global approach to model validation. Finally, a coarse investigation of the simulations of the thermocline depth anomalies with respect to the mean seasonal cycle su ggests that, because of the small signal-to-noise ratio, most models h ave only little skill in the 12 degrees N-12 degrees S domain, but mor e in the equatorial waveguide. The linear model, however, does not see m to have predicative skill for these noisy anomalies.