An intercomparison between four tropical ocean models Thermocline variability

Abstract

A multivariate model testing procedure is used to intercompare several tropical ocean models, using the evolution of the thermocline depth during the 1982—1984 FOCAL/SEQUAL experiment in the equatorial Atlantic as observational basis. Four models of increasing complexity are considered: Cane’s linear multimode model, a nonlinear 2-layer model developed at LODYC, the KNMI GCM and the LODYC GCM. Some of the uncertainties in the atmospheric forcing are taken into account by forcing the models 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 model response are substantial, none of the models is fully consistent with the observations, i.e., within the error bars. The more complex models represent the thermocline depth variations significantly better than the simpler ones, in particular when the comparison is done over broad geographical areas. When the whole intercomparison domain (12°N-12°S) is considered, the LODYC GCM performs better than the KNMI GCM, both for the yearly mean conditions and for the variations around the 3-year mean, while the 2 models have comparable performance in the equatorial waveguide (3°N-3°S). In view of its simplicity, the linear model shows much skill, in particular for simulating the long-term mean. The 2-layer model poorly represents the long-term mean position of the thermocline, but represents well the variations around the 3-year mean, outperforming the linear model in the equatorial waveguide. Along meridional sections, model performance may 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 suggests that, because of the small signal-to-noise ratio, most models have only little skill in the 12°N-12°S domain, but more in the equatorial waveguide. The linear model, however, does not seem to have predicative skill for these noisy anomalies.