Sensitivity and realism of wind-driven tropical ocean models

The degree of realism of several equatorial ocean models is investigated by comparing modeled and observed mean seasonal cycles in the tropical Atlantic for two variables that play a role in sea surface temperature dynamics: 0/400 db dynamic height and surface current. The model reality intercomparisons are made using a multivariate statistical testing procedure that takes into account the uncertainties of the atmospheric and oceanic data. In particular, the uncertainties in the model response due to random errors in the wind forcing and its interannual variability are considered and the uncertainty in the bulk formulation for the wind stress is represented. Two simplified equatorial ocean models are considered: the linear multimode model of Cane (1984) and the two-layer primitive equation model of Andrich (1989). Some results are also given for the general circulation model of Philander and Pacanowski (1986), although the effect of forcing uncertainties could not be taken into account. It is found that the errors that were considered are unable to explain the large discrepancies between model simulations and observations. In, general, models represent better the 0/400 db dynamic height than the surface currents and the seasonal variations than the yearly mean. There is no striking difference in the performance of the three models for the dynamic height, but the general circulation model is superior for the surface currents, in particular near the western boundary. Better parameter tuning should, however, improve the performances of the two-layer model. It is also found that the sensitivity to forcing uncertainties is very model-dependent.