FINDING THE STEADY-STATE OF A GENERAL-CIRCULATION MODEL THROUGH DATA ASSIMILATION - APPLICATION TO THE NORTH-ATLANTIC OCEAN

An attempt is made to find a steady state of a general circulation mod el consistent with error estimates of the North Atlantic hydrography d uring 1981-1985 and with wind and thermodynamic boundary conditions. T he quadratic misfit of the model state from the data is minimized usin g the general circulation model together with its adjoint. A state whi ch is steady within limits of estimated observational error is found, but no such state is simultaneously consistent with the observed hydro graphic and surface flux fields. Model dynamics are able to sharpen gr adients which were overly smooth in the mapped data, producing a merid ional overturning cell with a maximum value of about 21 Sv. At equilib rium, the model must produce its own water masses and has a strong ten dency to go toward wintertime conditions. The consequence is an estima ted model surface temperature systematically lower than in the hydrogr aphy (which comes from all seasons). The western boundary current ther mocline is also both colder and fresher than the mapped hydrography, a nd the overall meridional heat transport is low (about 0.6 x 10(15) W) . The results suggest that the concept of a realistic steady state Nor th Atlantic circulation has reached the end of its utility, that model s with realistic property fluxes and divergences must have much higher resolution, and that the open-ocean boundary conditions must be formu lated as control variables.