DETERMINING THE MEAN, LARGE-SCALE CIRCULATION OF THE ATLANTIC WITH THE ADJOINT METHOD

A new model approach based on the adjoint formalism and aimed at assim ilating large sets of hydrographic data is presented. The goal of the model calculations is to obtain the mean, large-scale ocean circulatio n together with coefficients of iso- and diapycnal mixing and air-sea heat and freshwater fluxes. The final, optimal solution is required to be consistent with the principle of geostrophy and must predict reali stic distributions of temperature and salinity. The model covers the e ntire Atlantic and has realistic topography. Model resolution is nonun iform and ranges from 2.5-degrees to 10-degrees horizontally and from 60 m to 500 m in the vertical. Model velocities are initialized with g eostrophic flows calculated from original hydrographic station data; i nitial air-sea heat fluxes and mixing coefficients are taken from the literature. Mass, heat, and salt budgets are satisfied exactly, and fo r given boundary conditions and model flows the model temperature and salinity distributions are calculated. Then horizontal flows and the a ir-sea fluxes are modified automatically (in directions provided by th e adjoint model) until deviations between model temperatures and salin ities and their measured counterparts (data) are minimized while keepi ng the vertical shear of the horizontal velocities (as given by the in itial geostrophic flows) largely unchanged. Model results show that th e simulated temperature and salinity fields can indeed be brought to c lose agreement with the observed distributions. Modifications to the i nitial horizontal flows needed to achieve this agreement are found to consist mainly of constant velocity shifts (reference velocity) in the vertical profiles. The final, optimal flow field thus is consistent w ith geostrophic dynamics, and the present model can be regarded as a n ew approach to the classical problem of calculating reference velociti es from hydrographic data. Meaningful results are found for air-sea he at fluxes and for the vertical velocities w.