Air-sea fluxes based on observed annual cycle surface climatology and ocean model internal dynamics: a precise, non-damping zero-phase-lag approach applied to the Mediterranean Sea

A new model-based method of determining the surface fluxes of heat and freshwater that are needed to force ocean models is presented. In contrast to deriving the fluxes from a simulation with a restoring surface boundary condition, the new method determines the fluxes as a residual within the framework of physically realistic and natural boundary conditions on the sea surface temperature (SST) and sea surface salinity (SSS). The fluxes are computed (diagnosed) in such a way that an ensemble average of the model-simulated annual cycles of SST and SSS match the observed climatological annual cycles of SST and SSS, respectively. The surface boundary condition on the SST implicitly includes a net radiative flux (diagnosed) and a physically realistic heat exchange with the atmosphere (restoring flux), while the boundary condition on the SSS is the real freshwater flux (diagnosed) as proposed by Huang (J. Phys. Oceanogr., 33 (1993) 2428). Apart from being based on physically realistic surface boundary conditions, the advantage of the method is that it results in a realistic model simulation of the observed annual cycle of SST and SSS with no artificial damping of surface watermass fronts. The resulting heat fluxes and freshwater sources are realistic if the observed climatological data and model internal physics are accurate. The performance of the method is demonstrated using the DieCAST ocean model adapted to the Mediterranean Sea where the obtained model fluxes are compared with observations.