The North Atlantic circulation derived from inverse models

Abstract

This thesis describes two inverse models solving for a quasi-stationary ocean circulation, and discusses the circulation in the North Atlantic as derived from them. Both models are based on the adjoint technique and use finite-element discretization to accurately represent the sloping bottom topography. First a finite element inverse section model FEMSECT (Losch et al., 2004) is presented together with results of applying it to Fram Strait. FEMSECT exploits the thermal wind relation to estimate the velocity with respect to some reference level, and seeks for a compromise in the least square sense between the hydrographic and mooring data. Its novel feature is the ability to take into account the bottom triangles. Such inverse models are a standard tool to derive ocean transports from hydrographic measurements. However, they are not able to take into account the continuity constraint. The inverse finite element ocean model (IFEOM) presented afterwards respects the continuity locally and globally and also exploits the flexibility of 3D finite element grids. It is based on a steady-state version of the finite element ocean general circulation model FEOM (Danilovet al., 2004a). A steady state velocity field is determined from the momentum equations by the density field, and the stationary equation for the potential density is accounted for as a soft constraint. The IFEOM solves for density by minimizing the misfit between it and the density data under strong momentum and weak tracer balance constraint. Using an additional deep pressure gradient constraint (below 2000 m) is suggested and shown to be crucial for keeping the integral properties of the diagnosed ocean circulation close to those of the forward run of FEOM.The circulation in the North Atlantic is estimated by assimilating several data sets. The results are encouraging and indicate that IFEOM can be used to assimilate a climatological circulation from high quality hydrographic measurements.