The North Atlantic Circulation in the Early 1980s-An Estimate from Inversion of a Finite-Difference Model

Abstract

A finite-difference model of the North Atlantic is constructed for the purpose Of making an estimate of the circulation through an inverse calculation. The database is eclectic, and includes hydrography, oxygen, nutrients current meter and float records, atmospheric momentum, heat and water vapor transfers as well as estimates of certain integral fluxes. Owing to the available hydrographic database, the model resolution is restricted to 1° at best, and is much coarser in many aspects. This limited resolution is a major obstacle to accurate estimates of climatological fluxes. In its final form, there are about 9000 constraints in 29 000 formal unknowns plus 9000 noise unknowns. The system is solved as a tapered least-squares system by a sparse conjugate gradient algorithm. With the exception of a few float velocities, all constraints are found to be consistent within error estimates. The model produces estimates of large-scale fluxes and flux divergences for all conventional properties including heat and nutrients as well as carbon dioxide and alkalinity. Meridional fluxes of carbon are found to be indistinguishable from zero, whereas the North Atlantic tends to export nutrients to the south, but carry heat to the north. Traditional oceanographic depictions of the circulation through combination of nonsynoptic data into steady models may have reached their useful limit in the present calculation, as the conflicts between the data and physical requirements become quantitatively apparent.