The inference of North Atlantic circulation patterns from climatological hydrographic data

Abstract

We present a review of β spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations. A specific technique is then designed and applied to the North Atlantic part of Levitus' (1982) climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat, salt, and potential vorticity. These quantities are determined on a 1° grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms. The estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters. The resulting circulation pattern bears strong resemblance to the classical view of the North Atlantic circulation as put foreward by Wüst (1935) and Defant (1941). The upper layers are dominated by the Gulf Stream/North Atlantic current system with a broad subtropical gyre recirculation. In the lower layer a western boundary current is fed from Norwegian Sea overflow penetrating the Gibbs fracture zone and partly circulating around the Labrador Sea. As a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities, in particular in the upper layer. The vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of Defant (1941). Diffusion coefficients were determined for an upper layer (depth of mixed layer to 800 m depth) and a lower layer (800 m to 2000 m). The spatial pattern of these coefficients correlates with maps of eddy activity, showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre. Average values in the lower layer of the quiet regions are 10−5 m²/s and 10² m²/s for the diapycnal and isopycnal diffusivity, respectively, and 10−1 m²/s for the vertical diffusivity of vorticity (which yields 10² m²/s for the lateral diffusivity of potential vorticity). Toward the regions of strong currents and in the upper layer these values roughly increase by an order of magnitude.