A diagnosis of the North Atlantic horizontal and vertical circulation with error estimates
Authors: Isayev Grigory , Levitus Sydney 
Published: February 2004

A diagnostic model for computation of the horizontal and vertical components of large‐scale, time‐averaged oceanic circulation from climatologically averaged observed data is presented that completely avoids the use of advection‐diffusion balance. First, within this model the total transport is determined from density, wind stress, and bottom topography following the Mellor et al. (1982) model for computation of the total transport stream function. Then the ocean bottom is chosen as a reference surface. The horizontal and vertical components of reference (bottom) velocity are expressed through the total transport and the standard oceanographic data indicated above. A new method for determining vertical velocity is presented allowing computation of the vertical component of bottom velocity that is as reliable as computation of the horizontal components of bottom velocity. The horizontal and vertical components of absolute geostrophic velocity at different levels are determined as the horizontal and vertical components of bottom velocity plus the geostrophic shears at these levels relative to the bottom. The geostrophic shear of horizontal velocity is determined by the thermal wind relations; the geostrophic shear of vertical velocity is determined by the geostrophic vorticity balance. The model is applied to the computation and analysis of the North Atlantic climatological absolute geostrophic circulation. All input and output data are on a 1°×1° grid. The computed reference (bottom) velocity field contains the large‐scale flows which in some areas considerably affect the absolute geostrophic velocity field in the upper layer, namely, the Iceland, Greenland, and Labrador Currents of up to 20 cm/s, the countercirculation north of the Gulf Stream of up to 15 cm/s−1, the Azores circulation of up to 5 cm/s, and the coherent flow of up to 7 cm/s south of 30°N, which we call the Eastern (east of the Mid‐Atlantic Ridge) and Western (west of the Mid‐Atlantic Ridge) loops. All bottom flows mentioned above follow ƒ/h (planetary vorticity, where ƒ is the Coriolis parameter and h is the variable ocean depth) contours, are counterclockwise, and obey the following rule: the flow is upslope where the meridional bottom velocity is southward and vice versa. The computed vertical component of bottom velocity is typically several times larger than the Ekman pumping. The signs of the vertical component of bottom velocity are dynamically consistent with the signs of the meridional transport driven by bottom pressure torque and the vertical component of absolute geostrophic velocity at the ocean surface agrees with the Ekman pumping. Relative errors of the horizontal and vertical components of bottom velocity in the areas of these bottom flows are ∼10–20%. The diagnostic computations, analysis of the basic dynamics, and the error estimates indicate the following features of the North Atlantic large‐scale circulation. The Gulf Stream system, the North Atlantic Current, and the subtropical gyre are mainly baroclinic. The Iceland, Greenland, and Labrador currents, the countercurrent north of the Gulf Stream, and the Azores circulation are mainly barotropic. In the latitudes between 20°N and 30°N there is the broad westward flow across the whole North Atlantic with a speed in the main thermocline of 5–10 cm/s−1. This flow has comparable baroclinic and barotropic components and is separate from the subtropical gyre. The subtropical gyre consists of two circulation cells. The two‐cell structure is expressed in the main thermocline both in the absolute velocity field and in the geostrophic shear relative to the bottom. It is determined by the climatological density field and the thermal wind relations. South of 30°N there is the nonconventional large‐scale westward bottom flow along ƒ/h contours of up to 7 cm/s−1. Reality of this bottom flow is supported by independent evidence based on sediment data.