The Atlantic Subtropical Front/Current Systems of Azores and St. Helena

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Abstract
A large-scale climatic ocean circulation model was used to study the Atlantic Ocean circulation. This inverse model is an extension of the β-spiral formulation presented in papers by Stommel and Schott with a more complete version of the vorticity equation, including relative vorticity in addition to planetary vorticity. Also, a more complete database for hydrological measurements in the Atlantic Ocean was used, including not only the National Oceanographic Data Center database but also World Ocean Circulation Experiment data and cruises near the Azores, Angola, and Guinea-Bissau. A detailed analysis of the Northern Hemisphere Azores Current and Front shows that this new database and the model results were able to capture all major features reported previously. In the Southern Hemisphere, the authors have identified fully and described the subtropical front that is the counterpart to the Azores Current, which they call the St. Helena Current and Front. Both current systems of both hemispheres have similar intensities, depth penetration, volume transports, and zonal flow. Both have associated subsurface adjacent countercurrent flows, and their main cores flow at similar latitudes (∼34°N for the Azores Current and 34°S for the St. Helena Current). It is argued that both current systems and associated fronts are the poleward 18°C Mode Water discontinuities of the two Atlantic subtropical gyres and that both originate at the corresponding hemisphere western boundary current systems from which they penetrate into the open ocean interior. Thus, both currents should have a similar forcing source, and their origin should not be linked to any geographical peculiarities.