Physical and biogeochemical fluxes and net budgets in the subpolar and temperate North Atlantic

Abstract

A transoceanic hydrographic section across the North Atlantic Subpolar gyre from Vigo (northwestern Iberian Peninsula) to Cape Farewell (south of Greenland) was sampled in summer 1997 as part of the World Ocean Circulation Experiment program (WOCE A25, 4x cruise). The circulation pattern across the 4x section is diagnosed using inverse methods. The flow is constrained with measured mass transports at specific sites, while conserving mass and salt for the region north of the section and forcing the silicate flux to a reasonable value. The fluxes of physical (heat and freshwater) and chemical (nutrients and oxygen) properties are estimated and decomposed into barotropic, baroclinic and horizontal components. The heat transport amounts to 0.65 ± 0.1 PW poleward, with 54% and 45% of the flux due to the baroclinic (or overturning) and horizontal circulation, respectively. From the salt conservation, an equatorward freshwater transport of-0.4 ± 1.5 Sv is estimated, resulting from net precipitation plus runoff over the North Atlantic Ocean north of the section. The Subpolar gyre exports nutrients and oxygen southward toward the Subtropical ocean at rates of -50 ± 19, -6 ± 2, -26 ± 15 and -1992 ± 440 kmol s-1 for nitrate, phosphate, silicate and oxygen, respectively. The main mechanism responsible for the nutrient transport is the overturning cell, whereas oxygen is mainly transported southward due to the large-scale horizontal circulation. Combining our fluxes with those from the 36N section (Rintoul and Wunsch, 1991) allows us to examine budgets of physical (heat and freshwater) and chemical (nitrogen and oxygen) properties for an enclosed area of the Subpolar and Temperate North Atlantic. The tentative nitrogen budget for the box between the 4x and the 36N sections suggests that the Temperate North Atlantic is exporting organic nitrogen toward the Subtropical and the Subpolar provinces, which is consistent with indirect evidence.