Abstract
Abstract. The circulation patterns and the impact of the lateral export of nutrients and organic matter off NW Africa are examined by applying an inverse model to two hydrographic datasets gathered in fall 2002 and spring 2003. These estimates show significant changes in the circulation patterns at central levels from fall to spring, particularly in the southern boundary of the domain related to zonal shifts of the Cape Verde Frontal Zone. Southward transports at the surface and central levels at 26∘ N are 5.6±1.9 Sv in fall and increase to 6.7±1.6 Sv in spring; westward transports at 26∘ W are 6.0±1.8 Sv in fall and weaken to 4.0±1.8 Sv in spring. At 21∘ N a remarkable temporal variability is obtained, with a northward mass transport of 4.4±1.5 Sv in fall and a southward transport of 5.2±1.6 Sv in spring. At intermediate levels important spatiotemporal differences are also observed, and it must be highlighted that a northward net mass transport of 2.0±1.9 Sv is obtained in fall at both the south and north transects. The variability in the circulation patterns is also reflected in lateral transports of inorganic nutrients (SiO2, NO3, PO4) and dissolved organic carbon (DOC). Hence, in fall the area acts as a sink of inorganic nutrients and a source of DOC, while in spring it reverses to a source of inorganic nutrients and a sink of DOC. A comparison between nutrient fluxes from both in situ observations and numerical modeling output is finally addressed.
Highlights
The North Atlantic subtropical gyre (NASG) is one of the most important components in the thermohaline circulation
The main goal of this paper is to present an in situ hydrographic database and to estimate lateral mass as well as inorganic nutrients (INs) and dissolved organic carbon (DOC) transports during fall and spring seasons south of the Canary Islands in the context of a highly variable environment featured by the Canary Eddy Corridor, the upwelling off northwest Africa and the Cape Verde Frontal Zone (CVFZ)
An inverse box model has been applied in the eastern North Atlantic to estimate mass, nutrient and organic matter transports during spring and fall
Summary
The North Atlantic subtropical gyre (NASG) is one of the most important components in the thermohaline circulation. It presents a well-known intensification in its western margin, the Gulf Stream, with maximum velocities up to 2 m s−1 (Halkin and Rossby, 1985). The currents observed in this western margin of the gyre occupy a small horizontal extension compared to that of the currents in the eastern side, resulting in an asymmetric gyre (Stramma, 1984; Tomczak and Godfrey, 2003). Käse and Siedler (1982) found strikingly intense currents south of the Azores connected to the Gulf Stream and suggested that part of the recirculation of the NASG occurs southward in the vicinity of the African coast. Several surveys based on both in situ and remote sensing observations contributed to defining the general characteristics for the average flow of the region (Käse and Siedler, 1982; Stramma, 1984; Käse et al, 1986; Stramma and Siedler, 1988; Mittelstaedt, 1991; Zenk et al, 1991; Fiekas et al, 1992; Hernández-Guerra et al, 1993)
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