No abstract

[1] The upper branch of the meridional overturning circulation in the North Atlantic is fed by cross-equatorial transport of various water masses from the Southern Hemisphere. Here, we study the large-scale spreading of South Atlantic Water (SAW) into the western tropical North Atlantic from the equator to 25°N. The fractions of SAW in the upper ocean water masses are quantified using a water mass analysis applied on a data set of conductivity-temperature-depth data from the Hydrobase project and the Argo float program. To fill gaps in the data coverage and to gain insight into the mechanisms involved, the observations are complemented with results from the high-resolution Family of Linked Atlantic Model Experiments model ( 1 12°) , which has been shown to realistically simulate the inflow of SAW into the Caribbean. The analysis reveals the mean SAW propagation pathways in the North Atlantic and identifies the regions of largest variability. High SAW fractions in the thermocline and central water layers are limited to the region south of 10°N, where the water body consists of 80%-90% SAW. Thus, the zonal currents in the equatorial gyre are mainly formed of SAW. The weaker currents in the intermediate layer combined with a northward excursion of the North Equatorial Current allow the SAW in this layer to intrude farther north compared to the layers above. The transition into North Atlantic Water occurs gradually from 12°N to 20°N in the intermediate layer.

[1] Upwelling velocities w in the equatorial band are too small to be directly observed. Here, we apply a recently proposed indirect method, using the observed helium isotope ( 3 He or 4 He) disequilibria in the mixed layer. The helium data were sampled from three cruises in the eastern tropical Atlantic in September 2005 and June/July 2006. A onedimensional two-box model was applied, where the helium air-sea gas exchange is balanced by upwelling from

The cold upwelling waters of the eastern tropical oceans not only interact with the atmospheric circulation via changing the sea surface temperatures but also influence the biological activity via affecting the nutrient and oxygen contents of the upwelling waters. While the sources of the equatorial upwelling associated with the Equatorial Undercurrent (EUC) have been studied extensively, the relevance of the northern and southern off‐equatorial undercurrents (NEUC, SEUC) for the off‐equatorial upwelling regions has remained unclear. In this study we use output from a high‐resolution, 1/12° model (FLAME) to investigate the mean pathways and variability of the off‐equatorial undercurrents (OEUCs) in the Atlantic. In particular, a calculation of Lagrangian trajectories helps to gain insight into the source waters of the OEUCs and their connection to the upwelling regions in the eastern tropical Atlantic. In the model solution the sources of both OEUCs belong almost exclusively to the Southern Hemisphere. The pathways of the source waters are found to be governed by strong recirculations between the different eastward and westward zonal currents because of intense eddy motions associated with the tropical instability wave activity. Whereas the SEUC is predominantly fed through the recirculation in the ocean interior, the NEUC is also fed by a weak inflow from the western boundary current. Investigation of the fate of the NEUC shows only a weak direct supply to the upwelling in the Guinea Dome and along the African coast but a significant contribution to the equatorial upwelling.