On the spreading of South Atlantic Water into the Northern Hemisphere

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 degrees 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 degrees), 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 degrees 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 degrees N to 20 degrees N in the intermediate layer.