The South Atlantic Ocean plays an important role in the Atlantic meridional overturning circulation (AMOC), connecting it to the Indian and Pacific Oceans as part of the global overturning circulation system; yet the detailed time mean circulation structure in this region and the large-scale spatial pattern of the AMOC variability remain unclear. Using model outputs from a 60-year, eddying global ocean-sea ice simulation validated against observations at a zonal section at 34°S, a meridional section at 65°W in the Drake Passage, and a meridional section southwest of Africa, we show that the upper limb of the AMOC originates primarily from the Agulhas leakage and that, while the cold Pacific water from the Drake Passage does not contribute significantly to the AMOC, it does play a role in setting the temperature and salinity properties of the water masses in the subtropical South Atlantic. We also find that the North Atlantic deep water (NADW) in the lower limb of the AMOC flows southward as a deep western boundary current all the way to 45°S and then turns eastward to flow across the Mid-Atlantic Ridge near 42°S, and that the recirculation around the Vitoria-Trindade seamount chain brings some NADW into the Brazil Basin interior. Finally, we find that the modeled AMOC variability is coherent on interannual to decadal timescales from 35°S to about 35°N, where diapycnal water mass transformations between the upper and lower limbs of the AMOC are expected to be small.

The nearly four-decades-long quasi-continuous daily measurements of the Florida Current (FC) volume transport at 27ºN represents the longest climate record of a boundary current in existence. Given the extremely high utility of this submarine cable-collected time series for monitoring the Atlantic meridional overturning circulation, as well as for improving understanding and prediction of the regional weather, climate phenomena, coastal sea-level, and ecosystem dynamics, efforts are underway to establish a suitable backup observing system in case the cable becomes inoperable in the future. This study explores the utility of along-track satellite altimetry measurements since 1993 as a potential cable backup by establishing the relationship between the cross-stream sea surface height gradients and the FC volume transport derived from cable measurements and ship sections. We find that despite the lower temporal resolution, satellite altimetry can indeed serve as a decent but limited backup observing system. The FC transport inferred from satellite altimetry captures about 60% of the variability observed in the concurrent cable estimates, and the estimated error bars for the altimetry-derived transport are larger than those of the cable transport (2.1 Sv versus 1.5 Sv). We nevertheless demonstrate that satellite altimetry reproduces the seasonal, intra-seasonal, and inter-annual variability of the FC transport fairly well, as well as large transport anomalies during extreme weather events, such as tropical storms and hurricanes. The altimetry-derived transport can be provided in near-real time and serve the need to fill in data gaps in the cable record and assess its quality over time.

The properties and generation mechanisms of the Florida Current subseasonal variability (20 – 100 days) are evaluated from in-situ and satellite observations. The Florida Current volume transport estimates from submarine cable measurements reveal that subseasonal variability accounts for 37% of the total transport variance. It is most active through September - November and marked by quasi-monthly variation. Here we show that coastal-trapped waves generated by alongshore winds off the southern Mid-Atlantic Bight coast are the primary driver of the Florida Current transport subseasonal variability. In contrast, the role of local winds is insignificant. The subseasonal coastal-trapped waves cover a waveguide from Cape May to Port Isabel within 15 days with an average phase speed of 2.5 ± 0.4 m s-1. While transiting in the Florida Straits, the subseasonal waves modulate the Florida Current transport by up to 2.6 Sv, on average, close to the standard deviation of the total transport variability of 3.4 Sv. Under strong stratification in the Florida Straits, manifested in the Rossby deformation radius exceeding the cross-shelf length scale by a factor of 5, the waves exhibit Kelvin wave properties expected from theory. As the waves propagate into the Gulf of Mexico, their energy substantially dissipates. The wave amplitude at Cape May of up to 15 cm is more than five times higher than at Port Isabel.