Changes in the Atlantic Meridional Overturning Circulation (AMOC) and associated Meridional Heat Transport (MHT) can affect climate and weather patterns, regional sea levels, and ecosystems. However, despite its importance, direct observations of the AMOC are still limited spatially and temporally, particularly in the South Atlantic. The main goal of this study is to implement a cost-effective trans-basin section to estimate for the first time the AMOC at 22.5°S, using only sustained ocean observations. For this, an optimal mapping method that minimizes the difference between surface in-situ dynamic height and satellite altimetry was developed to retrieve monthly temperature and salinity profiles from Argo and XBT data along the 22.5°S section. The mean AMOC and MHT for 22.5°S were estimated as 15.55±2.81 Sv and 0.68±0.18 PW, respectively, and are stronger during austral fall/winter and weaker in spring. The high-resolution XBT data available at the western boundary are vital for capturing the highly variable Brazil Current, and our section shows a significant improvement when compared to Argo database. The mean values, interannual and seasonal time series of AMOC and MHT were compared with other products. At 22.5S the North Atlantic Deep Water is divided into two cores that flow along both western and eastern boundaries near 2500 m depth. Our results suggest a greater influence of western boundary system on the AMOC variability at 22.5°S; highlight the importance of high resolution in situ data for AMOC estimations; and contribute for a better understanding of AMOC and MHT variability in the South Atlantic.

Marine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short-lived nature and dependence on air-sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3–5 years. The pattern has a significant in-phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward-propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 3–9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic.

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 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.

Ice sheet melting into the Southern Ocean can change the formation and properties of the Antarctic Bottom Water (AABW). Climate models generally mimic ice sheet melting by adding uniformly-distributed freshwater fluxes in the Southern Ocean. Uniform fluxes misrepresent the heterogeneous Antarctic ice sheet melting patterns, and could bias AABW representation in models. We use a global ocean and sea-ice model to explore whether the spatial distribution and increases in freshwater fluxes can alter AABW properties, formation, and the Antarctic sea-ice area. We find that a realistic spatially varying flux sustains AABW with higher salinities compared to simulations with uniform meltwater fluxes. Finally, we show that a ~ 20% increase in ice sheet melting can trigger AABW freshening and Antarctic sea-ice expansion rates similar to those observed in the Southern Ocean since 1980, suggesting that the increasing Antarctic meltwater discharge can drive the observed AABW freshening and the Antarctic sea-ice expansion.