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Upper ocean fronts are dynamically active features of the global ocean playing a key role in the air‐sea exchanges of properties and their transport in the ocean interior. With scales ranging from the submesoscale (0.1–10 km) to the mesoscale (10–100s km) and a temporal variability from hours to months, collecting in situ observations of these structures is challenging and this has limited our understanding of their associated processes and impacts. During the EUREC4A‐OA/ATOMIC field experiment, which took place in the northwest tropical Atlantic in January–February 2020, a large number of uncrewed platforms, including five Saildrones, were deployed to provide a detailed picture of the upper‐ocean fine‐scale variability. This region is strongly influenced by the outflow of the Amazon River, even in winter, which is the minimum outflow season. Here, the generation of fine‐scale horizontal thermohaline gradients is driven by the stirring of this freshwater river input by large anticyclonic eddies, the so‐called North Brazil Current Rings. Vertical shear estimates using the Saildrones ADCP show that partial temperature compensation occurs along restratifying submesoscale salinity‐dominated fronts. The distribution of surface along‐track gradients, as sampled by different horizontal length‐scales, reveals the prevalence of submesoscale fronts. This is supported by a flattening of the spectral slopes of surface density at the submesoscale. This study emphasizes the need to resolve the upper ocean at high spatial resolution to understand its impact on the broader circulation and to properly represent air‐sea interactions. Oceanic eddies and filaments that range between 10 and 100 km in size can be identified in the study region of the northwestern tropical Atlantic using ocean color as viewed from space (a proxy for chlorophyll‐a). The ocean color maps show that these eddies and filaments are associated with the detachment from the shelf of a freshwater Amazon plume and its interaction with the larger oceanic motions O$\mathcal{O}$(100 km). Field observations from different measurements acquired from research vessels and five uncrewed surface vehicles (USVs) reveal the prevalence and scale of upper ocean fronts, whose magnitude results from the combined effect of temperature and salinity. The Amazon freshwater plume is key to the formation of strong salinity‐driven density fronts. However, when looking at O$\mathcal{O}$(1 km) scales, we detect horizontal temperature variations along slumping fronts that partially counteract the effect of salinity. This leads to a damping of the lateral density fronts. This study contributes to the development of a detailed picture of the ocean fine scales, which is necessary to improve our understanding of air‐sea interactions over frontal regions. Vertical shear sections from the Saildrone ADCP data reveal that temperature compensation develops along restratifying submesoscale frontsTemperature and salinity spectra show an increased temperature contribution to salinity‐dominated fronts at the submesoscaleA flattening of the density spectra from ~−3 to ~−2 at the submesoscale (1–10 km) is observed in the Boulevard des Tourbillons Vertical shear sections from the Saildrone ADCP data reveal that temperature compensation develops along restratifying submesoscale fronts Temperature and salinity spectra show an increased temperature contribution to salinity‐dominated fronts at the submesoscale

The transfer of Indian Ocean thermocline and intermediate waters into the South Atlantic via the Agulhas leakage is generally believed to be primarily accomplished through mesoscale eddy processes, essentially anticyclones known as Agulhas Rings. Here we take advantage of a recent eddy tracking algorithm and Argo float profiles to study the evolution and the thermohaline structure of one of these eddies over the course of 1.5 years (May 2013–November 2014). We found that during this period the ring evolved according to two different phases: During the first one, taking place in winter, the mixing layer in the eddy deepened significantly. During the second phase, the eddy subsided below the upper warmer layer of the South Atlantic subtropical gyre while propagating west. The separation of this eddy from the sea surface could explain the decrease in its surface signature in satellite altimetry maps, suggesting that such changes are not due to eddy dissipation processes. It is a very large eddy (7.1×1013m3in volume), extending, after subduction, from a depth of 200–1,200 m and characterized by two mode water cores. The two mode water cores represent the largest eddy heat and salt anomalies when compared with the surrounding. In terms of its impact over 1 year, the north‐westward propagation of this long‐lived anticyclone induces a transport of 2.2 Sv of water, 0.008 PW of heat, and 2.2×105kg s−1of salt. These results confirm that Agulhas Rings play a very important role in the Indo‐Atlantic interocean exchange of heat and salt. The transfer of the Indian Ocean waters to the South Atlantic Ocean is generally believed to be primarily accomplished through anticyclonic eddies known as Agulhas Rings. There have been various studies investigating these eddies from sparse in situ and satellite observations. Here we take the advantage of the complementary information provided by eddies detected from satellite maps and free‐drifting profiling floats measuring the upper 2,000 m of the ocean, to reconstruct the internal structure of one of them. This reconstruction shows that this Agulhas Ring quits the surface layer and slowly sinks into the subsurface as it drifts westward into the Atlantic Ocean. The ring is a very large eddy extending down to 1,200 m of depth and characterized by two cores showing temperature anomalies up to 4 °C when compared with the surrounding South Atlantic waters. These results confirm that Agulhas Rings play a very important role in the Indo‐Atlantic interocean exchange of heat and salt. Lagrangian reconstruction of an Agulhas Ring sampled by an Argo float for more than 1.5 yearsIn situ evidence of the subduction of an Agulhas RingThe reconstructed Agulhas Ring is characterized by two mode water cores

The Indo‐Atlantic interocean exchanges achieved by Agulhas Rings are tightly linked to global ocean circulation and climate. Yet they are still poorly understood because they are difficult to identify and follow. We propose here an original assessment on Agulhas Rings, achieved by TOEddies, a new eddy identification and tracking algorithm that we applied over 24 years of satellite altimetry. Its main novelty lies in the detection of eddy splitting and merging events. These are particularly abundant and significantly impact the concept of a trajectory associated with a single eddy, which becomes less obvious than previously admitted. To overcome this complication, we have defined a network of segments that group together in relatively complex trajectories. Such a network provides an original assessment of the routes and the history of Agulhas Rings. It links 730,481 eddies into 6,363 segments that cluster into Agulhas Ring trajectories of different orders. Such an order depends on the affiliation of the eddies and segments, in a similar way as a tree of life. Among them, we have identified 122 order 0trajectories that can be considered as the major trajectories associated to a single eddy, albeit it has undergone itself splitting and merging events. Despite the disappearance of many eddies in the altimeter signal in the Cape Basin, a significant fraction can be followed from the Indian Ocean to the South Brazil Current with, on average, 3.5 years to cross the entire South Atlantic. Mesoscale eddies are ubiquitous structures in the ocean and are one of the major sources of ocean variability. They play a crucial role in physically shaping the ocean general circulation, in transporting and mixing energy, chemicals, and other materials within and among ocean basins. This should be true, in particular, south of Africa where the largest mesoscale eddies, the so‐called Agulhas Rings, are shed from the Agulhas Current into the Cape Basin conveying Indian warm and salty waters into the Southeast Atlantic Ocean. However, due to their small‐scale and highly variable nature, ocean eddies are inadequately sampled and poorly reproduced in numerical models. Hence, we still lack a good assessment of their population and an appropriate understanding of their dynamics and exact role in the Earth's climate. We propose here an original assessment on Agulhas Rings achieved by a tracking algorithm that we applied over 24 years of satellite altimetry. Its main novelty lies in the detection of eddy separation and coalescence events that replace the concept of trajectories by the consideration of an eddy network. Such a network provides an original assessment of the routes and history of Agulhas Rings longer and more complex than previously described. New eddy tracking method including eddy merging and splitting improves assessments from satellite‐mapped fieldsAgulhas Rings show very complex behavior with much longer lifespan and geographical reach than previously thoughtThey play a major role in efficiently connecting western boundary currents of the South Indian and Atlantic Oceans

An enhanced Altimetry Gravest Empirical Mode (AGEM), including both adiabatic and diabatic trends, is developed for the Antarctic Circumpolar Current (ACC) south of Africa using updated hydrographic CTD sections, Argo data, and satellite altimetry. This AGEM has improved accuracy compared to traditional climatologies and other proxy methods. The AGEM for the Atlantic Southern Ocean offers an ideal technique to investigate the thermohaline variability over the past two decades in a key region for water mass exchanges and transformation. In order to assess and attribute changes in the hydrography of the region, we separate the changes into adiabatic and diabatic components. Integrated over the upper 2000 dbar of the ACC south of Africa, results show mean adiabatic changes of 0.16 ± 0.11°C decade−1and 0.006 ± 0.014 decade−1, and diabatic differences of −0.044 ± 0.13°C decade−1and −0.01 ± 0.017 decade−1for temperature and salinity, respectively. The trends of the resultant AGEM, that include both adiabatic and diabatic variability (termed AD‐AGEM), show a significant increase in the heat content of the upper 2000 dbar of the ACC with a mean warming of 0.12 ± 0.087°C decade−1. This study focuses on the Antarctic Intermediate Water (AAIW) mass where negative diabatic trends dominate positive adiabatic differences in the Subantarctic Zone (SAZ), with results indicating a cooling (−0.17°C decade−1) and freshening (−0.032 decade−1) of AAIW in this area, whereas south of the SAZ positive adiabatic and diabatic trends together create a cumulative warming (0.31°C decade−1) and salinification (0.014 decade−1) of AAIW. An adiabatic‐diabatic Altimetry Gravest Empirical Mode is developed for the ACC south of AfricaAnomalies separated in adiabatic and diabatic components exposing trends in frontal zonesAntarctic Intermediate Water may be a sensitive indicator of climate change in the region

Variations in the mass and heat transported by the meridional overturning circulation (MOC) have important, well‐documented, influences on global and regional climate, weather, ecosystems, and coastal sea levels. However, continuous, high‐frequency, observations of these quantities have been limited to date. Multiple years of full‐depth daily observations from moored instruments in the South Atlantic at 34.5°S are combined with satellite observations to resolve the volume transports in both the upper and abyssal MOC cells, and the associated full‐depth meridional heat transport (MHT), on daily to interannual timescales. A newly developed method for combining satellite sea level observations with historical hydrographic measurements was used to estimate daily full‐depth ocean profiles of temperature in the ocean interior where mooring coverage is sparse. The average MHT during 2013–2017 is 0.5 PW, with a daily standard deviation of 0.8 PW. The MHT variability is most strongly driven by the geostrophic relative velocity contributions (horizontal density‐gradient changes). This variability is highly correlated with the volume transport variability of the MOC upper cell (r= 0.96) and modestly anti‐correlated (r= −0.52) with the abyssal cell variations. An empirical relationship between the MHT and MOC values was developed allowing the reconstruction of a longer MHT time series including the pilot array period (2009–2010). Seasonal variation of the MHT is significant, and results from strong variations of all terms (Ekman, barotropic, and baroclinic). Although the 2013–2017 shows an increasing MHT trend (0.14 PW/year), the longer time period record suggests that the apparent trend may simply be interannual modulation of MHT at 34.5°S. Changes in the meridional overturning circulation, a large system of ocean currents driven by differences in temperature and salt content as well as the winds, are known to have significant influences on the global climate system. This study presents, for the first time, full‐depth, daily measurements of the volume and heat transported by this circulation system in the South Atlantic at 34.5°S based on direct observations. As the instruments anchored on the seafloor are too widely spaced in the basin interior, a new method for using satellite observations to estimate interior temperature profiles was developed and used. The roughly 4 years of continuous daily data reveal seasonal and interannual changes of these important flows. The observations also demonstrate that the volume and heat transports vary in a consistent manner with one another. This allows us to use some earlier moored observations from a pilot version of the array, extending the data record further back in time and producing ∼6 years of daily estimates of heat transport in the South Atlantic during 2009–2017, with a gap during 2010–2013. Implementation of a novel methodology for computing the daily meridional heat transport time series at 34.5°S from 2013 to 2017The heat transport has significant variability highly correlated with changes in the strength of the meridional overturning circulationA reconstructed/extended meridional heat transport time series allows analysis of seasonal to interannual changes and long‐term trend Implementation of a novel methodology for computing the daily meridional heat transport time series at 34.5°S from 2013 to 2017 The heat transport has significant variability highly correlated with changes in the strength of the meridional overturning circulation A reconstructed/extended meridional heat transport time series allows analysis of seasonal to interannual changes and long‐term trend

In February 2020, a 120‐km‐wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7°N/54°W‐56°W). It was initially stratified in the upper 10 m with a freshwater content of 2–3 m of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13–16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off‐shelf flow lasted 10 days, contributing to a plume area extending over 100,000 km2and associated with a 0.15 Sv (106m3s−1) freshwater transport. The off‐shelf plume was steered northward by a North Brazil Current ring up to 12°N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favorable wind direction closer to the Amazon estuary, which contributed to north‐westward freshwater transport on the shelf. Other such events of freshwater transport in January–March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favoring the north‐westward freshwater transport toward the shelf break. This study documents how freshwater from the Amazon reaches the deep ocean up to 12°N in the northwest tropical Atlantic in January–March. The classical view is that the water is channeled along the shelf to the Caribbean Sea. Here, we document a freshwater plume from in situ and satellite observations during the EUREC4A‐OA/ATOMIC program in 2020. This plume separated from the shelf near 55°W north of French Guiana on February 2–5. This fresher water was stirred by a North Brazil Current ring up to 12°N before mostly spreading westward. The near‐surface water was initially very stratified at least until 10‐m from the surface. More than 14 days later and 400 km farther north, salinity as low as 33.3 pss with mixing depths on the order of only 20‐m was still encountered. The total area of the freshwater plume reached 100,000 km2with a flow of freshwater on the order of 0.15 Sv (106m3s−1) during 10 days. This phenomenon seems to be triggered by changes in the wind direction on the shelf closer to the equator, and has also been observed in satellite products in 7 out of 10 years since 2010. First direct observation of a freshwater plume separating from the shelf near Demerara Rise in early FebruaryThe 2020 plume carried 0.15 Sv of freshwater northward during 10 days and after 14 days, covered more than 100,000 km2Plume probably initiated by wind direction shift closer to the equator, 5–7 days earlier and stirred by a North Brazil Current ring First direct observation of a freshwater plume separating from the shelf near Demerara Rise in early February The 2020 plume carried 0.15 Sv of freshwater northward during 10 days and after 14 days, covered more than 100,000 km2 Plume probably initiated by wind direction shift closer to the equator, 5–7 days earlier and stirred by a North Brazil Current ring

The variability of the Atlantic meridional overturning circulation (AMOC) has considerable impacts on the global climate system. Past studies have shown that changes in the South Atlantic control the stability of the AMOC and drive an important part of its variability. That is why significant resources have been invested in a South (S)AMOC observing system. In January 2017, the RV Maria S. Merianconducted the first GO‐SHIP hydrographic transect along the SAMOC‐Basin Wide Array (SAMBA) line at 34.5°S in the South Atlantic. This paper presents estimates of meridional volume, freshwater (MFT), and heat (MHT) transports through the line using the slow varying geostrophic density field and direct velocity observations. An upper and an abyssal overturning cell are identified with a strength of 15.64 ± 1.39 Sv and 2.4 ± 1.6 Sv, respectively. The net northward MHT is 0.27 ± 0.10 PW, increasing by 0.12 PW when we remove the observed mesoscale eddies with a climatology derived from the Argo floats data set. We attribute this change to an anomalous predominance of cold core eddies during the cruise period. The highest velocities are observed in the western boundary, within the Brazil and the Deep Western Boundary currents. These currents appear as a continuous deep jet located 150 km off the slope squeezed between two cyclonic eddies. The zonal changes in water masses properties and velocity denote the imprint of exchange pathways with both the Southern and the Indian oceans. The Atlantic meridional overturning circulation (AMOC) is a crucial element of the global ocean circulation and climate. It connects the Southern Ocean to the northern North Atlantic, and is responsible for the interhemispheric northward transport of heat and freshwater. The South Atlantic is a crossroad for water masses from the Southern, the Indian and the North Atlantic oceans. This paper analyzes the first full‐ocean‐depth trans‐basin measurements of the southernmost enclosed section of the Atlantic between South Africa and Brazil along 34.5°S. Our results confirm a northward transport of heat at this latitude. We also found a complex water mass structure and dynamics, characterized by intense boundary currents and mesoscale eddies. It is the sum of these elements that is not only crucial for the Atlantic but also for the global ocean circulation and climate. Overturning maximum is 15.64 ± 1.39 Sv; Meridional heat and freshwater transport are 0.27 ± 0.10 PW and 0.23 ± 0.02 Sv, respectivelyExcluding the mesoscale eddies from the section increased the meridional heat transport by 0.12 PWThe distribution of water masses and currents reflects the favorable position of the section for observing Overturning maximum is 15.64 ± 1.39 Sv; Meridional heat and freshwater transport are 0.27 ± 0.10 PW and 0.23 ± 0.02 Sv, respectively Excluding the mesoscale eddies from the section increased the meridional heat transport by 0.12 PW The distribution of water masses and currents reflects the favorable position of the section for observing

The Agulhas leakage of Indian Ocean waters into the Atlantic has been shown to be a key link in global ocean circulation and climate; an increased understanding of this process is therefore of more than just of local interest. While knowledge of the Agulhas leakage has increased over the past 30 years, the precise mechanisms at play and water properties transferred are still not sufficiently documented. This study covers the Agulhas rings heat content and transport, which contribute to the meridional overturning circulation of the global ocean during their route across the South Atlantic Ocean. By applying TOEddies, an eddy detection and tracking algorithm that takes into account eddy merging and splitting, to satellite altimetry maps combined with a colocation of the detected eddies with Argo floats vertical profiles, we are able to document the important heat anomaly characterizing Agulhas rings and their evolution across the Cape Basin and the South Atlantic Ocean. Notwithstanding that in situ data coverage is still limited at the ocean mesoscale, we have estimated an averaged Agulhas rings subsurface heat transport of 2.5 × 10−2PW and 1.5 × 10−2PW across the eastern and western South Atlantic Ocean, respectively. Mesoscale eddies are ubiquitous structures in the ocean. By containing more than half of the kinetic energy of the ocean circulation, they clearly shape it. They are also suspected to play a crucial role in transporting and mixing heat, salt, chemicals, and other materials within and among ocean basins. Because of this, they are alleged to impact the Earth climate and its changes. This should be true, in particular, south of Africa where the largest mesoscales eddies, the so‐called Agulhas rings, are shed from the Agulhas Current into the Cape Basin conveying Indian warm and salty waters into the southeast Atlantic Ocean. However, due to their small‐scale and highly variable nature, ocean eddies are inadequately sampled and poorly reproduced in numerical models. By using an eddy atlas built on satellite altimetry and associated to subsurface in situ observations acquired by the Argo profiling floats international program, we present new estimate of the role of Agulhas rings in advecting large quantity of heat across the South Atlantic basin. The results of our study are crucial to improve the understanding and modeling of ocean dynamics and the Earth climate. Most of the Agulhas rings are subsurface intensified in the central South Atlantic OceanNew method for individual reconstruction of heat content anomalies in well‐sampled eddies improves the assessment of eddy variabilityAn estimate of about 0.02 PW of heat flux achieved by the Agulhas Ring across the central South Atlantic is provided