The science guiding the EUREC⁴A campaign and its measurements is presented. EUREC⁴A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC⁴A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC⁴A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC⁴A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.

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 0 trajectories 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.

Deep Chlorophyll Maxima (DCM) are ubiquitous features in stratified oceanic systems. Their establishment and maintenance result from hydrographical stability favoring specific environmental conditions with respect to light and nutrient availability required for phytoplankton growth. This stability can potentially be challenged by mesoscale eddies impacting the water column's vertical structure and thus the environmental parameters that condition the subsistence of DCMs. Here, data from the global BGC‐Argo float network are collocated with mesoscale eddies to explore their impact on DCMs. We show that cyclonic eddies, by providing optimal light and nutrient conditions, increase the occurrence of DCMs characterized by Deep Biomass Maxima for phytoplankton. In contrast, DCMs in anticyclonic eddies seem to be driven by photoacclimation as they coincide with Deep Acclimation Maxima without biomass accumulation. These findings suggest that the two types of eddies potentially have different impacts on the role of DCMs in global primary production.

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 (7.1×10¹³ m³ in 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 2.2×10⁵ kg s⁻¹ of salt. These results confirm that Agulhas Rings play a very important role in the Indo‐Atlantic interocean exchange of heat and salt.

Summertime wildfire activity is increasing in boreal forest and tundra ecosystems in the Northern Hemisphere. However, the impact of long range transport and deposition of wildfire aerosols on biogeochemical cycles in the Arctic Ocean is unknown. Here, we use satellite-based ocean color data, atmospheric modeling and back trajectory analysis to investigate the transport and fate of aerosols emitted from Siberian wildfires in summer 2014 and their potential impact on phytoplankton dynamics in the Arctic Ocean. We detect large phytoplankton blooms near the North Pole (up to 82°N in the eastern Eurasian Basin). Our analysis indicates that these blooms were induced by the northward plume transport and deposition of nutrient-bearing wildfire aerosols. We estimate that these highly stratified surface waters received large amounts of wildfire-derived nitrogen, which alleviated nutrient stress in the phytoplankton community and triggered an unusually large bloom event. Our findings suggest that changes in wildfire activity may strongly influence summertime productivity in the Arctic Ocean.

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 × 10⁻² PW and 1.5 × 10⁻² PW across the eastern and western South Atlantic Ocean, respectively.