Cesar Barbedo Rocha

Oxygen concentrations are of fundamental importance for organisms and geochemical cycling in the oceans. Since the middle of the 20th century, oxygen concentrations have been declining in the open ocean and the coastal ocean, with implications for productive ecosystems. The northwest Atlantic (NWA) shelf from the coast of Labrador to Cape Hatteras is located near the intersection of subtropical and subpolar circulation in the Atlantic basin and is sensitive to climate variability. Here, we used in situ oxygen observations to determine the multidecadal trend of subsurface oxygen concentrations below the mixed layer on the NWA shelf from 1988 to 2019. Our results identified an overall decline in oxygen concentrations that varied across the region from 0.0 to −1.8 µ mol kg ⁻¹ yr ⁻¹ . These rates surpass the established Atlantic basin-wide trend. The greatest subregional oxygen loss occurred on the Eastern Scotian Shelf (−1.8 ± 0.2 µ mol kg ⁻¹ yr ⁻¹ ). By separating and comparing the contribution of oxygen solubility and apparent oxygen utilization (AOU) to the trends, the oxygen trends on the NWA shelf were determined to be driven by changes in the AOU. In the subregions where AOU dominates, the AOU change was driven by the decreased presence of Labrador Current water and the increased presence of Gulf Stream water in the region, while the rest was unexplained by our potential predictors, implicating local biological changes and changes in ice-cover.

We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and anticyclonic eddies near Hawai'i in the North Pacific Subtropical Gyre (NPSG) and explored mechanisms that sustain productivity in the cyclone after the initial intensification stage. The top of the nutricline was uplifted into the euphotic zone in the cyclone and depressed in the anticyclone. Subsurface nutrient concentrations and apparent oxygen utilization at the cyclone's inner periphery were higher than expected from isopycnal displacement, suggesting that shallow remineralization of organic material generated excess nutrients in the subsurface. The excess nutrients may provide a supply of subsurface nutrients to sustain productivity in maturing eddies. The shallow remineralization also raises questions regarding the extent to which cyclonic eddies promote deep carbon sequestration in subtropical gyres such as the NPSG. An upward increase in nitrate ¹⁵N/¹⁴N isotope ratios below the euphotic zone, indicative of partial nitrate assimilation, coincided with negative preformed nutrients—potentially signaling heterotrophic bacterial consumption of carbon‐rich (nitrogen‐poor) organic material. The ¹⁵N/¹⁴N of material collected in shallow sediment traps was significantly higher in the cyclone than in the anticyclone and showed correspondence to the ¹⁵N/¹⁴N ratio of the nitrate supply, which is acutely sensitive to sea level anomaly in the region. A number of approaches were applied to estimate the contribution of N₂ fixation to export production. Results among approaches were inconsistent, which we attribute to non‐steady state conditions during our observation period.

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic with the overall goal of understanding the role of fine-scale processes in internal ocean dynamics and air-sea interaction. Four oceanographic ships, the French Atalante, the German Maria S. Merian and Meteor, and the US Ron Brown, were closely coordinated with airborne observations and autonomous ocean platforms (gliders, ©Saildrones, Argo floats, and drifters) to simultaneously measure the ocean and atmosphere from east of Barbados to the northern border of French Guyana. The multiple observations of the ocean, atmosphere, and their interface have revealed more complex ocean dynamics than expected, in particular a strong interaction between the Amazon River outflow (despite its reduced winter discharge), the North Brazil Current (NBC), and several mesoscale eddies (including the highly energetic NBC rings). This leads to even richer submesoscale dynamics that shape an important fraction of the air-sea exchange of heat, momentum, and CO2, and efficiently isolates the NBC northward flow waters from intense and continuous interactions with the atmosphere. Owing to the many complementary observations from ships and autonomous platforms, we have been able to quantify some of these processes, including the diurnal cycle and the 3D dynamics of different mesoscale eddies, as well as to map and quantify different terms of the air-sea fluxes and their impacts on the marine atmospheric boundary-layer water budget. The results have been widely used not only to validate numerical simulations of the region, but also to guide their analyses and to improve various numerical parameterizations.The collection of these observations was the result of an important international coordination between many different groups of ocean and atmospheric scientists. In addition, the special strategy for targeted data collection of meso- and submesoscale processes relied on daily planning of the field experiment and on detailed analysis of the near-real-time satellite data and the observations already obtained during the experiment, which was essential for providing the right snapshots of the ocean and atmosphere for the quantification of many processes. The lessons learned from this experiment will be implemented and extended in the upcoming major high-resolution oceanographic endeavor, the WHIRLS experiment, which will take place in June-July 2025, southwest of Africa.

Ocean wind plays a fundamental role in the Earth's climate system by transporting heat, moisture, and gases globally while mediating their exchange with the ocean. How, and at what scales these processes occur is an active area of research, and modeling studies suggest that the strongest vertical transport and air‐sea exchanges occur at small scales. This paper introduces results from the S‐MODE campaign, during which a Doppler Scatterometer mapped 2D ocean vector wind on scales between 200 m and 100 km. While in an average sense, DopplerScatt wind data match classical spectra, atmospheric rolls, convection, and underlying ocean currents all drive unique wind spectra that are inconsistent with the average. Frequent observations of strong atmospheric divergence imply small‐scale vertical transport reaching 10 km/day. These results are not consistent with Kolmogorov and two‐dimensional turbulence theories due to anisotropy and divergence.

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.

The Northwest Tropical Atlantic (NWTA) is a region of complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8°N, which leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km². We show that these two processes influence the spatial variability of the region's surface latent heat flux (LHF). On the one hand, the presence of surface freshwater modifies the vertical stratification of the ocean, the mixed layer heat budget, and thus the air‐sea heat exchanges. On the other hand, NBC rings create a highly heterogeneous mesoscale sea surface temperature (SST) field that directly influences the near‐surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud‐Circulation Coupling in ClimAte ‐ Ocean Atmosphere (EUREC⁴A‐OA) and Atlantic Tradewind Ocean‐Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m², of which 100 W m² are associated with wind speed changes and 40 W m² with SST variations. Surface currents or heat release associated with stratification changes remain as second‐order contributions with LHF variations of less than 10 W m² each. This study highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales, although it is limited by the scarcity of collocated observations.

We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and anticyclonic eddies near Hawai’i in the North Pacific Subtropical Gyre (NPSG) and explored mechanisms that may sustain productivity in the cyclone after the initial intensification stage. The top of the nutricline was uplifted into the euphotic zone in the cyclone and depressed in the anticyclone. Subsurface nutrient concentrations and apparent oxygen utilization at the cyclone’s inner periphery were higher than expected from isopycnal displacement, suggesting that shallow remineralization of organic material generated excess nutrients in the subsurface. The excess nutrients may provide a supply of subsurface nutrients to sustain productivity in maturing eddies. The shallow remineralization also raises questions regarding the extent to which cyclonic eddies promote deep carbon sequestration in subtropical gyres such as the NPSG. An upward increase in nitrate 15N/14N isotope ratios below the euphotic zone, indicative of partial nitrate assimilation, coincided with negative preformed nutrients – potentially signaling heterotrophic bacterial consumption of carbon-rich (nitrogen-poor) organic material. The 15N/14N of material collected in shallow sediment traps was significantly higher in the cyclone than the anticyclone and showed correspondence to the 15N/14N ratio of the nitrate supply, which is acutely sensitive to sea level anomaly in the region. A number of approaches were applied to estimate the contribution of N2 fixation to export production; results among approaches were inconsistent, which we attribute to non-steady state conditions during our observation period.

Previous studies have reported temperature increase of bottom waters in the South Atlantic Ocean at rates of about 2 moC-1. Here we present data indicating four-year warming at rates near the interior of the South Atlantic of up to 17(+/- 0.9) moC-1. The data were sampled from January 2019 to January 2023, at two previously unobserved locations, at 31.13oW and 34.50oS (Site E) and 18.82oW and 34.50oS (Site F). Each mooring was equipped with two independent temperature sensors, one placed 1 m above the seafloor and the other 50 m above. Site E was moored at a nominal depth of 4006 m, under the influence of the Antarctic Bottom Water. Site F was at 3888 m, within a layer influenced by the North Atlantic Deep Water. The data show strong seasonality and significant variability at the semi-diurnal cycle. At both sites, the trends are positive and significant, with the higher rate at the deeper location. Assuming similar surface warming at the convection regions, with the signals being advected to the sampling sites, this could be indicating that climate signals acquired near the surface around Antarctica are reaching the abyssal South Atlantic earlier than those carried by the North Atlantic Deep Water. However, the large spatial heterogeneity and temporal variability of such signals suggest that other local processes are also important in determining the observed temperature variability.

The Northwest Tropical Atlantic (NWTA) is a region with complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8ºN that leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km ² . These two processes affect the spatial variability of the region’s surface latent heat flux (LHF). First, the presence of surface freshwater modifies the vertical stratification of the ocean limiting the amount of heat that can be released to the atmosphere. Second, they create a highly heterogeneous mesoscale sea-surface temperature (SST) field that directly influences near-surface atmospheric circulation. These effects are illustrated byd from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC ⁴ A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m ² , of which 100 W m ² are associated with wind speed changes and 40 W m ² with SST variations. Surface currents or stratification-change associated heat release remain as second-order contributions with LHF variations of less than 10 W m ² each. Although this study is limited by the paucity of collocated observations, it highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales.