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The health of the ocean, central to human well-being, has now reached a critical point. Most fish stocks are overexploited, climate change and increased dissolved carbon dioxide are changing ocean chemistry and disrupting species throughout food webs, and the fundamental capacity of the ocean to regulate the climate has been altered. However, key technical, organizational, and conceptual scientific barriers have prevented the identification of policy levers for sustainability and transformative action. Here, we recommend key strategies to address these challenges, including (1) stronger integration of sciences and (2) ocean-observing systems, (3) improved science-policy interfaces, (4) new partnerships supported by (5) a new ocean-climate finance system, and (6) improved ocean literacy and education to modify social norms and behaviors. Adopting these strategies could help establish ocean science as a key foundation of broader sustainability transformations.

The intense rainfall in the tropical Atlantic spatially overlaps with the spread of the Amazon plume. Based on remote‐sensed sea surface salinity and rainfall, we removed the contribution of rainfall to the apparent Amazon plume area, thus refining the quantification of its extension (0.84 ± 0.06 × 106km2to 0.89 ± 0.06 × 106km2). Despite the previous overestimation of the Amazon plume area due to the influence of rainfall (>16%), our calculated annual CO2flux based on rainfall‐corrected sea surface CO2fugacity confirms that the Amazon River plume is an atmospheric CO2sink of global importance (−7.61 ± 1.01 to −7.85 ± 1.02 Tg C yr−1). Yet we show that current sea‐air CO2flux assessments for the tropical Atlantic could be overestimated in about 10% by neglecting the CO2sink associated to the Amazon plume. Thus, including the Amazon plume, the sea‐air CO2exchange for the tropical Atlantic is estimated to be 81.1 ± 1.1 to 81.5 ± 1.1 Tg C yr−1. The Amazon plume area has been overestimated in >16% due to rainfallThe atmospheric CO2sink associated to the Amazon plume is of global importanceThe sea‐air CO2exchange for the tropical Atlantic needs reevaluation

The western boundary current system off Brazil is a key region for diagnosing variations of the Atlantic meridional overturning circulation (AMOC) and the southern subtropical cell. In July 2013 a mooring array was installed off the coast at 11°S similar to an array installed between 2000 and 2004 at the same location. Here we present results from two research cruises and the first 10.5 months of moored observations in comparison to the observations a decade ago. Average transports of the North Brazil Undercurrent and the Deep Western Boundary Current (DWBC) have not changed between the observational periods. DWBC eddies that are predicted to disappear with a weakening AMOC are still present. Upper layer changes in salinity and oxygen within the last decade are consistent with an increased Agulhas leakage, while at depths water mass changes are likely related to changes in the North Atlantic as well as tropical circulation changes. Study of interannual to decadal variability of NBUC transports from observations and a modelTransports over the period 2000–2004 and 2013–2014 have not changed significantlyWater mass changes at the western boundary are related to Atlantic circulation changes

The western Tropical Atlantic Ocean is a biogeochemically complex region due to the structure of the surface current system and the large freshwater input from the Amazon River coupled with the dynamics of precipitation. Such features make it difficult to understand the dynamics of the carbon cycle, leading to contrasting estimates in sea‐air CO2exchanges in this region. Here, we demonstrate that these contrasting estimates occur because the western Tropical Atlantic Ocean can be split into three distinct sub‐regions in terms of the sea‐air CO2exchanges. The sub‐region under the North Brazil Current domain acts as a weak annual CO2source to the atmosphere, with low interannual variability. The sub‐region under the North Equatorial Current influence acts as an annual CO2sink, with great temporal variability. The third sub‐region under the Amazon River plume influence shows greater interannual variability of CO2exchanges, but it always acts as a net oceanic sink for CO2. Despite this large spatial variability, the entire region acts as a net annual CO2sink of −1.6 ± 1.0 mmol m−2day−1. Importantly, the Amazon River plume waters drive 87% of the CO2uptake in the western Tropical Atlantic Ocean. In addition, we found a significant increasing trend in sea surface CO2partial pressure in the North Brazil Current and North Equatorial Current waters. Such trends are more pronounced than the increase in atmospheric CO2partial pressure, revealing the sensitivity of carbon dynamics in these sub‐regions to global climate change. Sea‐air CO2release (uptake) shown in the North Brazil Current (North Equatorial Current) domainStrong CO2uptake under the Amazon River plume influence, which accounts for 87% of the CO2uptake by the western Tropical Atlantic OceanIncreasing trend of sea surface CO2partial pressure in the North Brazil Current and the North Equatorial Current waters Sea‐air CO2release (uptake) shown in the North Brazil Current (North Equatorial Current) domain Strong CO2uptake under the Amazon River plume influence, which accounts for 87% of the CO2uptake by the western Tropical Atlantic Ocean Increasing trend of sea surface CO2partial pressure in the North Brazil Current and the North Equatorial Current waters

The Craik-Leibovich equations including vortex forcing terms and a pressure gradient force in x-momentum are integrated numerically. Some typical environmental problems with strong longitudinal currents are analysed.Results are compared with Leibovich & Paolucci's stability analysis which allows to identify situations where Langmuir cells exist. An establishing characteristic time for the setting of cells has the same order of magnitude as in field observations. Its variation with Langmuir number (La) and nondimensional depth is pointed out.A scaling analysis of the momentum equations is made for vanishing La. Influence of the longitudinal current intensities over Langmuir circulations structure is showed and confirmed by numerical results : more energetic convective cells are verified for situations where stronger longitudinal velocities are present. Results indicate how the redistribution of the shear stress induced by secondary flows is important for mixing in the water column and for coastal sediment transport.

The causes of the seasonal cycle of vertical turbulent cooling at the base of the mixed layer are assessed using observations from moored buoys in the tropical Atlantic Intertropical Convergence Zone (ITCZ) (4°N, 23°W) and trade wind (15°N, 38°W) regions together with mixing parameterizations and a one‐dimensional model. At 4°N the parameterized turbulent cooling rates during 2017–2018 and 2019 agree with indirect estimates from the climatological mooring heat budget residual: both show mean cooling of 25–30 W m −2during November–July, when winds are weakest and the mixed layer is thinnest, and 0–10 W m −2during August–October. Mixing during November–July is driven by variability on multiple time scales, including subdiurnal, near‐inertial, and intraseasonal. Shear associated with tropical instability waves (TIWs) is found to generate mixing and monthly mean cooling of 15–30 W m −2during May–July in 2017 and 2019. At 15°N the seasonal cycle of turbulent cooling is out of phase compared to 4°N, with largest cooling of up to 60 W m −2during boreal fall. However, the relationships between wind speed, mixed layer depth, and turbulent mixing are similar: weaker mean winds and a thinner mixed layer in the fall are associated with stronger mixing and turbulent cooling of SST. These results emphasize the importance of seasonal modulations of mixed layer depth at both locations and shear from TIWs at 4°N. In the tropics, vertical turbulent mixing transports heat from the surface waters to the deeper ocean, regulating sea surface temperature (SST). Turbulent mixing has been shown to cause pronounced cooling of SST in the eastern equatorial Pacific and Atlantic Oceans. This cooling is driven to a large extent by strong vertical shear of the zonal equatorial currents, which give rise to instability and mixing. In this study the seasonal cycles of turbulent cooling are assessed at two off‐equatorial locations in the tropical North Atlantic. Despite the different conditions at these two locations, it is found that turbulent cooling at both is strongest during the time of year when the mean winds are weakest and the mixed layer is thinnest. This is surprising, since it might be expected that stronger mean winds and associated deepening of the mixed layer would directly force much of the mixing and turbulent cooling at off‐equatorial locations. Instead, the shallow mean mixed layer, which is more responsive to surface forcing, sets the stage for stronger bursts of cooling. In addition, intraseasonal variations in shear and mixing from tropical instability waves are important at 4°N. These results will be useful for evaluating the mixing parameterizations used in numerical models. The seasonal cycle of vertical turbulent cooling was assessed at 4°N, 23°W and 15°N, 38°WTurbulent cooling of sea surface temperature is strongest during the seasons when mean winds are weakest and the mixed layer is thinnestTropical instability waves contributed 15–30 W m −2of monthly mean cooling at 4°N, 23°W during May–July 2017 and May–July 2019