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In the easternmost portion of the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE–SW to the Equator, where São Tomé Island sits in the path of the Equatorial Undercurrent (EUC). In this study, we explore the meso‐to‐large‐scale effects of local flow‐topography interactions that escalate from the EUC encounter with the Gulf of Guinea islands. A mean shipboard Acoustic Doppler Current Profiler section captures the EUC as a strong subsurface jet that ultimately hits São Tomé. Motivated by these observations, we ran two ocean general circulation model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (PV) from these simulations show that the EUC bifurcates at 6°E, triggering mesoscale activity that spreads the EUC waters. On both sides of São Tomé, the EUC branches roll up into eddies that propagate westward. These low‐PV anticyclones carry high salinity through the tropical Atlantic, introducing PV anomalies along eddy corridors mirrored by the Equator. The formation of such eddies is affected by diapycnal and isopycnal mixing and friction, and their distribution between hemispheres is intrinsically related to the location of São Tomé. The eddies are modulated at both seasonal and interannual scales. A strong EUC generates numerous and stronger eddies, while seasonal equatorial upwelling indirectly hinders their formation. Convergence of the EUC upstream of São Tomé reveals intense downwelling and freshening, whereas divergence and upwelling are associated with the EUC negotiating the island. Plain Language Summary: In the eastern portion of the tropical Atlantic, in the Gulf of Guinea, Bioko Island marks the beginning of an island chain that stretches NE–SW to the Equator, where São Tomé Island sits in the path of the strongest current in the tropical Atlantic, the Equatorial Undercurrent (EUC). The EUC connects the tropical Atlantic, transporting high‐salinity waters eastward. In this study we explore the effects of flow‐topography interactions on the EUC circulation. Velocity observations capture the EUC as a strong current that hits São Tomé Island. Motivated by these observations, we ran two model simulations that differ by the presence versus absence of the Gulf of Guinea islands. Diagnostics of salinity and potential vorticity (a dynamical quantity that relates to the circulation) show that the EUC bifurcates at 6°E, spreading the EUC waters. On both sides of São Tomé Island, the EUC branches form eddies that propagate westward through the tropical Atlantic. The formation and distribution of such eddies present an asymmetry between hemispheres due to the EUC interaction with São Tomé. On regional scales, a particle‐tracking experiment reveals sinking before the particles reach São Tomé, with mixing and freshening of the EUC waters as they flow around the island. Key Points: Velocity observations show that the Equatorial Undercurrent (EUC) hits São Tomé Island at 6°E, forcing the jet to bifurcate earlier than if the island did not existThe EUC dynamics and the location of São Tomé impose an asymmetry to the bifurcation, affecting net zonal fluxes in the tropical AtlanticThe EUC‐island encounter triggers vertical advection and transformation of the EUC water via diapycnal and isopycnal mixing and friction [ABSTRACT FROM AUTHOR]

Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo floats, we analyze the isopycnal θ/S anomalies (θ'/S') within newborn eddies in the tropical Atlantic Ocean (TAO). Our results show that on density‐coordinates, both anticyclonic eddies (AEs) and cyclonic eddies (CEs) can exhibit positive, negative, or nonsignificant θ'/S'. Almost half of the sampled eddies do not have significant θ'/S' at their generation site. The other half exhibits significant positive or negative θ', up to ±0.5°C typically. More than 70% of them have a subsurface signature. Refined analyses of the vertical structure of new‐born eddies in three selected subregions show the dominance of cold (warm) subsurface AEs (CEs) likely due to isopycnal advection of large‐scale potential vorticity (PV) and θ. PV is a key parameter to analyze eddy generation and dynamics. Isopycnal advection, friction or diapycnal mixing can be involved in the generation of PV anomalies from which vortices can then emerge. However, it is difficult to evaluate PV at mesoscale. In this study, we propose to combine θ'/S' and PV anomalies to analyze which process could be involved in its transformation in the ocean. We argue that eddies created by diapycnal mixing or isopycnal advection of water‐masses are associated with PV anomalies and significant θ'/S'. In contrast, a frictional forcing also creates PV anomalies but without modifying θ/S. Even though our results remain qualitative, the proposed diagnostics can be of interest to validate realistic models and then use them to analyze the PV anomaly sources. Plain Language Summary: Mesoscale vortices are common features in the global ocean, having typical length scales of 10–100 km and lifespans from several weeks to several months or even years for the more energetic structures. The vorticity of these quasicircular rotating structures emerging from large‐scale currents can be formed by several physical processes: isopycnal advection of a water mass, friction (wind stress, bottom stress), or diapycnal mixing. In this study, we first analyze the vertical structure of temperature/salinity in the eddies at their generation site. Based on the analysis of satellite and in situ data in the tropical Atlantic Ocean, we show that ∼50% of the eddies do not present significant isopycnal temperature anomalies when they are generated. In contrast, the remaining ∼50% of eddies present significant isopycnal temperature anomalies, but with unexpected numerous cold anticyclones and warm cyclones. We then discuss the link between the potential vorticity and isopycnal temperature anomalies, showing combinations of the previous physical processes involved have to be invoked to explain the observed eddy characteristics. Key Points: At their generation site, 50% of observed eddies have nonsignificant isopycnal temperature/salinity (θ/S) anomalies in the tropical Atlantic Ocean (TAO)On density coordinates, both cyclonic eddy (CEs) and anticyclonic eddies (AEs) can exhibit significant positive, negative, or nonsignificant isopycnal θ/S anomaliesWe discuss the relationship between isopycnal θ/S and PV anomalies and how they can inform us on their generation mechanisms [ABSTRACT FROM AUTHOR]

Relevance of a biogeochemical dataset in deciding the candidate models and estimating the parameters for three different models of biological nitrogen flux in a water column is analyzed by using the data from the French JGOFS EUMELI cruises and the SeaEiFS ocean color sensor. The model with dissolved organic matter exhibited reasonable annual behavior and possible role of deep nitrate data was seen due to large seasonal changes in deep nitrate data.

Spatio-temporal variability of the biological front in the Agulhas Current system comparing SeaWiFS and modeled chlorophyll fields produced by the monthly atmospheric NCEP reanalysis over a period of four years October 1997 to October 2001is examined. The model reproduces the annual cycle of the observed chlorophyll within the Agulhas Current system biogeochemical province and the modeled fields display slightly higher annual mean and amplitude.

We study mesoscale eddy characteristics in the tropical Atlantic Ocean, using 23 years of daily altimetry sea level anomalies. Eddies are mainly generated in the eastern boundary upwelling systems and in the Brazil Current region. Their westward propagation speed reaches 20 cm/s in equatorial areas, decreasing with latitudes. They present typical amplitudes of 1‐5 cm. The largest and most energetic eddies are observed in the equatorial region, especially in the North Brazil Current (NBC) retroflection. The seasonal cycle of the eddy characteristics shows higher amplitudes along the NBC retroflection and the western part of the North Equatorial Countercurrent. A new criterion, based on altimetry data, determines the probability that barotropic instability of mean surface currents is responsible for eddy generation. We find a strong likelihood that the latter plays a key role along the North Equatorial Countercurrent, whereas other mechanisms must be invoked for the NBC region. Key Points: Mesoscale eddies in the tropical Atlantic ocean have amplitudes, radii, and kinetic energies of 1–5 cm, 30–100 km, and 50–100 cm2/s 2, respectivelySeasonal cycle of eddy properties has the highest amplitude in the NBC retroflection and in the western part of the NECCBarotropic instability is the main mechanism for eddy generation in the western part of the NECC [ABSTRACT FROM AUTHOR]

The Indian and French Space Agencies, ISRO and CNES, have conceptualized a space-borne Thermal Infrared Reflectance (TIR) mission, TRISHNA (Thermal infRared Imaging Satellite for High-resolution Natural Resource Assessment). The primary design drivers of TRISHNA are the monitoring of (i) terrestrial water stress and use, and of (ii) coastal and continental water. A suit of four TIR bands and six optical bands is planned. The TIR bands will be centred at 8.6 μm, 9.1 μm, 10.3 μm and 11.5 μm to provide noon-night global observations at 57m nadir resolution over land and coastal regions. The field of view (FOV) is ±34° and the orbit of 761 km altitude was designed to allow 3 sub-cycle acquisitions during the 8-day cycle. The optical bands correspond to blue, green, red, and NIR plus two SWIR bands at 1.38 μm and 1.61 μm. The green, red, NIR and the 1.61 μm SWIR bands will have better radiometry quality than those of AWiFS. ISRO and CNES will develop optical and TIR payloads, respectively. Assessing evapotranspiration and furthermore Gross and Net Primary Productivity (GPP and NPP) will in turn assist in quantifying water use in rainfed and irrigated agriculture, water stress and water use efficiency, with expected applications to agricultural drought and early warning, crop yield prediction, water allocation, implementation of water rights, crop insurance business and agro-advisories to farmers. The other scientific objectives of TRISHNA are also briefly described. TRISHNA instrument will fly aboard a ISRO spacecraft scheduled to be launched from 2024 for a minimum period of 5 years' mission lifetime. [ABSTRACT FROM AUTHOR]

An accurate quantification of the role of the ocean as source/sink of greenhouse gases (GHGs) requires to access the high-resolution of the GHG air-sea flux at the interface. In this paper we present a novel method to reconstruct maps of surface ocean partial pressure of CO2 (pCO2) and air-sea CO2 fluxes at super resolution (4 km, i.e., 1/32° at these latitudes) using sea surface temperature (SST) and ocean color (OC) data at this resolution, and CarbonTracker CO2 fluxes data at low resolution (110 km). Inference of super-resolution pCO2 and air-sea CO2 fluxes is performed using novel nonlinear signal processing methodologies that prove efficient in the context of oceanography. The theoretical background comes from the microcanonical multi-fractal formalism which unlocks the geometrical determination of cascading properties of physical intensive variables. As a consequence, a multi-resolution analysis performed on the signal of the so-called singularity exponents allows for the correct and near optimal cross-scale inference of GHG fluxes, as the inference suits the geometric realization of the cascade. We apply such a methodology to the study offshore of the Benguela area. The inferred representation of oceanic partial pressure of CO2 improves and enhances the description provided by CarbonTracker, capturing the small-scale variability. We examine different combinations of ocean color and sea surface temperature products in order to increase the number of valid points and the quality of the inferred pCO2 field. The methodology is validated using in situ measurements by means of statistical errors. We find that mean absolute and relative errors in the inferred values of pCO2 with respect to in situ measurements are smaller than for CarbonTracker. [ABSTRACT FROM AUTHOR]

The Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of the global catches in the ocean. Often associated with Oxygen Minimum Zones (OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle. Important bioavailable N loss due to denitrification and anammox processes as well as greenhouse gas emissions (e.g, N2O) occur also in these EBUS. However, their dynamics are currently crudely represented in global models. In the climate change context, improving our capability to properly represent these areas is crucial, due to anticipated changes in the winds, productivity, and oxygen content. We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated OMZs. We implemented this model in a 3-D realistic coupled physical/biogeochemical configuration in the Namibian upwelling system (Northern Benguela) using the high-resolution hydrodynamical model ROMS. We present here a validation using in situ and satellite data as well as diagnostic metrics, and sensitivity analyses of key parameters and N2O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N2O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate and Chl a concentrations, and the rates of microbial processes (e.g. NH+4 and NO-2 oxidation, NO-3 reduction and anammox) as well. Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking and nitrification play a key role for the low-oxygen water content, N loss and N2O concentrations in the OMZ. Moreover, the importance of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is highlighted to improve the representation of microbial activity linked with OMZ. The simulated minimum oxygen concentrations are driven by the poleward meridional advection of oxygen-depleted waters offshore of 300m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. [ABSTRACT FROM AUTHOR]

Rossby waves appear to have a clear signature on surface chlorophyll concentrations which can be explained by a combination of vertical and horizontal mechanisms. In this study, we investigate the role of the different physical processes in the north Atlantic to explain the surface chlorophyll signatures and the consequences on primary production, using a 3-D coupled physical/biogeochemical model for the year 1998. The analysis at 20 given latitudes, mainly located in the subtropical gyre, where Rossby waves are strongly correlated with a surface chlorophyll signature, shows the important contribution of horizontal advection and of vertical advection and diffusion of inorganic dissolved nitrogen. The main control mechanism differs according to the biogeochemical background conditions of the area. The surface chlorophyll anomalies, induced by these physical mechanisms, have an impact on primary production. We estimate that Rossby waves induce, locally in space and time, increases (generally associated with the chlorophyll wave crest) and decreases (generally associated with the chlorophyll wave trough) in primary production, ~±20% of the estimated background primary production. This symmetrical situation suggests a net weak effect of Rossby waves on primary production. [ABSTRACT FROM AUTHOR]

An eddy-permitting coupled ecosystem-circulation model including dissolved organic matter is used to estimate the dissolved organic nitrogen (DON) supply sustaining primary production in the subtropical North Atlantic Ocean. After an analysis of the coupled model performances compared to the data, a sensitivity study demonstrates the strong impact of parameter values linked to the hydrolysis of particulate organic nitrogen and remineralisation of dissolved organic nitrogen on surface biogeochemical concentrations. The physical transport of dissolved organic nitrogen contributes to maintain the level of primary production in this subtropical gyre. It is dominated by the meridional component. We estimate a meridional net input of 0.039 molN.m-2. yr-1 over the domain (13°- 35° N and 71-40° W) in the subtropical gyre. This supply is driven by the Ekman transport in the southern part and by non-Ekman transport (meridional current components, eddies, meanders and fronts) in the northern part of the subtropical gyre. At 12° N, our 15 estimate (18 kmolN.s-1) confirms the estimation (17.9 kmolN.s-1) made by Roussenov et al. (2006) using a simplified biogeochemical model in a large scale model. This DON meridional input is within the range (from 0.05 up to 0.24 molN.m-2.yr-1) (McGillicuddy and Robinson, 1997; Oschlies, 2002) of all other possible mechanisms (mesoscale activity, nitrogen fixation, atmospheric deposition) fuelling primary production in the sub tropical gyre. The present study confirms that the lateral supply of dissolved organic nitrogen might be important in closing the N budget over the North Atlantic Ocean and quantifies the importance of meridional input of dissolved organic nitrogen. [ABSTRACT FROM AUTHOR]

How do Rossby waves influence primary production in the North Atlantic Ocean? Rossby waves have a clear signature on surface chlorophyll concentrations which can be explained by a combination of vertical and horizontal mechanisms (reviewed in Killworth et al., 2004). In this study, we aim to investigate the role of the different physical processes to explain the surface chlorophyll signatures and the consequences on primary production using a 3-D coupled physical/biogeochemical model for the year 1998. The analysis at 20 given latitudes, mainly located in the subtropical gyre, where Rossby waves are strongly correlated with a surface chlorophyll signature, shows that vertical and horizontal processes are involved in the surface chlorophyll anomalies. Furthermore, the ecosystem response is, as expected, stronger when vertical input of dissolved inorganic nitrogen is observed. The surface chlorophyll anomalies, induced by these physical mechanisms, have an impact on primary production. We then estimate that Rossby waves induce, locally in space and time, increases (generally associated with the wave crest) and decreases (generally associated with the wave trough) in primary production (↑±20% of the estimated primary production). This symmetrical situation suggests a net weak effect of Rossby waves on primary production. [ABSTRACT FROM AUTHOR]

Surface ocean biogeochemistry and photochemistry regulate ocean-atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or p CO 2 ) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N 2 O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes., Competing Interests: We declare we have no competing interests., (© 2020 The Authors.)

We investigated the seasonal and annual variability of surface sulphur plumes in the northern Benguela upwelling system off Namibia because of their significant impacts on the marine ecosystem, fishing industry, aquaculture farming and tourism due to their toxic properties. We identified the sulphur plumes in ocean colour satellite data of the medium resolution imaging spectrometer (MERIS) for the 2002-2012 time period using the differences in the spectral properties of Namibian Benguela optical water types. The sulphur events have a strong seasonal cycle with pronounced main and off-seasons forced by local and remote-driven processes. The main peak season is in late austral summer and early austral autumn at the beginning of the annual upwelling cycle caused by increasing equatorwards alongshore winds. The sulphur plume activity is high between February and April during the seasonal oxygen minimum associated with the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the Angola Benguela Frontal Zone, the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters as well as the seasonal arrival of the downwelling coastal trapped waves. The off-season is in austral spring and early austral summer during increased upwelling intensity and enhanced oxygen supply. The annual variability of sulphur events is characterized by very high activities in years 2004, 2005 and 2010 interrupted by periods of lower activity in years 2002 to 2003, 2006 to 2009 and 2011 to 2012. This result can be explained by the relative contributions or adding effects of local and remote-driven forces (from the equatorial area). The probability for the occurrence of sulphur plumes is enhanced in years with a lower annual mean of upwelling intensity, decreased oxygen supply associated with decreased lateral ventilation of bottom waters, more southern position of the Angola Benguela Frontal Zone, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves. Understanding of the variability and forcing processes of the toxic sulphur events will help in the future to monitor and forecast them as well as to manage their social and economic consequences in the northern Benguela upwelling system off Namibia.

Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels., (© 2014 John Wiley & Sons Ltd.)