South Atlantic Subtropical Gyre Research Articles

Controls on distributions of aluminium, manganese and cobalt in the South Atlantic Ocean along GEOTRACES transect GA10

Trace metals (TMs) manganese (Mn), cobalt (Co), and aluminium (Al) have important geochemical and biological roles in the ocean. Here, we present full depth profiles of dissolved (d) and particulate Al, Mn, and Co along the latitude of 40 °S in the South Atlantic Ocean from the GEOTRACES GA10 cruises that operated in austral spring 2010 and summer 2011. The region is characterized by enhanced primary productivity and forms a key transition zone between the Southern Ocean and South Atlantic Subtropical Gyre. The mean concentrations of dAl, dCo, and dMn (±standard deviation) were 3.36 ± 2.65 nmol kg−1, 35.3 ± 17.6 pmol kg−1, and 0.624 ± 1.08 nmol kg−1, respectively. Their distributions in surface waters were determined by external sources and complex internal biogeochemical processes. Specifically, surface ocean dCo was controlled by the interplay between phytoplankton uptake, remineralization and external inputs; dMn was likely determined by the formation and photoreduction of Mn-oxides; and dAl was supplied by atmospheric deposition and removed by scavenging onto particles. Fluvial and sedimentary inputs near the Rio de La Plata estuary and benthic sources from the Agulhas Bank resulted in elevated dTM concentrations in near-shore surface waters. These externally sourced dTMs were effectively delivered to the open ocean by offshore diffusion and/or advection, and potentially facilitated enhanced primary productivity along the transect.The distributions of dTMs at depth were predominantly controlled by the mixing of North Atlantic Deep Water (NADW) and waters of Antarctic origin (e.g., Upper Circumpolar Water (UCDW) and Antarctic Bottom Water (AABW)). The calculated endmember concentrations of dAl and dCo in NADW showed minor decreases in the SASTG following north–south transport, suggesting removal rates of 0.064 nM/year and 0.035–0.075 pM/year, respectively. The endmember concentration of dCo in AABW was maintained at ∼30 pmol kg−1 without evidence for scavenging removal in the Southern Ocean and SASTG (time frame >400 years). The concentrations of dMn in NADW and AABW were between 0.1 and 0.16 nmol kg−1, and any elevated dMn concentrations were ascribed to local external inputs (e.g., from sediments in the Argentine Basin and hydrothermal activity near the Mid-Atlantic Ridge). Hence, four controlling factors (sources, internal cycling, water mass mixing and time) need to be considered when assessing TM distributions in the global ocean, even for TMs that are vulnerable to scavenging removal processes. Because the deep waters formed in high latitude oceans are crucial components of the global thermohaline overturning system, any processes (e.g., glacier melting, upwelling and sinking, and biological activity) that impact the preformed dTM concentrations in high latitude oceans will determine the downstream dTM distributions. Therefore, the sources and sinks of TMs and associated biological activity in high latitude oceans could engender basin to global scale impacts on seawater distributions of Al, Co, and Mn and their stoichiometric relationships with macronutrients, and the global biogeochemical cycles of these scavenged-type TMs.

The South Atlantic Circulation Between 34.5°S, 24°S and Above the Mid‐Atlantic Ridge From an Inverse Box Model

AbstractThe South Atlantic Ocean plays a key role in the heat exchange of the climate system, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic Subtropical gyre at 34.5°S, 24°S, and 10°W, we identify water masses and compute absolute geostrophic circulation using inverse modeling. In the upper layers, the currents describe the South Atlantic anticyclonic gyre with the northwest flowing Benguela Current (26.3 ± 2.0 Sv at 34.5°S, and 21.2 ± 1.8 Sv at 24°S) flowing above the Mid‐Atlantic Ridge (MAR) between 22.4°S and 28.4°S (−19.2 ± 1.4 Sv), and the southward flowing Brazil Current (−16.5 ± 1.3 Sv at 34.5°S, and −7.3 ± 0.9 Sv at 24°S); the deep layers feature the southward transports of Deep Western Boundary Current (−13.9 ± 3.0 Sv at 34.5°S, and −8.7 ± 3.8 Sv at 24°S) and Deep Eastern Boundary Current (−15.1 ± 3.5 Sv at 34.5°S, and −16.3 ± 4.7 Sv at 24°S), with the interbasin west‐to‐east flow close to 24°S (7.5 ± 4.4 Sv); the abyssal waters present northward mass transports through the Argentina Basin (5.6 ± 1.1 Sv at 34.5°S, and 5.8 ± 1.5 Sv at 24°S) and Cape Basin (8.6 ± 3.5 Sv at 34.5°S–3.0 ± 0.8 Sv at 24°S) before returning southward (−2.2 ± 0.7 Sv at 24°S to −7.9 ± 3.6 Sv at 34.5°S), without any interbasin exchange across the MAR. In addition, we compute the upper AMOC strength (14.8 ± 1.0 and 17.5 ± 0.9 Sv), the equatorward heat transport (0.30 ± 0.05 and 0.80 ± 0.05 PW), and the freshwater flux (0.18 ± 0.02 and −0.07 ± 0.02 Sv) at 34.5°S and 24°S, respectively.

How does the phytoplankton–light feedback affect the marine N2O inventory?

Abstract. The phytoplankton–light feedback (PLF) describes the interaction between phytoplankton biomass and the downwelling shortwave radiation entering the ocean. The PLF allows the simulation of differential heating across the ocean water column as a function of phytoplankton concentration. Only one third of the Earth system models contributing to the 6th phase of the Coupled Model Intercomparison Project (CMIP6) include a complete representation of the PLF. In other models, the PLF is either approximated by a prescribed climatology of chlorophyll or not represented at all. Consequences of an incomplete representation of the PLF on the modelled biogeochemical state have not yet been fully assessed and remain a source of multi-model uncertainty in future projection. Here, we evaluate within a coherent modelling framework how representations of the PLF of varying complexity impact ocean physics and ultimately marine production of nitrous oxide (N2O), a major greenhouse gas. We exploit global sensitivity simulations at 1∘ horizontal resolution over the last 2 decades (1999–2018), coupling ocean, sea ice and marine biogeochemistry. The representation of the PLF impacts ocean heat uptake and temperature of the first 300 m of the tropical ocean. Temperature anomalies due to an incomplete PLF representation drive perturbations of ocean stratification, dynamics and oxygen concentration. These perturbations translate into different projection pathways for N2O production depending on the choice of the PLF representation. The oxygen concentration in the North Pacific oxygen-minimum zone is overestimated in model runs with an incomplete representation of the PLF, which results in an underestimation of local N2O production. This leads to important regional differences of sea-to-air N2O fluxes: fluxes are enhanced by up to 24 % in the South Pacific and South Atlantic subtropical gyres but reduced by up to 12 % in oxygen-minimum zones of the Northern Hemisphere. Our results, based on a global ocean–biogeochemical model at CMIP6 state-of-the-art level, shed light on current uncertainties in modelled marine nitrous oxide budgets in climate models.

Projected Atlantic overturning slow-down is to be compensated by a strengthened South Atlantic subtropical gyre

The upper limb of the Atlantic meridional overturning circulation brings shallow interocean contributions to replenish the North Atlantic export of deepwaters. It is primarily formed in the southern South Atlantic where the converging entrainment of Pacific and Indian Ocean waters meet and incorporate into the South Atlantic subtropical gyre. Here, we use Community Earth System Model 1 Large Ensemble simulation results along 1920–2100 to investigate the response of the Atlantic meridional overturning circulation upper limb and the South Atlantic subtropical gyre to future human-induced climate warming under business-as-usual greenhouse gas emissions. In terms of flow redistribution, we find that the Atlantic meridional overturning circulation upper limb weakens not because less waters are being imported from the Pacific and Indian basins — but because waters are being mostly directed to recirculate in the southwestern portion of a distorted South Atlantic subtropical gyre, turning back southward after reaching the South Atlantic western boundary.

The Chlorophyll-a Content Distribution in the Atlantic Ocean in December 2019 — January 2020 according to Ship Measurements at the Different Hydrometeorological Conditions

The horizontal and vertical chlorophyll-a concentrations (Chl) distributions along the ship route of 79th cruise of R/V “Akademik Mstislav Keldysh” in the Atlantic Ocean in December 2019—January 2020 were obtained. Analysis of the observations was conducted for the biogeochemical provinces which were determined by means of the different hydrometeorological conditions for the phytoplankton community functioning.Three methods of Chl measurements were applied: standard extraction method of the seawater samples, the fluorometer underway measurements in the flow-through water system at the depth of 5 m and in-situ profiling fluorometer measurements at stations. Also, measurements of the seawater temperature and salinity in the flow-through water system, the CTD and optical profiling, and ARGO floats were used.The maximum Chl values in the near-surface ocean layer are observed at zones of divergence in the Canary upwelling systems (CNRY) (1.3 mg/m3) and in the Falkland (Malvinas) Current system (4.8 mg/m3), and also in the Drake Strait (1.2 mg/m3). The least Chl values (0.01–0.12 mg/m3) are observed in the tropical, equatorial areas and in the South Atlantic Subtropical Anticyclonic Gyre (SASTG) at the ship route.Vertical Chl profiles with absence of the significant Chl maximum were found. In the CNRY region the Chl values were not changed much within the euphotic zone. While in the region of the Antarctic Circumpolar Current in the Drake Strait and in the offshore stream of the Falkland Current the quasi-homogeneous Chl profiles were observed in the upper layer which is almost 2 times deeper than the euphotic zone.In the profiles with the significant local Chl maximum the most Chl value of 3.5 mg/m3 is observed at the depth about 12 m in the upwelling region on the Patagonian Shelf. In other regions the depth of Chl maximum localization (0.2–0.9 mg/m3) varies widely from 25 to 120 m. In the western part of the SASTG region the absolute Chl maxima were found at the depth of 120 m, that is deeper, then the euphotic zone which is equal to 100 m.

What are small-size microplastic distributions telling us?

Writing in this issue of Global Change Biology, Shiye Zhao and co-authors report a microplastic soup extending thousands of meters below the South Atlantic Subtropical Gyre. This discovery contributes two pieces to the missing plastic puzzle- (1) nets have been under-sampling the smallest microplastic size fraction, which is actually not missing when using high-volume filtration, and (2) small microplastics in particular find their way below the surface. But their results also contribute several new questions, because some of what they found is quite surprising. This article is a Research Article on Zhao et al., https://doi.org/10.1111/gcb.16089.

Large quantities of small microplastics permeate the surface ocean to abyssal depths in the South Atlantic Gyre

Hundreds of studies have surveyed plastic debris in surface ocean gyre and convergence zones, however, comprehensive microplastics (MPs, ≤5mm) assessments beneath these surface accumulation areas are lacking. Using in situ high-volume filtration, Manta net and MultiNet sampling, combined with micro-Fourier-transform-infrared imaging, we discovered a high abundance (up to 244.3 pieces per cubic meter [n m-3 ]) of small microplastics (SMPs, characteristically <100μm) from the surface to near-sea floor waters of the remote South Atlantic Subtropical Gyre. Large horizontal and vertical variations in the abundances of SMP were observed, displaying inverse vertical trends in some cases. SMP abundances in pump samples were more than two orders of magnitude higher than large microplastics (LMPs, >300μm) concurrently collected in MultiNet samples. Higher-density polymers (e.g., alkyd resins and polyamide) comprised >65% of the total pump sample count, highlighting a discrepancy between polymer compositions from previous ocean surface-based surveys, typically dominated by buoyant polymers such as polyethylene and polypropylene. Contrary to previous reports stating LMP preferentially accumulated at density gradients, SMP with presumably slower sinking rates are much less influenced by density gradients, thus resulting in a more even vertical distribution in the water column, and potentially longer residence times. Overall, our findings suggest that SMP is a critical and largely underexplored constituent of the oceanic plastic inventory. Additionally, our data support that weak current systems contribute to the formation of SMP hotspots at depth, implying a higher encounter rate for subsurface particle feeders. Our study unveils the prevalence of plastics in the entire water column, highlighting the urgency for more quantification of the deep-ocean MP, particularly the smaller size fraction, to better understand ecosystem exposure and to predict MP fate and impacts.

Calibrating Trilobatus sacculifer Na/Ca Ratios From Atlantic Core‐Tops as a Proxy for Sea Surface Salinity

AbstractRecent studies found a positive correlation between seawater salinity and Na/Ca ratios in foraminiferal calcite, suggesting a potential new paleosalinity proxy (Bertlich et al., 2018, https://doi.org/10.5194/bg-15-5991-2018; Hauzer et al., 2021, https://doi.org/10.1029/2021pa004218; Mezger et al., 2016, https://doi.org/10.1002/2016pa003052; Wit et al., 2013). To test this new proxy in an open‐ocean setting, Na/Ca ratios from the surface‐dwelling planktic foraminifera Trilobatus sacculifer were measured from a suite of nine sediment core‐tops spanning from the North Atlantic subtropical gyre to the South Atlantic subtropical gyre across a salinity range of 1.6. Results show a positive correlation between habitat salinity (HS) and T. sacculifer Na/Ca (Na/Ca (mmol/mol) = 0.516 (HS (±0.17)) – 12.934; R2 = 0.81, p &lt; 0.005). We also find no relationship between T. sacculifer Na/Ca and sample size or habitat temperature, indicating Na/Ca is predominantly controlled by salinity. Additionally, we generate the first downcore record of sea surface salinity (SSS) in the Florida Straits over the last deglaciation based on T. sacculifer Na/Ca variability. Results show agreement between our Na/Ca record and a previously published deglacial δ18Osw record that is also thought to reflect SSS variability from the same core (Schmidt &amp; Lynch‐Stieglitz, 2011, https://doi.org/10.1029/2011PA002157). Both records indicate an abrupt increase in SSS during the Younger Dryas (11.7–12.9 kyr). Converting the Florida Straits T. sacculifer Na/Ca ratios to SSS using our new Atlantic core‐top calibration indicates a maximum salinity change of ∼2.2 across the last deglaciation, in agreement with modeling study estimates of salinity change across this period.

Revisiting the Atlantic South Equatorial Current

AbstractThe southern branch of the South Equatorial Current (SSEC) is the northern limit of the South Atlantic Subtropical Gyre. When this current reaches Brazil around 14°S it bifurcates into a southward flow as the Brazil Current (BC) and the surface portion of the northward flowing North Brazil Undercurrent (NBUC). The SSEC system is a key component of the western boundary supply, influencing the NBUC/BC variability and, therefore, global climate through the Meridional Overturning Circulation. In this study, using altimetry satellite data and reanalyzes outputs (1993–2018), we revisit the SSEC mean state and show this current arriving at the South Atlantic western boundary as a multi‐banded flow with surface signatures resulting from different subsurface cores. These bands have velocities between 0.02 and 0.07 m s−1 and, as shown by ADCP data from the PIRATA project, their signature in synoptic scenarios is obscured by eddies and waves with velocities between 0.1 and 0.3 m s−1. In addition, the SSEC annual cycle analysis shows that the seasonality of the bands is out of phase with each other, presenting westward transport anomalies between 0.4 and 2.6 Sv. Finally, our results show that the seasonality of this multi‐banded flow both defines where the BC is born, and modulates the seasonality of semi‐permanent mesoscale eddies off Brazil.

Tropical Atlantic stratification response to late Quaternary precessional forcing

The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward cross-equatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This cross-equatorial transport is influenced by the thermocline circulation and stratification. Although seasonal thermocline stratification in the WTA is precession-driven, the existence of an orbital pacemaker of changes in the entire WTA upper ocean stratification, which comprises the main thermocline, remains elusive. Here, we present a 300 ka-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (δ18O) and Mg/Ca of planktonic foraminifera. Our Δδ18O record between Globigerinoides ruber and Globorotalia truncatulinoides, representing upper ocean stratification, shows a robust precession pacing, where strong stratification was linked to high summer insolation in the Northern Hemisphere (precession minima). Mg/Ca-based temperatures support that stratification is dominated by changes in thermocline temperature. We present a new mechanism to explain changes in WTA stratification, where during the Northern Hemisphere summer insolation maxima, the Intertropical Convergence Zone shifts northward, developing a negative wind stress curl anomaly in the tropical Atlantic. This, in turn, pulls the main thermocline up and pushes the South Atlantic Subtropical Gyre southwards, increasing the stratification to the north of the gyre. This mechanism is supported by experiments performed with the Community Earth System Model (CESM1.2). Finally, we hypothesize that the precession-driven WTA stratification may affect the cross-equatorial flow into the North Atlantic.

Meridional changes in the South Atlantic Subtropical Gyre during Heinrich Stadials

Subtropical ocean gyres play a key role in modulating the global climate system redistributing energy between low and high latitudes. A poleward displacement of the subtropical gyres has been observed over the last decades, but the lack of long-term monitoring data hinders an in-depth understanding of their dynamics. Paleoceanographic records offer the opportunity to identify meridional changes in the subtropical gyres and investigate their consequences to the climate system. Here we use the abundance of planktonic foraminiferal species Globorotalia truncatulinodes from a sediment core collected at the northernmost boundary of the South Atlantic Subtropical Gyre (SASG) together with a previously published record of the same species from the southernmost boundary of the SASG to reconstruct meridional fluctuations of the SASG over last ca. 70 kyr. Our findings indicate southward displacements of the SASG during Heinrich Stadials (HS) 6-4 and HS1, and a contraction of the SASG during HS3 and HS2. During HS6-4 and HS1, the SASG southward displacements likely boosted the transfer of heat to the Southern Ocean, ultimately strengthening deep-water upwelling and CO2 release to the atmosphere. We hypothesize that the ongoing SASG poleward displacement may further increase oceanic CO2 release.

Variability and Trends of the South Atlantic Subtropical Gyre

AbstractPrevious studies have suggested that the South Atlantic subtropical gyre has been expanding, shifting poleward, and intensifying. We investigate these possibilities from an observational standpoint by analyzing the seasonal and interannual variability, and the long‐term trends, in sea surface height, steric height, and ocean mass. For our analysis, we employ satellite altimetry from AVISO (1993–2018), steric height fields reconstructed from the Roemmich‐Gilson Argo Climatology (2004–2016), and ocean mass fields from GRACE (2002–2019). Seasonal variabilities in the gyre position and size are influenced by the positions of the northern and eastern boundaries, as well as the Brazil‐Malvinas Confluence. The gyre strength follows the seasonal heating and cooling of the upper ocean and is largely influenced by variability in the sea surface height gyre maximum. Variability in the ocean mass is driven by the wind stress curl and out of phase with the steric signal. Interannual changes to the southern boundary and strength of the gyre are related to the wind stress curl. We find a significant southward migration of the southern gyre boundary but no trends in the gyre size or gyre strength from 1993 to 2018.

Microbial ecology of the South Atlantic Subtropical Gyre: a state-of-the-art review of an understudied ocean region

Microbial ecology of the South Atlantic Subtropical Gyre: a state-of-the-art review of an understudied ocean region

An assessment of Brazil Current surface velocity and associated transport near 22°S: XBT and altimetry data

The use of Expendable Bathythermographs (XBTs) is widespread throughout the world’s oceans. 15 XBT high-density transects are located only in the Atlantic Ocean. However, the only transect that exclusively monitors the Brazil Current (BC) is the NOAA/AOML AX97 transect, running from Rio de Janeiro to Trindade Island. The BC is the Western Boundary Current linked to the circulation of the South Atlantic subtropical gyre. Surface velocity field based on the AX97 XBT data reference transect and two different altimetry datasets (AVISO and ATOBA) are compared with the objective of assessing the impact of different horizontal resolution altimetry products on averaged BC dynamical features. Furthermore, a sensibility study is performed considering different approaches to estimate BC volume transports. Data from 43 AX97 cruises, covering the period from Jan/2004 to Dec/2013 were used. AVISO (ATOBA) data are daily (weekly) and present a spatial resolution of 1/4° (1/12°). AX97 geostrophic velocity values were calculated based on XBT data using the isopycnal of σθ=26.8 kg/m3 as the reference level of no motion, whereas the altimetry surface geostrophic velocities were obtained based on maps of absolute dynamic topography. About 80.5% (74.4%) of the time, at least one altimetry dataset observed the BC core at AX97 region. ATOBA dataset presented a better performance if compared to AVISO when the BC is highly unstable. Also, in coastal areas, where the flow frequently changes direction, a high-resolution altimetry product (ATOBA) tends to represent the horizontal structure of the BC better than AVISO, where BC horizontal pattern is smoother. Finally, the merge between AX97 data and altimetry datasets presented some inconsistencies when surface currents are in opposite directions. It is recommended to combine AX97 data with another type of data acquired in deeper regions, where near-zero velocities will propagate a smaller error along the water column.

Evolution of the Thermohaline Structure of One Agulhas Ring Reconstructed from Satellite Altimetry and Argo Floats

AbstractThe 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.

Metabolite composition of sinking particles differs from surface suspended particles across a latitudinal transect in the South Atlantic

AbstractMarine sinking particles transport carbon from the surface and bury it in deep‐sea sediments, where it can be sequestered on geologic time scales. The combination of the surface ocean food web that produces these particles and the particle‐associated microbial community that degrades them creates a complex set of variables that control organic matter cycling. We use targeted metabolomics to characterize a suite of small biomolecules, or metabolites, in sinking particles and compare their metabolite composition to that of the suspended particles in the euphotic zone from which they are likely derived. These samples were collected in the South Atlantic subtropical gyre, as well as in the equatorial Atlantic region and the Amazon River plume. The composition of targeted metabolites in the sinking particles was relatively similar throughout the transect, despite the distinct oceanic regions in which they were generated. Metabolites possibly derived from the degradation of nucleic acids and lipids, such as xanthine and glycine betaine, were an increased mole fraction of the targeted metabolites in the sinking particles relative to surface suspended particles, while algal‐derived metabolites like the osmolyte dimethylsulfoniopropionate were a smaller fraction of the observed metabolites on the sinking particles. These compositional changes are shaped both by the removal of metabolites associated with detritus delivered from the surface ocean and by production of metabolites by the sinking particle‐associated microbial communities. Furthermore, they provide a basis for examining the types and quantities of metabolites that may be delivered to the deep sea by sinking particles.

On the translation of Agulhas rings to the western South Atlantic Ocean

The shedding of Agulhas rings is the primary process connecting the Indian and Atlantic oceans. The rings transport warm and salty waters that feed the surface limb of the Atlantic Meridional Overturning Circulation. Early studies suggest that Agulhas rings decay and diffuse their contents within the South Atlantic subtropical gyre. In this paper, we update the ring census using an automated algorithm to detect and track eddies over more than 23 years of satellite altimetry data (1993–2016) and calculate their main characteristics. While 140 rings spawned from the Agulhas Retroflection, their following splitting and merging resulted in 74 long-lived rings that crossed the Walvis Ridge and translated towards the west. Eventually, three rings reached the western boundary. For one of them, we use in situ measurements to document its interaction with the Brazil Current and two cyclonic eddies, which resulted in a current velocity increase by three times. Although already hypothesized, this interaction had not been demonstrated with in situ evidence until now.

South Atlantic Subtropical Gyre Late Twentieth Century Changes

AbstractThis study builds upon existing research suggesting recent changes in the circulation of global subtropical gyres with respect to the South Atlantic Ocean using simulation results from the ocean component of the Community Earth System Model version 1—the Parallel Ocean Program version 2. The results point to an intensification of the total anticyclonic circulation of the subtropical gyre and a southward displacement of the system, as revealed by the wind stress curl, sea surface height, and barotropic stream function fields. Increased values of these variables were found within the dynamical limits of the South Atlantic Subtropical Gyre (SASG), while their basin‐scale structure seemed to be concurrently drifting poleward. The southern branch of the South Equatorial Current (sSEC) marks the northern boundary of the SASG. When reaching the South American coast, it bifurcates into the North Brazil Undercurrent to the north and the Brazil Current to the south. The sSEC bifurcation latitude (SBL) dictates the partition between the waters flowing poleward to recirculate within the SASG and those flowing toward the equatorial region and the Northern Hemisphere. A southward migration of the SBL at a rate of −0.11°± 0.03°/year was observed, associated with a substantial increase in the equatorward advection of waters within the sSEC‐SBL‐North Brazil Undercurrent system.

Strong Mixing and Recirculation in the Northwestern Argentine Basin

AbstractThe Atlantic component of the Meridional Overturning Circulation (AMOC) is a key contributor to the global meridional transport of volume, salt, and heat, and thus plays a central role in global climate. As part of ongoing efforts to monitor the intensity and variability of the AMOC in the South Atlantic, hydrographic sections have been regularly occupied since 2009 near the western boundary along a zonal line at 34.5°S. Here this high‐quality, high‐resolution data set is analyzed to establish the average hydrographic conditions of the northwestern Argentine Basin and the water mass spatial and temporal variability. The water mass analysis also reveals the pathways of the flow in this region, which are further corroborated by full‐depth direct velocity measurements. The repeated hydrographic sections capture an extremely rich vertical structure, characterized by seven distinct water mass layers of northern and southern origin, each with unique property signatures. Almost all of these layers exhibit a sharp zonally banded structure, which is indicative of recirculation cells offshore from the western boundary. The circulation at intermediate levels includes a previously undetected recirculation cell confined very close to the western boundary and superimposed on the classical intermediate water pathway beneath the South Atlantic subtropical gyre. The deep level flow is characterized by the Deep Western Boundary Current (DWBC) and a northward recirculation ~500 km east from the slope.

A synthesis of the environmental response of the North and South Atlantic Sub-Tropical Gyres during two decades of AMT

Anthropogenically-induced global warming is expected to decrease primary productivity in the subtropical oceans by strengthening stratification of the water column and reducing the flux of nutrients from deep-waters to the sunlit surface layers. Identification of such changes is hindered by a paucity of long-term, spatially-resolved, biological time-series data at the basin scale. This paper exploits Atlantic Meridional Transect (AMT) data on physical and biogeochemical properties (1995–2014) in synergy with a wide range of remote-sensing (RS) observations from ocean colour, Sea Surface Temperature (SST), Sea Surface Salinity (SSS) and altimetry (surface currents), combined with different modelling approaches (both empirical and a coupled 1-D Ecosystem model), to produce a synthesis of the seasonal functioning of the North and South Atlantic Sub-Tropical Gyres (STGs), and assess their response to longer-term changes in climate. We explore definitive characteristics of the STGs using data of physical (SST, SSS and peripheral current systems) and biogeochemical variables (chlorophyll and nitrate), with inherent criteria (permanent thermal stratification and oligotrophy), and define the gyre boundary from a sharp gradient in these physical and biogeochemical properties. From RS data, the seasonal cycles for the period 1998–2012 show significant relationships between physical properties (SST and PAR) and gyre area. In contrast to expectations, the surface layer chlorophyll concentration from RS data (CHL) shows an upward trend for the mean values in both subtropical gyres. Furthermore, trends in physical properties (SST, PAR, gyre area) differ between the North and South STGs, suggesting the processes responsible for an upward trend in CHL may vary between gyres. There are significant anomalies in CHL and SST that are associated with El Niño events. These conclusions are drawn cautiously considering the short length of the time-series (1998–2012), emphasising the need to sustain spatially-extensive surveys such as AMT and integrate such observations with models, autonomous observations and RS data, to help address fundamental questions about how our planet is responding to climate change. A small number of dedicated AMT cruises in the keystone months of January and July would complement our understanding of seasonal cycles in the STGs.