North Brazil Current Retroflection Research Articles

Seasonal and Interannual Variability in the Distribution and Removal of Terrigenous Dissolved Organic Carbon in the Amazon River Plume

AbstractThe Amazon River is a large source of terrigenous dissolved organic carbon (tDOC) to the Atlantic Ocean. The fate of this tDOC in the ocean remains unclear despite its importance to the global carbon cycle. Here, we used two decades of satellite ocean color to describe variability in tDOC in the Amazon River plume. Our analyses showed that tDOC distribution has a distinct seasonal pattern, reaching northwest toward the Caribbean during high discharge periods, and moving eastward entrained in the North Brazil Current retroflection during low discharge periods. Elevated tDOC content extended beyond the shelfbreak in all months of the year, suggesting that cross‐shelf carbon transport occurs year‐round. Maximum variability was found at the plume core, where seasonality accounted for 40% of the total variance, while interannual variability accounted for 15% of the variance. Our results revealed a seasonal pattern in tDOC removal over the shelf with increased consumption in May when river discharge is high. Anomalies in tDOC removal over the shelf with respect to the seasonal cycle were significantly correlated with anomalies in tDOC concentration offshore of the shelfbreak with a lag of 30–40 days, so that anomalously high inshore tDOC removal was associated with anomalously low tDOC content offshore. This suggests that variability in the offshore transport of tDOC in the Amazon River plume is modulated by interannual changes in tDOC removal over the shelf.

Late summer northwestward Amazon plume pathway under the action of the North Brazil Current rings

The North Brazil Current (NBC) flows offshore of the mouth of the Amazon River and seasonally sheds anticyclonic rings (NBC rings) that propagate northwestward and interact with the Amazon River plume (ARP). Mesoscale features have a high temporal variability that is hard to monitor from current weekly and monthly sea surface salinity (SSS) satellite fields. Novel SSS fields with a higher temporal resolution analyzed together with satellite geostrophic currents, chlorophyll-a, and wind speed and in-situ data from the “Microbiomes cruise” on the SV Tara in August–September 2021 revealed a late summer freshwater pathway, which was not well documented in earlier studies. By combining these datasets, we improved the characterization of summer ARP pathways. In 2021, the ARP was a succession of freshwater patches cut off from the main plume by the NBC rings. A patch of about 200.000 km2 with salinity <33.5 pss was observed in September 2021, bringing 0.5 Sv of Amazon water northwestward in a period where the mean ocean currents lead to eastward transport. This patch was shallow, very stratified, and it created a surface steric-height anomaly that was identified as an anticyclonic feature in altimetric sea level products. Once separated from the NBC retroflection, it was mainly driven by Ekman currents. Other similar patches were observed during the 2021 summer, leading to a strong intermittency of the ARP transport. They strongly contributed to make 2021 the year with the largest northwestward freshwater transport in late summer within the 2010–2021 time-period investigated. This freshwater transport pathway is important for all plume-related phenomena, and show the ability of combined SMOS and SMAP data to accurately represent the day-to-day SSS variability.

Vertical Interaction Between NBC Rings and Its Implications for South Atlantic Water Export

AbstractNorthwestward‐propagating North Brazil Current (NBC) Rings are often destroyed upon reaching the Caribbean islands, carrying South Atlantic waters into the North Atlantic gyre and connecting the two branches of the Atlantic Meridional Overturning Circulation. Recent observations of NBC rings reported surface and subsurface cores separated by strong stratification. As independent structures, the subsurface eddies often appeared below—even if not aligned with—the surface rings. Motivated by these findings, this paper investigates the occurrence and consequences of vertical coupling between surface and subsurface NBC rings by applying a simplified theoretical model to a realistic numerical simulation. Eddy tracking in this complex simulation reveals around 1,600 instantaneous observations where eddies overlap. At each observation, we assess the eddies' vertical interaction with a 21/2‐layer quasi‐geostrophic framework. This interaction boils down to either coupling between the layers or a vertical splitting of the system, depending on eddy strength versus background shear. The effects of coupling include long‐lived eddies that can travel longer distances. These effects are particularly important for the lower layer, where differences between coupled and non‐coupled eddies are striking: about 40% of eddies with coupling travel 500 km, with ∼10% carrying South Atlantic Water up to the Caribbean sea. Without coupling, eddies propagate less than 150 km. During coupling, lower‐layer eddies could be driven by their surface counterparts to move offshore and bypass a topographic choke point, thus leaving the NBC retroflection region. Although still speculative, this idea explains the link between coupling and the increase in both lifetime and distance traveled for these eddies.

Seasonal Eddy Variability in the Northwestern Tropical Atlantic Ocean

Abstract Eddies in the northwestern tropical Atlantic Ocean play a crucial role in transporting the South Atlantic Upper Ocean Water to the North Atlantic and connect the Atlantic and the Caribbean Sea. Although surface characteristics of those eddies have been well studied, their vertical structures and governing mechanisms are much less known. Here, using a time-dependent energetics framework based on the multiscale window transform, we examine the seasonal variability of the eddy kinetic energy (EKE) in the northwestern tropical Atlantic. Both altimeter-based data and ocean reanalyses show a substantial EKE seasonal cycle in the North Brazil Current Retroflection (NBCR) region that is mostly trapped in the upper 200 m. In the most energetic NBCR region, the EKE reaches its minimum in April–June and maximum in July–September. By analyzing six ocean reanalysis products, we find that barotropic instability is the controlling mechanism for the seasonal eddy variability in the NBCR region. Nonlocal processes, including advection and pressure work, play opposite roles in the EKE seasonal cycle. In the eastern part of the NBCR region, the EKE seasonal evolution is similar to the NBCR region. However, it is the nonlocal processes that control the EKE seasonality. In the western part of the NBCR region, the EKE magnitude is one order of magnitude smaller than in the NBCR region and shows a different seasonal cycle, which peaks in March and reaches its minimum in October–November. Our results highlight the complex mechanisms governing eddy variability in the northwestern tropical Atlantic and provide insights into their potential changes with changing background conditions.

Revisiting the tropical Atlantic western boundary circulation from a 25-year time series of satellite altimetry data

Abstract. Geostrophic currents derived from altimetry are used to investigate the surface circulation in the western tropical Atlantic over the 1993–2017 period. Using six horizontal sections defined to capture the current branches of the study area, we investigate their respective variations at both seasonal and interannual timescales, as well as the spatial distribution of these variations, in order to highlight the characteristics of the currents on their route. Our results show that the central branch of the South Equatorial Current and its northern branch near the Brazilian coast, the North Brazil Current component located south of the Equator, and the Guyana Current have similar annual cycles, with maxima (minima) during late boreal winter (boreal fall) when the Intertropical Convergence Zone is at its southernmost (northernmost) location. In contrast, the seasonal cycles of the North Brazil Current branch located between the Equator and 7–8∘ N, its retroflected branch, the northern branch of the South Equatorial Current to the west of 35∘ W, and the North Equatorial Countercurrent show maxima (minima) during late boreal summer (boreal spring), following the remote wind stress curl strength variation. West of 32∘ W, an eastward current (the Equatorial Surface Current, ESC) is observed between 2–2∘ N, identified as the equatorial extension of the retroflected branch of the North Brazil Current. It is part of a large cyclonic circulation observed between 0–6∘ N and 35–45∘ W during boreal spring. We also observed a secondary North Brazil Current retroflection flow during the second half of the year, which leads to the two-core structure of the North Equatorial Countercurrent and might be related to the wind stress curl seasonal changes. To the east, the North Equatorial Countercurrent weakens and its two-core structure is underdeveloped due to the weakening of the wind stress. At interannual scales, depending on the side of the Equator examined, the North Brazil Current exhibits two opposite scenarios related to the phases of the tropical Atlantic Meridional Mode. At 32∘ W, the interannual variability of the North Equatorial Countercurrent and of the northern branch of the South Equatorial Current (in terms of both strength and/or latitudinal shift) are associated with the Atlantic Meridional Mode, whereas the variability of the Equatorial Surface Current intensity is associated with both the Atlantic Meridional Mode and Atlantic Zonal Mode phases.

Forcing mechanisms of the circulation on the Brazilian Equatorial Shelf

The Brazilian Equatorial Shelf (BES) is a very dynamic region due to the influence of various forcings: trade winds, macrotides, the Amazon River, and the North Brazil Current (NBC). To investigate each forcing’s role in BES circulation, a control simulation and three sensitivity simulations (without rivers, without tides, and without winds) were performed using the Regional Ocean Modeling System (ROMS) with a horizontal resolution of 1/24° and realistic forcings. The results showed that the NBC advected low-salinity waters from the Amazon River northwestward to Caribbean Sea and occasionally eastward through the NBC retroflection. The NBC exhibited the strongest flow when all forcings were included in the simulation. On the other hand, southeasterly winds inhibited the NBC retroflection system and, thus reducing its contribution to North Equatorial Counter Current (NECC) and Equatorial Undercurrent (EUC). The simulation without tides showed a plume more advected by the NBC and constricted to the first 10 m of the water column, while in the simulation without wind the plume was less advected northwestward by the weakened NBC and spread more toward the shelf break. The correlation analysis highlighted the complexity of the system, the complementarity of the forcings and their time-variable interdependence. The three sensitivity simulations and correlation analysis demonstrated the wind to be the main mechanism of the circulation in the BES. The sensitivity simulations also gave insight of how circulation would respond in a future scenario if climate changes persist. Previous studies suggest that the reduced Amazon runoff may indirectly weaken Atlantic ITCZ and thus trade winds causing a weakened circulation in tropical Atlantic. Our results show that this weakened circulation would advect even less fresh water northward, worsening the disturbances in North Atlantic due to the reduced freshwater supply.

Eastward propagating surface salinity anomalies in the tropical North Atlantic

ABSTRACT Upper ocean variations across the tropical Atlantic are strongly seasonal due to corresponding seasonality in surface forcing and continental runoff. This implies that many features in regional upper ocean state or transport anomalies may also be seasonally locked. In the boreal summer and autumn, remote sensing sea surface salinity (SSS) observations show the presence of eastward propagating anomalies concurrent with the seasonal development of fresh Amazon plume and acceleration of the eastward North Equatorial Countercurrent. Interannual variations in these eastward cross-Atlantic SSS signals are investigated in connection with their forcing by wind and circulation patterns. Satellite data show that these SSS anomalies are advected zonally across the entire Atlantic. It is suggested that they originate due to wind-induced changes in the Amazon plume areal extent, which are notorious in the North Brazil Current retroflection. Satellite SSS is instrumental for exploring such signals because in-situ observations do not always capture them due to the limitation in resolved spatial and temporal scales.

Amazon river plume influence on planktonic decapods in the tropical Atlantic

The western tropical Atlantic is strongly influenced by the Amazon, receiving the full discharge from the largest river basin of the world. In order to ascertain the coastal-oceanic gradient in abundance and composition of planktonic decapod communities along the Amazon River Plume (ARP) and its retroflection, 33 plankton samples were obtained along three sampling transects: Coastal and oceanic area influenced by the ARP (Coastal IARP and Ocean IARP, respectively), and oceanic waters without ARP influence (Ocean). A total of 13,117 specimens, belonging to 33 taxa, were analyzed. The distribution of planktonic decapod communities was strongly influenced by the Amazon discharge, showing the following patterns: (1) The coastal-oceanic gradients in decapod abundance are similar to other shelf areas of eastern South America, with higher abundance in the area under ARP influence, (2) meroplanktonic decapods dominate in coastal and some oceanic areas due to the offshore transport of coastal organisms along the ARP retroflection, and (3) the ARP is clearly responsible for the observed differences in community structure between the three transects. Furthermore, this study highlights the importance of coastal and shelf environments as main sources of planktonic decapods for pelagic ecosystems in the tropical Atlantic during the period of strong North Brazil Current retroflection.

Eddies in the Tropical Atlantic Ocean and Their Seasonal Variability

AbstractWe 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.

Seasonal Variability of Retroflection Structures and Transports in the Atlantic Ocean as Inferred from Satellite-Derived Salinity Maps

Three of the world’s most energetic regions are in the tropical and South Atlantic: the North Brazil Current Retroflection, the Brazil-Malvinas Confluence, and the Agulhas Current Retroflection. All three regions display offshore diversions of major boundary currents, which define the intensity of the returning limb of the Atlantic meridional overturning circulation. In this work, we use a sea-surface salinity (SSS) satellite product, combined with a high-resolution numerical model and in situ measurements, in order to explore the seasonal variation of the surface currents and transports in these three regions. The analysis of the model output shows that the SSS patterns reflect the surface velocity structure, with the largest horizontal SSS gradients coinciding with those areas of highest velocity and the most predominant velocity vector being 90° anticlockwise (clockwise) from the horizontal SSS gradient in the northern (southern) hemisphere. This information is then applied to the SSS satellite product to obtain maps of water velocity and salt transports, leading to a quantitative tool to estimate both water and salt transports in key regions of the world ocean.

EMPIRICAL ORTHOGONAL FUNCTION ANALYSIS OF SATELLITE-DERIVED CURRENTS IN THE TROPICAL ATLANTIC

An Empirical Orthogonal Function (EOF) analysis was performed using 22 years of monthly surface currents in order to investigate the main modes of variability of the Tropical Atlantic surface circulation based on satellite data. The EOF1 of the zonal component captures more than 50% of the total variance and represents the central branch of the South Equatorial Current (cSEC). The EOF2 of the zonal component (~8% variance) represents a zonal dipole with a positive signal to the west, and a negative one to the east in the Atlantic Cold Tongue (ACT). The meridional EOF1 (18% variance), with alternating positive and negative structures in the western tropical North Atlantic, representing the high dynamics associated to the North Brazil Current retroflection and its anticyclonic eddies. The meridional EOF2 (~ 12% variance) represents the high variability of the NBC, with stronger intermittent signals propagating from the eastern Atlantic. In the EOF1 of currents speed predominates the cSEC pattern (30%) and the EOF2 (8%) represents the zonal dipole between South Atlantic Warm Pool and the ACT. These dominant modes of surface current variability contribute to the understanding of the ocean-atmosphere interaction processes, zonal and meridional transport variability, and the surface dynamics in the TA. Keywords: EOF Analysis, Remote Sensing, Ocean Currents, Oceanic Dynamics, Tropical Atlantic

Sea Surface Salinity Signature of the Tropical Atlantic Interannual Climatic Modes

AbstractThe characteristic sea surface salinity (SSS) patterns associated with the tropical Atlantic meridional and equatorial interannual modes are extracted from in situ observations, by a statistical analysis performed on the 1980–2012 period. These SSS signatures of the interannual climatic modes are reproduced in a regional numerical simulation. For each mode, oceanic and/or atmospheric processes driving the SSS signature are identified through a mixed‐layer salt budget in the validated model. During a positive meridional mode in spring, a northward shift of the Intertropical Convergence Zone and related precipitation maximum creates a south‐north dipole of positive‐negative SSS anomalies around the equator. Western boundary currents strengthen and advect relatively fresh equatorial waters, which creates negative SSS anomalies in the north and south west tropical Atlantic. Meridional and vertical advection create positive SSS anomalies off the Congo River. During a positive equatorial mode in summer, a southward shift of the Intertropical Convergence Zone‐related rainfall maximum creates a south‐north dipole of negative‐positive SSS anomalies between the equator and 10°N. Meridional advection also contributes to the positive SSS anomalies between 5°N and 10°N. Vertical advection and diffusion at the mixed‐layer base create positive SSS anomalies between 5°S and the equator. Horizontal advection creates large SSS anomalies in the North Brazil Current retroflection region, negative along the coast and positive further offshore. The SSS signatures of the meridional and equatorial modes described above are well captured by the Soil Moisture–Ocean Salinity satellite during the 2010 and 2012 events.

Holocene sedimentation on the distal Amazon Fan/Demerara Abyssal Plain

We have studied the rock magnetic characteristics of 31 multicores and 7 gravity cores from deep-sea sediments of the distal Amazon Fan and Demerara Abyssal Plain. This study has permitted us to delineate the space/time pattern of Holocene physical sedimentation along the northeastern margin of South America associated with the Amazon River sediment plume. The rock magnetic variability is correlatable over the entire study region. This degree of correlation has permitted us to develop a composite picture of Holocene rock magnetic (and overall physical clastic) sedimentation. 26 radiocarbon dates have been combined with the rock magnetic correlations to provide a composite Holocene chronostratigraphy for that variability. We have built isopach maps of overall sedimentation for three windows in the Holocene (3000, 4500, and 7000 calendar years BP). The overall sedimentation in the center of our region is ~5 cm/kyr. But, sedimentation rates are faster closer to the coastline and associated with the distal clastic flux from the Amazon Plume. There is a secondary high in sediment accumulation under the North Brazil Current retroflection. Our maps suggest that the retroflection has been active at the millennial scale for at least the last 7000 years. We estimate that the mid-Holocene (~4.5–9000 calendar years BP) pattern of sedimentation was distinctly different from that of the last few thousand years. The clastic sediments were ~25% reduced in concentration (with the biogenic sediments similarly increased by ~25%), they were finer–grained, and significantly less rich in ferric iron phases (hematite/goethite). We think these three covarying clastic parameters are due to an interval of reduced clastic flux from the Amazon River Plume with a reduced proportion of Amazon Basin sediments (as opposed to Andes-derived sediments). We hypothesize this was due to significantly drier Amazon Basin environmental conditions during that time interval.

Simulating transport pathways of pelagic Sargassum from the Equatorial Atlantic into the Caribbean Sea

Since 2011, beach inundation of massive amounts of pelagic Sargassum algae has occurred around the Caribbean nations and islands. Previous studies have applied satellite ocean color to determine the origins of this phenomenon. These techniques, combined with complementary approaches, suggest that, rather than blooms originating in the Caribbean, they arrive from the Equatorial Atlantic. However, oceanographic context for these occurrences remains limited. Here, we present results from synthetic particle tracking experiments that characterize the interannual and seasonal dynamics of ocean currents and winds likely to influence the transport of Sargassum from the Equatorial Atlantic into the Caribbean Sea. Our findings suggest that Sargassum present in the western Equatorial Atlantic (west of longitude 50°W) has a high probability of entering the Caribbean Sea within a year’s time. Transport routes include the Guiana Current, North Brazil Current Rings, and the North Equatorial Current north of the North Brazil Current Retroflection. The amount of Sargassum following each route varies seasonally. This has important implications for the amount of time it takes Sargassum to reach the Caribbean Sea. By weighting particle transport predictions with Sargassum concentrations at release sites in the western Equatorial Atlantic, our simulations explain close to 90% of the annual variation in observed Sargassum abundance entering the Caribbean Sea. Additionally, results from our numerical experiments are in good agreement with observations of variability in the timing of Sargassum movement from the Equatorial Atlantic to the Caribbean, and observations of the spatial extent of Sargassum occurrence throughout the Caribbean. However, this work also highlights some areas of uncertainty that should be examined, in particular the effect of “windage” and other surface transport processes on the movement of Sargassum. Our results provide a useful launching point to predict Sargassum beaching events along the Caribbean islands well in advance of their occurrence and, more generally, to understand the movement ecology of a floating ecosystem that is essential habitat to numerous marine species.

Mechanisms of the 40–70 Day Variability in the Yucatan Channel Volume Transport

AbstractThe Yucatan Channel connects the Caribbean Sea with the Gulf of Mexico and is the main outflow region of the Caribbean Sea. Moorings in the Yucatan Channel show high‐frequent variability in kinetic energy (50–100 days) and transport (20–40 days), but the physical mechanisms controlling this variability are poorly understood. In this study, we show that the short‐term variability in the Yucatan Channel transport has an upstream origin and arises from processes in the North Brazil Current. To establish this connection, we use data from altimetry and model output from several high resolution global models. A significant 40–70 day variability is found in the sea surface height in the North Brazil Current retroflection region with a propagation toward the Lesser Antilles. The frequency of variability is generated by intrinsic processes associated with the shedding of eddies, rather than by atmospheric forcing. This sea surface height variability is able to pass the Lesser Antilles, it propagates westward with the background ocean flow in the Caribbean Sea and finally affects the variability in the Yucatan Channel volume transport.

Investigation of the Intra-Annual Variability of the North Equatorial Countercurrent/North Brazil Current Eddies and of the Instability Waves of the North Tropical Atlantic Ocean Using Satellite Altimetry and Empirical Mode Decomposition

AbstractThe intra-annual variability of the tropical Atlantic Ocean north of the equator is investigated with satellite altimetry mean sea level anomaly data and with an algorithm based on empirical mode decomposition (EMD) methods. Two regions of high variability are identified. The first region, between 3° and 12°N, is characterized by the presence of westward-propagating eddies linked to the North Brazil Current (NBC) retroflection in the vicinity of the Brazilian coast. They show a strong annual cycle. In this paper the EMD algorithm points out that this signal is frequency modulated shifting from large length-scale structures in October to smaller ones in March. Consequently, the number of “eddies” per year can be aliased, according to the time and location of sampling, and can impact the percentage they explain of the interhemispheric exchange of mass and heat associated with the meridional overturning circulation’s upper limb. A scenario concerning this dynamics is proposed. The second region reveals the presence of westward-propagating instability waves centered north of the equator (3°–7°N) between 50° and 10°W. These instability waves are also frequency modulated and show a strong seasonal cycle with maximum amplitude around August.

A Synoptic Assessment of the Amazon River-Ocean Continuum during Boreal Autumn: From Physics to Plankton Communities and Carbon Flux.

The Amazon generates the world's largest offshore river plume, which covers extensive areas of the tropical Atlantic. The data and samples in this study were obtained during the oceanographic cruise Camadas Finas III in October 2012 along the Amazon River-Ocean Continuum (AROC). The cruise occurred during boreal autumn, when the river plume reaches its maximum eastward extent. In this study, we examine the links between physics, biogeochemistry and plankton community structure along the AROC. Hydrographic results showed very different conditions, ranging from shallow well-mixed coastal waters to offshore areas, where low salinity Amazonian waters mix with open ocean waters. Nutrients, mainly and , were highly depleted in coastal regions, and the magnitude of primary production was greater than that of respiration (negative apparent oxygen utilization). In terms of phytoplankton groups, diatoms dominated the region from the river mouth to the edge of the area affected by the North Brazil Current (NBC) retroflection (with chlorophyll a concentrations ranging from 0.02 to 0.94 mg m−3). The North Equatorial Counter Current (NECC) region, east of retroflection, is fully oligotrophic and the most representative groups are Cyanobacteria and dinoflagellates. Additionally, in this region, blooms of cyanophyte species were associated with diatoms and Mesozooplankton (copepods). A total of 178 zooplankton taxa were observed in this area, with Copepoda being the most diverse and abundant group. Two different zooplankton communities were identified: a low-diversity, high-abundance coastal community and a high-diversity, low-abundance oceanic community offshore. The CO2 fugacity (fCO2sw), calculated from total alkalinity (1,450 < TA < 2,394 μmol kg−1) and dissolved inorganic carbon (1,303 < DIC < 2,062 μmol kg−1) measurements, confirms that the Amazon River plume is a sink of atmospheric CO2 in areas with salinities <35 psu, whereas, in regions with salinities >35 and higher-intensity winds, the CO2 flux is reversed. Lower fCO2sw values were observed in the NECC area. The ΔfCO2 in this region was less than 5 μatm (−0.3 mmol m−2 d−1), while the ΔfCO2 in the coastal region was approximately 50 μatm (+3.7 mmol m−2 d−1). During the cruise, heterotrophic and autotrophic processes were observed and are indicative of the influences of terrestrial material and biological activity, respectively.

Seasonal and interannual variability of sea‐air CO2 fluxes in the tropical Atlantic affected by the Amazon River plume

AbstractCO2 fugacities obtained from a merchant ship sailing from France to French Guyana were used to explore the seasonal and interannual variability of the sea‐air CO2 exchange in the western tropical North Atlantic (TNA; 5–14°N, 41–52°W). Two distinct oceanic water masses were identified in the area associated to the main surface currents, i.e., the North Brazil Current (NBC) and the North Equatorial Current (NEC). The NBC was characterized by permanent CO2 oversaturation throughout the studied period, contrasting with the seasonal pattern identified in the NEC. The NBC retroflection was the main contributor to the North Equatorial Counter Current (NECC), thus spreading into the central TNA, the Amazon River plume, and the CO2‐rich waters probably originated from the equatorial upwelling. Strong CO2 undersaturation was associated to the Amazon River plume. Total inorganic carbon drawdown due to biological activity was estimated to be 154 µmol kg−1 within the river plume. As a consequence, the studied area acted as a net sink of atmospheric CO2 (from −72.2 ± 10.2 mmol m−2 month−1 in February to 14.3 ± 4.5 mmol m−2 month−1 in May). This contrasted with the net CO2 efflux estimated by the main global sea‐air CO2 flux climatologies. Interannual sea surface temperature changes in the TNA caused by large‐scale climatic events could determine the direction and intensity of the sea‐air CO2 fluxes in the NEC. Positive temperature anomalies observed in the TNA led to an almost permanent CO2 outgassing in the NEC in 2010.

The influence of salinity on tropical Atlantic instability waves

AbstractSea surface salinity (SSS) data derived from the Aquarius/SAC‐D satellite mission are analyzed along with other satellite and in situ data to assess Aquarius' capability to detect tropical instability waves (TIWs) and eddies in the tropical Atlantic Ocean and to investigate the influence that SSS has on the variability. Aquarius data show that the magnitude of SSS anomalies associated with the Atlantic TIWs is ±0.25 practical salinity unit, which is weaker than those in the Pacific by 50%. In the central equatorial Atlantic, SSS contribution to the mean meridional density gradient is similar to sea surface temperature (SST) contribution. Consequently, SSS is important to TIW‐related surface density anomalies and perturbation potential energy (PPE). In this region, SSS influences surface PPE significantly through the direct effect and the indirect effect associated with SSS‐SST covariability. Ignoring SSS effects would underestimate TIW‐related PPE by approximately three times in the surface layer. SSS also regulates the seasonality of the TIWs. The boreal‐spring peak of the PPE due to SSS leads that due to SST by about one month. Therefore, SSS not only affects the spatial structure, but the seasonal variability of the TIWs in the equatorial Atlantic. In the northeast Atlantic near the Amazon outflow and the North Brazil Current retroflection region and in the southeast Atlantic near the Congo River outflow, SSS accounts for 80–90% of the contribution to mean meridional density gradient. Not accounting for SSS effect would underestimate surface PPE in these regions by a factor of 10 and 4, respectively.

Terrigenous input off northern South America driven by changes in Amazonian climate and the North Brazil Current retroflection during the last 250 ka

Abstract. We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial periods. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. Increased Amazonian rainfall reflects the intensification of the South American monsoon in response to enhanced land–ocean thermal gradient and moisture convergence. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganizations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.