Eastern Tropical North Atlantic Research Articles

The emission, transport, and impacts of the extreme Saharan dust storm of 2015

Abstract. ​​​​​​​Each summer, the Saharan Air Layer (SAL) transports massive amounts of mineral dust across the Atlantic Ocean, affecting weather, climate, and public health over large areas. Despite the considerable impacts of African dust, the causes and impacts of extreme trans-Atlantic African dust events are not fully understood. The “Godzilla” trans-Atlantic dust event of 2020 has been extensively studied, but little is known about other similar events. Here, we examine the June 2015 event, the second strongest trans-Atlantic African dust event that occurred during the summers from 2003–2022. This event was characterized by moderately high dust emissions over western North Africa and an extremely high aerosol optical depth (AOD) over the tropical North Atlantic. The high dust loading over the Atlantic is associated with atmospheric circulation extremes similar to the Godzilla event. Both the African easterly jet (AEJ) and Caribbean low-level jet (CLLJ) have greatly intensified, along with a westward extension of the North Atlantic subtropical high (NASH), all of which favor the westward transport of African dust. The enhanced dust emissions are related to anomalously strong surface winds in dust source regions and reduced vegetation density and soil moisture across the northern Sahel. The dust plume reduced net surface shortwave radiation over the eastern tropical North Atlantic by about 25 W m−2 but increased net longwave flux by about 3 W m−2. In contrast to the Godzilla event, the 2015 event had minor air quality impacts on the US, partially due to the extremely intensified CLLJ that dispersed the dust plume towards the Pacific.

Distribution of polysaccharidic and proteinaceous gel−like particles in three cyclonic eddies in the Eastern Tropical North Atlantic

Transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP), two prominent classes of gel−like particles in the ocean primarily produced by phytoplankton, play crucial roles in ecological and biogeochemical processes, influencing microbial nutrition, growth, and particle aggregation. The distribution of these particles is intricately linked to the spatiotemporal dynamics of phytoplankton. Mesoscale cyclonic eddies (CEs) are known to stimulate phytoplankton growth and influence particle transport, but their effects on TEP and CSP remain to be determined. In the Eastern Tropical North Atlantic (ETNA), we examined three CEs: one off the Mauritanian coast during summer (Mau), one offshore during winter (Sal), and another near Brava island during winter. Mau and Brava CEs were in their intensification/maturity phase, while the Sal CE was in its decay phase. Both TEP and CSP concentrations correlated with primary productivity, but TEP increased with chlorophyll−a concentration, whereas elevated CSP coincided also with the highest abundance of pico−nanophytoplankton (<20 µm), mainly Synechococcus. Both gels exhibited a positive correlation with bacterial biomass production, indicating their consumption by heterotrophic bacteria. TEP total area in the epipelagic waters of all CEs (Mau, Brava, and Sal) was elevated compared to surrounding waters, with on average 4, 2.5, and 1.6−fold higher values, respectively. However, no significant difference in TEP size distribution was observed within any CEs and their surroundings. Similarly, CSP total area increased in the epipelagic waters of Mau and Brava CEs, with on average 5 and 2.4−fold higher values, respectively, compared to surrounding waters. CSP particles were notably larger in these two eddies, while the Sal CE showed no significant difference from surrounding waters in CSP abundance and size. Overall, TEP and CSP exhibited distinct responses to CEs, with increased concentrations during their intensification/maturation stage and remineralization dominating during their decaying stage.

Impact of the North Atlantic Oscillation on the Decadal Variability of the Upper Subtropical‐Tropical Atlantic Ocean

AbstractIn the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical‐tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006–2022. Relative to the negative NAO phase (2009–2010), during the positive NAO phase (2014–2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite‐derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical‐tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20°N and 20°S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0°, 23°W, likely via the pulling effect of the subtropical cells.

Dependency of simulated tropical Atlantic current variability on the wind forcing

Abstract. The upper wind-driven circulation in the tropical Atlantic Ocean plays a key role in the basin-wide distribution of water mass properties and affects the transport of heat, freshwater, and biogeochemical tracers such as oxygen or nutrients. It is crucial to improve our understanding of its long-term behaviour, which largely relies on model simulations and applied forcing due to sparse observational data coverage, especially before the mid-2000s. Here, we apply two different forcing products, the Coordinated Ocean-ice Reference Experiments (CORE) v2 and the Japanese 55-year Reanalysis (JRA55-do) surface dataset, to a high-resolution ocean model. Where possible, we compare the simulated results to long-term observations. We find large discrepancies between the two simulations regarding the wind and current field. In the CORE simulation, strong, large-scale wind stress curl amplitudes above the upwelling regions of the eastern tropical North Atlantic seem to cause an overestimation of the mean and seasonal variability in the eastward subsurface current just north of the Equator. The wind stress curl of JRA55-do forcing shows much finer structures, and the JRA55-do simulation is in better agreement with the mean and intraseasonal fluctuations in the subsurface current found in observations. The northern branch of the South Equatorial Current flows westward at the surface just north of the Equator. On interannual to decadal timescales, it shows a high correlation of R=0.9 with the zonal wind stress in the CORE simulation but only a weak correlation of R=0.35 in the JRA55-do simulation. We also identify similarities between the two simulations. The strength of the eastward-flowing North Equatorial Counter Current located between 3 and 10° N covaries with the strength of the meridional wind stress just north of the Equator on interannual to decadal timescales in the two simulations. Both simulations present a comparable mean, seasonal cycle and trend of the eastward off-equatorial subsurface current south of the Equator but underestimate the current strength by half compared to observations. In both simulations, the eastward-flowing Equatorial Undercurrent weakened between 1990 and 2009. In the JRA simulation, which covers the modern period of observations, the Equatorial Undercurrent strengthened again between 2008 to 2018, which agrees with observations, although the simulation underestimates the strengthening by over a third. We propose that long-term observations, once they have reached a critical length, need to be used to test the quality of wind-driven simulations. This study presents one step in this direction.

The 1991–2020 sea surface temperature normals

AbstractThe 1991–2020 climate normals for sea surface temperature (SST) are computed based on the NOAA Daily Optimum Interpolation SST dataset. This is the first time that high‐resolution SST normals with global coverage can be achieved in the satellite SST era. Normals are one of the fundamental parameters in describing and understanding weather and climate and provide decision‐making information to industry, public, and scientific communities. This product suite includes SST mean, standard deviation, count and extreme parameters at daily, monthly, seasonal and annual time scales on 0.25° spatial grids. The main feature of the SST mean state revealed by the normals is that in the Tropics, the Indo‐Pacific Ocean is dominated by the warm pool (SST ≥ 28°C) while the eastern Pacific is characterized by the cold tongue (SST ≤ 24°C); in the midlatitudes, SSTs are in zonal patterns with high meridional gradients. Daily SST standard deviations are generally small (<1.0°C) except in frontal zones (>1.5°C) mostly associated with ocean currents such as the Gulf Stream, Kuroshio and Equatorial Currents. Compared to the 1982–2011 climatology, the 1991–2020 mean SSTs increased over most global areas but obvious cooling is seen in the Southern Ocean, eastern tropical South Pacific Ocean and North Atlantic warming hole. The Indo‐Pacific warm pool (IPWP) is found to have strengthened in both intensity and coverage since 1982–2011. By a count parameter criterion of ≥300 days annually with SST ≥ 28°C, the IPWP coverage increased 33% from 1982–2011 to 1991–2020. The global mean SST of 1991–2020 is warmer than that of 1982–2011, and the warming rate over 1991–2020 doubles that over 1901–2020.

Dissolved Organic Matter Fluorescence as a Tracer of Upwelling and Microbial Activities in Two Cyclonic Eddies in the Eastern Tropical North Atlantic

AbstractMesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post‐upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with parallel factor analysis, two humic‐like, and two protein‐like components. Humic‐like FDOM components correlated to optode‐based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g., temperature) and to dissolved inorganic nitrogen. The tryptophan‐like FDOM components correlated with the carbon and nitrogen content of semi‐labile DOM, phytoplankton biomass, community respiration, and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies.

Increase in Cape Verde hurricanes during Atlantic Niño

At seasonal-to-interannual timescales, Atlantic hurricane activity is greatly modulated by El Niño–Southern Oscillation and the Atlantic Meridional Mode. However, those climate modes develop predominantly in boreal winter or spring and are weaker during the Atlantic hurricane season (June–November). The leading mode of tropical Atlantic sea surface temperature (SST) variability during the Atlantic hurricane season is Atlantic Niño/Niña, which is characterized by warm/cold SST anomalies in the eastern equatorial Atlantic. However, the linkage between Atlantic Niño/Niña and hurricane activity has not been examined. Here, we use observations to show that Atlantic Niño, by strengthening the Atlantic inter-tropical convergence zone rainband, enhances African easterly wave activity and low-level cyclonic vorticity across the deep tropical eastern North Atlantic. We show that such conditions increase the likelihood of powerful hurricanes developing in the deep tropics near the Cape Verde islands, elevating the risk of major hurricanes impacting the Caribbean islands and the U.S.

Influence of the ITCZ and OMZ on the isotopic composition of suspended particulate matter in the western tropical North Atlantic

The tropical North Atlantic Ocean is a complex region where the intertropical convergence zone (ITCZ) and the oxygen minimum zone (OMZ) play important roles in the control of the physical and biogeochemical processes and affect the efficiency of the carbon pump. Changes in the composition of the particulate organic matter (POM) have been extensively studied in the eastern tropical North Atlantic and recent findings suggested less effective remineralization in the OMZ by comparison with well‑oxygenated waters. However, dynamics of the POM in the western tropical North Atlantic, including the effect of the hypoxic condition on the isotopic signature of the POM, has been poorly understood. This study uses data collected in October 2018 during the PIRATA XVIII cruise (38°W) to describe the biogeochemical variability and isotopic composition of suspended POM in the water column, with emphasis on the effect of the ITCZ and the OMZ on the POM. The ITCZ, located between 5°N and 10°N, was responsible for the low sea surface salinity and local upwelling, thus favoring the high biological productivity found in this band. Although a clear relation between δ13CPOC values and the ITCZ position was not found, low δ15NTPN values (∼2‰) and high silicate concentration observed on the surface between 4°N-6°N suggested that dust deposition and its associated enhancement of the diazotrophic activity may be important in the southernmost part of the ITCZ. Preferential remineralization of fractions rich in 13C (proteins, carbohydrates) and nitrogen compounds resulted in low δ13CPOC and high C:N values observed at depth. However, these alterations were attenuated in the OMZ, suggesting that reduced remineralization expected in waters with low oxygen concentrations may also affect the isotopic POM signature. Additionally, the minimum δ15NTPN value (∼1‰) observed in the core of the OMZ confirms that, although microbial activity may be important in this region, nitrogen-loss processes are not predominant in the western tropical Atlantic.

Northeastern Tropical Atlantic SST and Sahel Rainfall Variability

Northeastern Tropical Atlantic SST and Sahel Rainfall Variability

Eddy-enhanced primary production sustains heterotrophic microbial activities in the Eastern Tropical North Atlantic

Abstract. Mesoscale eddies modulate the ocean's physical, chemical, and biological properties. In cyclonic eddies (CEs), nutrient upwelling can stimulate primary production by phytoplankton. Yet, how this locally enhanced autotrophic production affects heterotrophy and consequently the metabolic balance between the synthesis and the consumption of dissolved organic matter (DOM) remains largely unknown. To fill this gap, we investigated the horizontal and vertical variability in auto- and heterotrophic microbial activity (biomass production and respiration) within a CE that formed off Mauritania and along the ∼ 900 km zonal corridor between Mauritania and the Cape Verde islands in the Eastern Tropical North Atlantic (ETNA). Our results show how the physical disturbances caused by the CE affected the biomass distribution of phyto- and bacterioplankton and their metabolic activities. The injection of nutrients into the sunlit surface resulted in enhanced autotrophic pico- and nanoplankton abundance and generally increased autotrophic activity as indicated by chlorophyll a (Chl a) concentration, primary production (PP), and extracellular release rates. However, the detailed eddy survey also revealed an uneven distribution of these variables with, for example, the highest Chl a concentrations and PP rates occurring near and just beyond the CE's periphery. The heterotrophic bacterial activity was similarly variable. Optode-based community respiration (CR), bacterial respiration (BR) estimates, and bacterial biomass production (BP) largely followed the trends of PP and Chl a. Thus, a submesoscale spatial mosaic of heterotrophic bacterial abundance and activities occurred within the CE that was closely related to variability in autotrophic production. Consistent with this, we found a significant positive correlation between concentrations of semi-labile dissolved organic carbon (SL-DOC; here the sum of dissolved hydrolysable amino acids and dissolved combined carbohydrates) and BR estimates. Extracellular release of carbon as indicated by primary production of dissolved organic carbon (PPDOC) was variable with depth and laterally and not always sufficient to compensate the bacterial carbon demand (BCD: BR + BP), with PPDOC accounting for between 28 % and 110 % of the BCD. Bacterial growth efficiency (BGE: BP / BCD) ranged between 1.7 % and 18.2 %. We estimated the metabolic state to establish whether the CE was a source or a sink of organic carbon. We showed that the CE carried a strong autotrophic signal in the core (PP / CR > 1). Our results suggest that submesoscale (0–10 km) processes lead to highly variable metabolic activities in both photoautotrophic and heterotrophic microorganisms. Overall, we revealed that the CEs not only trap and transport coastal nutrients and organic carbon to the open ocean but also stimulate phytoplankton growth, generating freshly produced organic matter during westward propagation. This drives heterotrophic processes and may contribute to the previously observed net heterotrophy in open Atlantic surface waters.

A Multiscale‐Model‐Based Near‐Term Prediction of Tropical Cyclone Genesis Frequency in the Northern Hemisphere

AbstractNear‐term prediction of tropical cyclone (TC) activity is of great importance to formulate the long‐term plans for mitigating the potential TC‐induced damages. Here, we introduce a multi‐timescale regression model of TC genesis frequency (TCGF), which includes contributions by three interannual modes (El Niño‐Southern Oscillation, sea surface temperature anomalies over the eastern Indian Ocean and tropical North Atlantic), two interdecadal modes (the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation), and a global warming mode. The model is shown to be able to capture well the present‐day multi‐timescale changes in TCGF in the major TC basins in the Northern Hemisphere. By combining the model and 100‐member simulations by the Max Planck Institute Earth System Model, we predict a robust increase in TCGF over the eastern North Pacific, an insignificant increase over the western North Pacific, and little change over the North Atlantic during 2020–2030 relative to 2009–2019.

Interdecadal changes in SST variability drivers in the Senegalese-upwelling: the impact of ENSO

Sea surface temperature (SST) variability in the North Eastern Tropical Atlantic has its center of action in the Senegalese–Mauritanian upwelling system, where its drivers are wind-induced ocean dynamics and air–sea thermodynamic processes. Thus, a better understanding of the local wind variations, together with their predictability, contributes to a more comprehensive assessment of the SST variability in that region. In this study, we use monthly data from two ocean reanalyses, SODA and ORAS-5, and a regional forced-ocean simulation to characterize the interannual SST variability off the Senegalese Coast in the common period 1960–2008. Local indices of the mixed layer heat budget during the major upwelling season (February–March–April) exhibit pronounced interannual to decadal variability. We demonstrate that the local interannual SST variability undergoes inter-decadal fluctuation and concomitant changes in its local and remote drivers. Off-Senegal SST variability was largely controlled by wind-induced Ekman transport during the 1960s–1970s, acting under favorable thermocline and mixed layer conditions. However, from 1980s onwards, the drastically reduced Ekman impact observed on local SSTs is associated with a deeper thermocline. This shift in the effectiveness of the dynamic mechanisms coincides with a more active ENSO teleconnection with upwelling before the 1980s. An extended SODA record reveals that the multidecadal modulator of the ENSO impact on the North-eastern Tropical Atlantic resembles the negative phase of the Atlantic Multidecadal Variability. Our results bring to light the fundamental role played by the global decadal background state in the activation of the drivers and air-sea mechanisms responsible for generating the interannual off-Senegal SST variability.

Length-weight relationships of 55 mesopelagic fishes from the eastern tropical North Atlantic: Across- and within-species variation (body shape, growth stanza, condition factor).

We present estimates of length-weight relationships (LWRs) of 55 mesopelagic fish species of 13 taxonomic families based on data collected in the eastern tropical North Atlantic (ETNA) in March/April 2015. Our data include novel records for 19 species, while for 25 species LWRs are based on the most robust sample sizes, and for 21 species they are based on the most representative size ranges available up to now. In 31 species, body lengths were within the maximum range of body lengths recorded in the area, with new records of maximum lengths for 13 species. Most values for b fell between 2.5 and 3.5 with a mean exponent b of 3.08 (median 3.12) and a mean a of 0.0172 (median 0.0113). Body shape as covariate ('elongated', 'fusiform' and 'short-deep') strongly determined the variation in log a as a function of parameter b. For the mesopelagic fish species investigated, the form factor a3.0 indicated a significant increase of median a3.0 from 'elongated' to 'fusiform' to 'short-deep' body shapes. Large variability existed in parameter b between species of the same taxonomic family. Isometric growth was indicated in only nine species, whereas a positive allometry was suggested in 22 species. Using segmented regression analysis, we investigated ontogenetic variation in LWRs in 30 species. Of these, 20 species showed a breakpoint in LWR, whereby nearly equal numbers exhibited an increase or a decrease in slope following the breakpoint. Seven out of nine species showed significant regional variation in the slope of the relationship of the relative condition factor Krel vs. body length between two or more regions of the ETNA [eastern and western part of the oxygen minimum zone (LO-E, LO-W), northern and central equatorial region (EQ-N, EQ-C)]. A conspicuous pattern was an increase in Krel with body size in the LO-E (in six out of eight species), whereas in the LO-W and the equatorial regions the majority of species showed a related decrease. These findings support the idea that growth patterns in mesopelagic fishes in tropical regions show species-specific ecological niche and life-history adaptations that are finely tuned to small-scale regional environmental conditions. Comparison of our data with those of other studies emphasises that, regarding the small adult sizes of many mesopelagic fish species, estimates of LWR parameters are strongly influenced by sampled size distributions.

Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

Community structure of mesopelagic fishes constituting sound scattering layers in the eastern tropical North Atlantic

Mesopelagic sound scattering layers (SSLs) are predominantly associated with mesopelagic fish taxa with gas-inflated swimbladders that perform active nocturnal diel vertical migration (DVM), like species of the families Myctophidae, Gonostomatidae, Sternoptychidae and Phosichthyidae. Larger-sized species with limited, partial, asynchronous and non-migratory migration pattern (non-DVM) and without gas-bearing swimbladders are presumably invisible by hydroacoustic methods operating at lower frequencies (18–38 kHz). Their vertical migration behaviour and functional role in the mesopelagic community remain largely disregarded. The present study investigated (i) the taxonomic and functional mesopelagic fish composition (migration behaviour and feeding guild as traits) of the main SSLs and (ii) regional variation in the vertical community structure related to the presence of the oxygen minimum zone (OMZ) in the eastern tropical North Atlantic. We analysed hydroacoustic and biological trawl data collected concurrently with a pelagic midwater trawl in distinct SSLs between 45 and 680 m depth by means of ordination, cluster and compositional analyses. Whereas daytime hydroacoustic backscatter profiles demonstrated higher mean Sv (38 kHz) backscatter at shallower depths at low-oxygen (LO) station 309 compared to the equatorial (EQ) station 330, night-time hydroacoustic profiles at LO stations could not be directly related to OMZ conditions. Catch abundances and biomass showed no clear regional pattern between EQ and LO stations. The analysis of trawl samples emphasised the importance of non-DVM species to the composition of tropical SSLs and further indicated regional and vertical variation in the mesopelagic fish community structure. At all stations, non-DVM species made up the largest proportions of total tow community composition (abundance and biomass) in the principal and secondary mesopelagic SSL during night-time (375–680 m) and during most daytime tows (LO: 390–555 m, EQ: 325–500 m). Our analysis suggests a more pronounced gradient structure in the vertical community composition of the LO compared to the EQ. On the one hand, shallower tows at eastern LO stations 306 and 311 were clustered with comparatively deeper tows. The eastern LO stations 306–315 had the highest numbers of non-DVM species in the principal epi- and mesopelagic SSL and the largest proportions of mesopelagic predatory species in the principal mesopelagic SSL. On the other hand, only the dominant DVM-species of the LO (i.e., Nannobrachium isaacsi, Ceratoscopelus warmingii, Notoscopelus resplendens, Lepidophanes guentheri and Hygophum macrochir) also showed consistent presence in the principal mesopelagic SSL during night-time. Moreover, our study demonstrates the usefulness of using biomass as additional parameter, when investigating functional aspects of mesopelagic community structure.

Major Element Signatures of Silicate Dust Deposited on the West African Margin: Links With Transport Patterns and Provenance Regions

AbstractMineral dust deposition characteristics are poorly constrained, even in the Northeastern Tropical Atlantic Ocean, which is immediately downwind of the Saharan desert and the largest marine repository of aeolian dust in the world. Here, we report on a 2‐year (March 2013–February 2015) time series of deposited dust on the Senegalese margin. This record enables us to document the chemical variability (major elements) of the settling Saharan dust (<30 µm silicate fraction, i.e., carbonate‐free) at a resolution varying from one week to one day, along with the deposition flux. This continuous time series reveals a greater geochemical diversity than previously reported, particularly during the dry winter‐spring season when continental trade winds sweep across vast regions of West Africa at low atmospheric levels before reaching the sampling site. By contrast, during major deposition events, which make up for most of the yearly flux, our record shows that Saharan dust chemical composition displays much narrower ranges. Trajectory analyses indicate that these relatively well‐defined chemical signatures are due to the limited number of provenance sectors involved during major deposition occurrences. The chemical characterization of the dust deposited during these events, hence, allows identifying the major element fingerprint of the related source regions, the most important one being a sizable area at the border of Algeria and Mali including the Tanezrouft desert north of the Taoudeni basin. Also, since these major events are associated with major Saharan outbreaks, they provide estimations of the prevailing elemental signatures for Saharan dust impacting the Northeastern Tropical Atlantic Ocean.

In situ observations show vertical community structure of pelagic fauna in the eastern tropical North Atlantic off Cape Verde

Distribution patterns of fragile gelatinous fauna in the open ocean remain scarcely documented. Using epi-and mesopelagic video transects in the eastern tropical North Atlantic, which features a mild but intensifying midwater oxygen minimum zone (OMZ), we established one of the first regional observations of diversity and abundance of large gelatinous zooplankton. We quantified the day and night vertical distribution of 46 taxa in relation to environmental conditions. While distribution may be driven by multiple factors, abundance peaks of individual taxa were observed in the OMZ core, both above and below the OMZ, only above, or only below the OMZ whereas some taxa did not have an obvious distribution pattern. In the eastern eropical North Atlantic, OMZ expansion in the course of global climate change may detrimentally impact taxa that avoid low oxygen concentrations (Beroe, doliolids), but favour taxa that occur in the OMZ (Lilyopsis, phaeodarians, Cydippida, Colobonema, Haliscera conica and Halitrephes) as their habitat volume might increase. While future efforts need to focus on physiology and taxonomy of pelagic fauna in the study region, our study presents biodiversity and distribution data for the regional epi- and mesopelagic zones of Cape Verde providing a regional baseline to monitor how climate change may impact the largest habitat on the planet, the deep pelagic realm.

Evidence of Stable Foraminifera Biomineralization during the Last Two Climate Cycles in the Tropical Atlantic Ocean

Planktonic foraminiferal biomineralization intensity, reflected by the weight of their shell calcite mass, affects global carbonate deposition and is known to follow climatic cycles by being increased during glacial stages and decreased during interglacial stages. Here, we measure the dissolution state and the mass of the shells of the planktonic foraminifera species Globigerina bulloides from a Tropical Eastern North Atlantic site over the last two glacial–interglacial climatic transitions, and we report no major changes in plankton calcite production with the atmospheric pCO2 variations. We attribute this consistency in foraminifera calcification to the climatic and hydrological stability of the tropical regions. However, we recorded increased shell masses midway through the penultimate deglaciation (Termination II). In order to elucidate the cause of the increased shell weights, we performed δ18O, Mg/Ca, and μCT measurements on the same shells from a number of samples surrounding this event. Compared with the lighter ones, we find that the foraminifera of increased weight are internally contaminated by sediment infilling and that their shell masses respond to local surface seawater density changes.

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernandez-Leon et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

Fluctuations of the Atlantic North Equatorial Undercurrent and Associated Changes in Oxygen Transports

AbstractAlthough the core velocity of the Atlantic North Equatorial Undercurrent (NEUC) is low (0.1−0.3 m s−1), it has been suggested to act as an important oxygen supply route towards the oxygen minimum zone in the eastern tropical North Atlantic. For the first time, the intraseasonal to interannual NEUC variability and its impact on oxygen are investigated based on shipboard and moored velocity observations around 5°N, 23°W. In contrast to previous studies that were mainly based on models or hydrographic data, we find hardly any seasonal cycle of NEUC transports in the central Atlantic. The NEUC transport variability is instead dominated by sporadic intraseasonal events. Only some of these events are associated with high oxygen levels suggesting an occasional eastward oxygen supply by NEUC transport events. Nevertheless, they likely contribute to the local oxygen maximum in the mean shipboard section along 23°W at the NEUC core position.