Subsurface Fine‐Scale Patterns in an Anticyclonic Eddy Off Cap‐Vert Peninsula Observed From Glider Measurements

Water masses and oceanic eddy regulation of larval fish assemblages along the Cape Verde Frontal Zone

An analysis of mixing in the frontal zone of South and North Atlantic Central Water off North-West Africa

This perspective paper aims at presenting the current knowledge on the processes of ocean acidification (OA) and coastal acidification (CA) in the Brazilian coastal ocean. We define and differentiate the processes of OA and CA: the first driven by the actual global increase of atmospheric carbon dioxide (CO2); the second driven by a combination of ocean uptake of atmospheric CO2 and other local/regional chemical additions or subtractions in aquatic ecosystems at the land-ocean interface. Regarding OA, we have centered our analysis on the data available for the main water masses along the Brazilian coast: South Atlantic Central Water (SACW), Tropical Water (TW), and Coastal Water (CW). The few data available for the shallow coastal waters (< 200 m depth) of the continental shelf reveal an increase in the anthropogenic component of the total dissolved inorganic carbon (DIC) pool in the SACW, with a decline in the ocean pH (over two decades; 1993-2013), and in the saturation state of calcium carbonate (CaCO3) minerals. We could not find OA trends for TW and CW because no data was available. Overall, the colder water masses (SACW, Plata Plume) have lower buffering capacity and simulations show that will potentially experience earlier negative OA impacts than the warmer waters masses (TW, Amazon Plume). Regarding CA, we have identified some local/regional studies investigating the carbonate chemistry in nearshore/estuarine ecosystems, particularly on the quantification of sources and sinks of CO2 , and determining short-term variabilities. Apparently, spreading coastal eutrophication in Brazil can enhances or reduces the process of OA, depending on the net ecosystem metabolism in combination with other chemical alterations. However, we could not find medium-long term acidification trends due to the limited data. There is a limited capacity to produce long time-series of carbonate chemistry parameters in key ecosystems and regions along the Brazilian coast. This lack of past information hinders and impairs the scientific community for identifying potential patterns of acidification along the Brazilian coast. We call for an urgent action in Brazil, with emphasis on the establishment of moored buoys/stations and/or scientific programs in the long term with continuous, real-time measurements of the main carbonate chemistry parameters. Keywords: ocean acidification, coastal acidification, coastal eutrophication, continental shelf, Southwestern Atlantic Ocean

Abstract. The organic carbon fluxes mediated by planktonic communities in two cyclonic eddies (CEs) and two anticyclonic eddies (AEs) at the Canary Eddy Corridor were studied and compared with the dynamics in two far-field (FF) stations located outside the eddies. We observed favorable conditions and signs for upwelling at the center of CEs and for downwelling and mixing at the centers of AEs. CEs were characterized by a higher concentration of nutrients and the highest concentration of chlorophyll a (chl a), associated with the highest abundance of microphytoplankton and diatoms. AEs displayed concentrations of chl a values and nutrients similar to those at the FF stations, except for the highest ammonium concentration occurring at AE and a very low concentration of phosphorus at FF stations. AEs were transient systems characterized by an increasing abundance of picophytoplankton and heterotrophic bacteria. While primary production was similar between the systems, the production of dissolved organic carbon (PDOC) was significantly higher in the AEs. Phytoplankton cell mortality was lowest in the CEs, and we found higher cell mortality rates at AE than at FF stations, despite similar chl a concentration. Environmental changes in the AEs have been significantly prejudicial to phytoplankton as indicated by higher phytoplankton cell mortality (60% of diatoms cells were dead) and higher cell lysis rates. The adverse conditions for phytoplankton associated with the early-stage anticyclonic systems, mainly triggered by active downwelling, resulted in higher cell mortality, forcing photosynthesized carbon to fuel the dissolved pool.

Abstract. A subsurface low oxygen zone is located in the eastern tropical North Atlantic Ocean (ETNA) in the upper ocean with the core of the hypoxic (O2 ≦60 µmol kg−1) oxygen minimum zone (OMZ) at 400 to 500 m depth. The subsurface circulation in the OMZ region is derived from observations and data assimilation results. Measurements in the ETNA of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at ∼ 8° N, 23° W (at ∼ 330 m depth) in November–December 2008, in November–December 2009 and October–November 2010 show the circulation in the upper part of the OMZ with spreading to the east in the North Equatorial Countercurrent (NECC) region and northwestward around the Guinea Dome. Three floats equipped with oxygen sensors deployed at ∼ 8° N, 23° W with parking depths at 330, 350 and 400 m depths were used to estimate velocity along the float trajectory at the surface and at the parking depth. At the 350 m park depth north of 9° N a cyclonic northwestward flow across the OMZ was observed. The northward drift of a float into the upper OMZ and a stronger cyclonic flow around the Guinea Dome seem to be connected to a strong Atlantic Meridional Mode (AMM) event in 2009. A near-surface cyclonic circulation cell east of the Cape Verde Islands reaches down into the OMZ layer. The circulation of the upper OMZ mirrors the near-surface circulation. Oxygen measurements from the cruises used here, as well as from other recent cruises up to the year 2014, confirm the continuous deoxygenation trend in the upper OMZ since the 1960s near the Guinea Dome. The three floats deployed with the tracer show spreading paths consistent with the overall observed tracer spreading. Oxygen sensors on the floats remained well calibrated for more than 20 months, and so the oxygen profiles can be used to investigate mesoscale eddy signatures. Mesoscale eddies may modify the oxygen distribution in OMZs. However, in general eddies are less energetic in the ETNA south of the Cape Verde Islands compared to similar latitudes in the eastern tropical South Pacific.

The physical processes driving the genesis of surface- and subsurface-intensified cyclonic and anticyclonic eddies originating from the coastal current system of the Mauritanian upwelling region are investigated using a high-resolution (∼1.5 km) configuration of GFDL’s Modular Ocean Model. Estimating an energy budget for the boundary current reveals a baroclinically unstable state during its intensification phase in boreal summer and which is driving eddy generation within the near-coastal region. The mean poleward coastal flow’s interaction with the sloping topography induces enhanced anticyclonic vorticity, with potential vorticity close to zero generated in the bottom boundary layer. Flow separation at sharp topographic bends intensifies the anticyclonic vorticity, and submesoscale structures of low PV coalesce to form anticyclonic vortices. A combination of offshore Ekman transport and horizontal advection determined the amount of South Atlantic Central Water (SACW) in an anticyclonic eddy. A vortex with a relatively dense and low PV core will form an anticyclonic mode water eddy, which will subduct along isopycnals while propagating offshore and hence be shielded from surface buoyancy forcing. Less contribution of dense SACW promotes the generation of surface anticyclonic eddies as the core is composed of a lighter water mass, which causes the eddy to stay closer to the surface and hence be exposed to surface buoyancy forcing. Simulated cyclonic eddies are formed between the rotational flow of an offshore anticyclonic vortex and a poleward flowing boundary current, with eddy potential energy being the dominant source of eddy kinetic energy. All three types of eddies play a key role in the exchange between the Mauritanian coastal currents system and the adjacent eastern boundary shadow zone region.

Event Abstract Back to Event Seasonal patterns of cephalopod paralarvae distribution in relation to oceanographic features off South Brazil Bight (SBB) Carolina C. Araújo1* and Maria D. Gasalla1 1 Instituto Oceanográfico, Brazil Cephalopods play an important role in marine food webs worldwide, supporting industrial and artisanal fisheries. However, the knowledge about early life stages of cephalopods and their mechanisms responsible for distribution, recruitment and variability in the South Brazil Bight (SBB) is still poorly understood. In SBB occurs waters from the mixing between Tropical Water (TW), warm and saline (T> 20 ° C and S> 36.40) transported in the surface layer of the Brazil Current (BC), the South Atlantic Central Water (SACW), relatively cold (T <20 ° C and S <36.4), transported CB in the lower layer, and the Coastal Water (CW), resulting from mixing continental freshwater discharge to the continental shelf, and the lower salinity (S <34) waters of the SBB. This area is influenced by local and remote winds, tides, baroclinic pressure gradients due to buoyancy advections and variability mesoscale of the BC. The BC produces a flow on the platform southward and away from the coast during the summer and a flow toward the north and the coast during the winter. Seasonal variability of BC causes a different behavior of water bodies, due to formation of vortices and eddies induced by changes in wind patterns. In summer, the Ekman transport generated by the dominance of northeast winds and training vortices in BC, moving surface water, promoting the intrusion and resurgence of SACW, which is rich in nutrients and responsible for the increase primary productivity. The penetration SACW during the summer stratification causes the formation of a strong thermocline, on the other hand, the water column becomes homogeneous during winter due to its retreat toward the slope. At Cabo Frio region where the continental shelf is narrowest the coastline direction changes from NE-SW to approximately E-W that favoring the upwelling phenomenon. Plankton samples obtained during the Project “Dinâmica do Ecossistema de Plataforma da Região Oeste do Atlântico Sul” (DEPROAS) from two surveys using bongo and multi plankton sample (MPS) nets, both with 333 µm mesh were examined in order to identify cephalopod paralarvae composition, distribution and abundance on fourteen transects perpendiculars from the coastline during summer (bongo: n=49; MPS: n=58) and winter (bongo: n=47; MPS: n=47) off SBB between Cabo de São Tomé and São Sebastião Island (22°-24°S) up to 200 m in relation to oceanographic features, such as water masses and mesoscale features. The samples were collected oblique (from near bottom to surface) and stratified (10 to 20 meters intervals) hauls at ~ 2 knots during 10 minutes. The mean water temperature for all the study area, from surface to 200 m depth, was significantly higher in the summer (~20.3°C) than in the winter (~20.3°C). During the summer the water column was strongly stratified, while during the winter the water column was relatively homogeneous. The T-S diagrams show the presence of three water masses in the area: Tropical Water (TW), South Atlantic Central Water (SACW) and Coastal Water (CW). The Tropical water mass occupied the layer between the surface and 100 m depth, mainly in the oceanic region. In the deep layer (> 100m depth) values below 18°C and salinity between 35 and 36 were registered, suggesting the presence of the SACW mass. The SACW was also found at the surface in the coastal region, Cabo Frio and indicating a coastal upwelling event. In the southern coastal area the temperature was about 20°C and salinity 34 during both summer and winter, indicating the presence of a CW mass. Overall 337 cephalopod paralarvae were identified representing seven families: Ancistrocheiridae (Ancistrocheirus sp), Argonautidae (Argonauta sp), Enoploteuthidae (Abralia sp and Abraliopsis sp), Loliginidae (Doryteuthis spp), Octopodidae (Octopus spp), Ommastrephidae (Illex sp, Stenoteuthis sp and Ommastrephes sp) and Sepiolidae (Heteroteuthis sp). A total of 199 (summer) and 68 (winter) of cephalopod paralarvae were collected with bongo net. During summer Argonautidae (Argonauta) was most frequent (44%) followed by Enoploteuthidae (18%), associated to stratified water columns formed with CW in the surface layers and SACW in deeper layers. During winter Octopodidae (Octopus) was most frequent (35,3%) followed by Enoploteuthidae (16,2%), associated to TW in the surface and SACW in deeper layers. Loliginidae paralarvae was found nearshore (until 100m isobath) associated to stratified water column formed by CW in surface layers and SACW in the deeper layers. Enoploteuthidae, Argonautidae and Ommastrephidae occurred mainly in the middle shelf (upper 100m isobath) related to waters masses (TW and SACW) transported by BC. A total of 50 (summer) and 20 (winter) of cephalopod paralarvae were collected by MPS net. Cephalopod paralarvae were most frequent until 40 m depth associated to thermocline. Enoploteuthidae was most frequent upper 20 m, indicating relation to warmer waters (TW) transported by BC. Overall, the density of cephalopod paralarvae correlates with oceanographic processes during summer when productivity is locally enhanced. The present study provides new information on cephalopods early life stage patterns in the area that will be useful to investigate recruitment patterns and fisheries assessment of cephalopods populations. Acknowledgements The authors are grateful to the responsible for the Biological Collection “Prof. E. F. Nonato” (Institute of Oceanography, University of São Paulo) for plankton samples. Keywords: cephalopods, paralarvae, Brazil Current, thermocline, distribution patterns Conference: XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016. Presentation Type: Oral Presentation Topic: 3. OCEANOGRAPHY, MODELLING AND ECOSYSTEM DYNAMICS Citation: Araújo CC and Gasalla MD (2016). Seasonal patterns of cephalopod paralarvae distribution in relation to oceanographic features off South Brazil Bight (SBB). Front. Mar. Sci. Conference Abstract: XIX Iberian Symposium on Marine Biology Studies. doi: 10.3389/conf.FMARS.2016.05.00088 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 14 May 2016; Published Online: 02 Sep 2016. * Correspondence: Ms. Carolina C Araújo, Instituto Oceanográfico, São Paulo, São Paulo, 05508-090, Brazil, carolcosta27@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. 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Abstract. Localized open-ocean low-oxygen “dead zones” in the eastern tropical North Atlantic are recently discovered ocean features that can develop in dynamically isolated water masses within cyclonic eddies (CE) and anticyclonic mode-water eddies (ACME). Analysis of a comprehensive oxygen dataset obtained from gliders, moorings, research vessels and Argo floats reveals that “dead-zone” eddies are found in surprisingly high numbers and in a large area from about 4 to 22° N, from the shelf at the eastern boundary to 38° W. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg−1 could be associated with 27 independent eddies (10 CEs; 17 ACMEs) over a period of 10 years. Lowest oxygen concentrations in CEs are less than 10 µmol kg−1 while in ACMEs even suboxic (&lt; 1 µmol kg−1) levels are observed. The oxygen minimum in the eddies is located at shallow depth from 50 to 150 m with a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly in the core depth range (50 and 150 m) for CEs (ACMEs) is −38 (−79) µmol kg−1. North of 12° N, the oxygen-depleted eddies carry anomalously low-salinity water of South Atlantic origin from the eastern boundary upwelling region into the open ocean. Here water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary. In contrast, the oxygen-depleted eddies south of 12° N carry weak hydrographic anomalies in their cores and seem to be generated in the open ocean away from the boundary. In both regions a decrease in oxygen from east to west is identified supporting the en-route creation of the low-oxygen core through a combination of high productivity in the eddy surface waters and an isolation of the eddy cores with respect to lateral oxygen supply. Indeed, eddies of both types feature a cold sea surface temperature anomaly and enhanced chlorophyll concentrations in their center. The low-oxygen core depth in the eddies aligns with the depth of the shallow oxygen minimum zone of the eastern tropical North Atlantic. Averaged over the whole area an oxygen reduction of 7 µmol kg−1 in the depth range of 50 to 150 m (peak reduction is 16 µmol kg−1 at 100 m depth) can be associated with the dispersion of the eddies. Thus the locally increased oxygen consumption within the eddy cores enhances the total oxygen consumption in the open eastern tropical North Atlantic Ocean and seems to be an contributor to the formation of the shallow oxygen minimum zone.

A shallow mesoscale anticyclonic eddy, observed south of the Canary Islands with satellite altimetry, has been intensively studied with multiparametric sampling. Hydrographic data from a CTD installed on an undulating Nu‐shuttle platform reveal the presence of a mesoscale anticyclonic eddy of ∼125 km diameter. The difference in sea level anomaly (SLA) between the interior and the edge of the eddy, as determined from altimetry, is ∼15 cm, which compares well with the maximum dynamic height differences as inferred using a very shallow reference level (130 m). Further, the associated surface geostrophic velocities, of about 35 cm s−1 in the northeast and southwest edges of the eddy, are in good agreement with direct velocity measurements from the ship. Deep rosette‐CTD casts confirm that the structure is a shallow eddy extending no deeper than 250 m before the fusion with another anticyclone. The SLA‐tendency (temporal rate of change of sea surface height) indicates a clear northwestward migration during the two first weeks of November 2008. Applying an eddy SSH‐based tracker, the eddy's velocity propagation is estimated as 4 km d−1. Use of the QG‐Omega equation diagnoses maximum downward/upward velocities of about ±2 m d−1. The instability of the Canary coastal jet appears to be the mechanism responsible for the generation of the shallow anticyclonic eddy.

Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic oxygen minimum zone

The sea level changes due to steric contributions have become more relevant in the past decades because oceans store the largest part of the heat from anthropogenic greenhouse gas emissions. Recent studies showed evidence that the sea level variation is mainly attributed to the first 2000 m of the water column. Therefore, our purpose was to quantify how much of the individual water masses variation is correlated to the sea level changes. To quantify the layer thickness variability of the Tropical Surface Water (TSW), South Atlantic Central Water (SACW), Antarctic Intermediate Water (AAIW), Circumpolar Water (CDW), the North Atlantic (NADW), and the lower layers (LOWER), we used WOA18 data, altimetric satellites data, and the outputs from ECCO and HYCOM models. The layer thickness had seasonal and interannual variability, but only the TSW, SACW, and AAIW were consistent between the models. The main process observed on ECCO analysis was an alternating expansion and contraction between consecutive water masses, while on HYCOM, only the LOWER expanded and the other water masses contracted. The TSW (ECCO) showed the largest contribution (44% to 90%) to the sea level, followed by the SACW (0% to 44%), AAIW (0% to 44%), UCDW (~11%), UNADW(~11%), and LOWER (5%) between 0ºS and 40ºS. The TSW was the only water mass whose relative explained variance decreased southward, while the other five water masses increased southward. The SACW and AAIW had a higher explained variance (11% - 88%) between 40ºS to 50ºS, and the LOWER had the highest value (&gt;44%) between 50ºS and 55ºS. HYCOM results were similar to ECCO, except that the TSW explained variance was higher (44% to 90%) only between 0ºS to 25ºS and SACW (44% to 90%) was between 25ºS to 45ºS. The correlation between the altimeter sea level and ECCO showed higher values (&gt;70%) from 0ºS 10ºS to 40ºW--15ºE. This indicates that the hypothesis of this work is valid for this region. Even though HYCOM presented a correlation higher than 75% for most of the basin, the layer thickness variability showed inconsistencies with previous studies.

Anticyclonic mesoscale eddies are often observed in the Balearic Sea (BS) toward the end of summer and autumn. In some years, these eddies become strong and persistent, modifying the local water mass properties. In this study, we analyze two of the most significant recent long‐lived anticyclonic eddies, occurring in 2010 and 2017, using data from a high‐resolution circulation model, altimetry and satellite‐borne sea surface temperature observations. These eddies lasted around 2 and 4 months, respectively, with a radius varying between 40 and 75 km. The generation and intensification mechanisms of these long‐lived anticyclonic eddies are studied by means of (a) energy conversion terms associated with eddy‐mean flow interaction and (b) model sensitivity tests. Results show that these eddies were formed and intensified through mixed barotropic and baroclinic instabilities. The former are produced under the action of intense northwesterly (NW) winds. The latter are related to the existence of an intense summer thermal front between the BS and the Gulf of Lion, and to northward inflows of relatively lower salinity waters. Both the wind events and the presence of the thermal front are necessary for the formation of the eddies. The intensification process varied between both events. While in 2010 it was driven by significant salinity gradients produced by northwards inflows, in 2017 it was produced by additional intense NW winds. Both long‐lived anticyclonic eddies created long‐lasting surface temperature anomalies up to 2.5°C, which have characteristics of local marine heatwaves.

Intermediate layer water masses are defined according to temperature‐salinity relationships derived from conductivity‐temperature‐depth (CTD) observations measured during four 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) hydrographic surveys. The intermediate layer, bounded by density surfaces of sigma theta 26.00 and 27.65 (approximately 150 and 1300 m deep, respectively), is conveniently divided into upper and lower layers by the relatively low salinity Antarctic Intermediate Water (AAIW), which is centered at sigma theta 27.25 (approximately 700 m deep). Approximately 60 ± 5% of the region's waters are traced to a southern hemisphere origin, indicating the importance of AAIW in the western tropical Atlantic's water mass structure. The southern source water masses, South Atlantic Central Water and AAIW, enter the WESTRAX region (west of 44°W and between the equator and 9°N) as part of the subthermocline North Brazil Current. Depending on the season, all or part of these southern waters retroflect anticyclonically through the region and flow eastward into the North Equatorial Undercurrent. The primary northern source water mass, North Atlantic Central Water, enters the northeastern corner of the WESTRAX region as part of a cyclonic branch of the North Equatorial Current (NEC) and converges with the southern water. This meeting produces mixture water masses which make up 45 ± 4% of the region in volume and are predominantly of a southern nature. The patterns of the mixture water masses which fill the areas between the source water masses suggest the importance of lateral mixing in this ocean region. Further, some of the mixture water in the upper layer appears to be part of the NEC, suggesting southern water recirculation in the tropical Atlantic gyre. A time dependent water mass box model of advective and mixing transports is used to suggest that lateral mixing dominates vertical mixing by a ratio of approximately 10 to 1. Typical box model results for the fall–winter 1990–1991 period indicate that 13 Sv of mixture water masses are produced through mixing (a sum of 9 Sv and 4 Sv from southern and northern source water masses, respectively), while a net 17 Sv of mixture water masses are exported from the region.

A deliberate tracer release experiment in 2008–2010 was used to study diapycnal mixing in the tropical northeastern Atlantic. The tracer (CF3SF5) was injected on the isopycnal surface σΘ = 26.88 kg m−3, which corresponds to about 330 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the vertically and laterally expanding tracer patch. The mean diapycnal mixing estimate over the entire region occupied by the tracer and the period of 30 months was found to be (1.19 ± 0.18) × 10−5 m2 s−1, or, alternatively, (3.07 ± 0.58) × 10−11 (kg m−3)2 s−1as computed from the advection‐diffusion equation in isopycnal coordinates with the thickness‐weighted averaging. The latter method is preferable in the regions of different stratification for it yields local diapycnal mixing estimates varying less with stratification than their Cartesian coordinate counterparts. Results of this study are comparable to the results of the North Atlantic tracer release experiment (NATRE). However, the internal wave‐wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, the diapycnal diffusivity found in this study is higher than parameterized by the low latitude of the site (4°N–12°N).

Anticyclonic mesoscale eddies are known to trap and modulate near-inertial kinetic energy (NIKE); however, the spatial distribution of NIKE within the eddy core and periphery, as well as the mechanisms driving its energy cascade to smaller scales, remains inadequately understood. This study analyzed the evolution of NIKE in anticyclonic eddies using satellite altimetry and field observations from four mooring arrays. By extracting near-inertial oscillations (NIOs) and subharmonic wave kinetic energy across mooring stations during the same period, we characterized the spatial structure of NIKE within the eddy field. The results revealed that NIKE was concentrated in the eddy core, where strong NIOs (peak velocity ~0.23 m/s) persisted for ~7 days, with energy primarily distributed at depths of 200–400 m and propagating inward from the periphery. Subharmonic waves fD1 generated by interactions between NIOs and diurnal tides highlighted the role of the vertical nonlinear term in energy transfer. A further analysis indicated that under vorticity confinement, NIKE accumulated in the core of the eddy and dissipated through shear instability and nonlinear wave interactions. The migrating anticyclonic eddy thus acted as a localized energy source, driving mixing and energy dissipation in the ocean interior.