Subtropical South Atlantic Research Articles

Spatial distribution of living coccolithophores along an east- west transect in the subtropical South Atlantic.

Spatial distribution of living coccolithophores along an east- west transect in the subtropical South Atlantic.

South Atlantic interocean exchange as the trigger for the Bølling warm event

The North Atlantic Ocean underwent an abrupt temperature increase of 9 °C at high latitudes within a couple of decades during the transition from Heinrich event 1 (H1) to the Bolling warm event, but the mechanism responsible for this warming remains uncertain. Here we address this issue, presenting high-resolution last deglaciation planktic and benthic foraminiferal records of temperature and oxygen isotopic composition of seawater (δ 18 Osw) for the subtropical South Atlantic. We identify a warming of ~6.5 °C and an increase in δ 18 O sw of 1.2‰ at the permanent thermocline during the transition, and a simultaneous warming of ~3.5 °C with no significant change in δ 18 O sw at intermediate depths. Most of the warming can be explained by tilting the South Atlantic east-west isopycnals from a flattened toward a steepened position associated with a collapsed (H1) and strong (Bolling) Atlantic meridional overturning circulation (AMOC). However, this zonal seesaw explains an increase of just 0.3‰ in permanent thermocline δ 18 O sw . Considering that δ 18 O sw at the South Atlantic permanent thermocline is strongly influenced by the inflow of salty Indian Ocean upper waters, we suggest that a strengthening in the Agulhas leakage took place at the transition from H1 to the Bolling, and was responsible for the change in δ 18 O sw recorded in our site. Our records high-light the important role played by Indian-Atlantic interocean exchange as the trigger for the resumption of the AMOC and the Bolling warm event.

Antarctic ozone depletion causes an intensification of the Southern Ocean super‐gyre circulation

Recent climate trends over the Southern Hemisphere (SH) summer feature a strengthening of the circumpolar westerly and a weakening of the midlatitude westerly extending from the stratosphere to Earth's surface. Much of the change is attributable to Antarctic ozone depletion. However, the consequential ocean circulation changes are unknown. Here I demonstrate that the observed surface wind changes have forced a southward shift and spin‐up of the super gyre, which links the subtropical South Pacific, Indian and Atlantic Ocean circulation, advecting more warm water southward. The circulation change includes a strengthening of the East Australian Current (EAC) flow passing through the Tasman Sea. The southward shift may be responsible for the observed unusually large warming in the SH midlatitude ocean and may contribute to the reported range extension to the south of many marine species in the South West Pacific.

Amino acid uptake of Prochlorococcus spp. in surface waters across the South Atlantic Subtropical Front

To test the hypothesis that surface-living Prochlorococcus spp. (Pro) cyanobacteria metabolism declines towards the boundaries of its natural habitat, a latitudinal transect of surface waters was sampled from the centre of the Southern Atlantic Gyre (SAG, 20 to 35° S) to the South Subtropical Frontal Zone (SSFZ, 35 to 45° S). Along this transect, amino acid uptake rates of Pro, Synechococcus spp. (Syn) and an average bacterioplankton cell were determined using 35S-methionine precursor and flow-cytometry sorting, with methionine uptake rate as an index of cellular metabolic activity. Methionine and possibly other amino acids were a very minor nutrient source for Syn, while their contribution to Pro production was significant. Contrary to expectations, the mean methionine uptake rate per Pro cell in the SSFZ was about 3 times higher than in the SAG. The uptake rates per unit Pro biomass were equal to or higher than that of an average bacterioplankton cell in both the SAG and SSFZ. About 20 and 5% of total bacterioplankton consumption of amino acids could be assigned to Pro in the SAG and SSFZ, respectively. Methionine and leucine turnover rates were 3.5 and 3 times higher in the SSFZ than in the SAG, respectively. These results suggest that Pro remained highly metabolically active and acquired more methionine at its habitat boundaries, despite higher rates of bacterioplankton activity and therefore greater competition, as well as exposure to deep water mixing, low light and low temperature conditions.

Simulations of the Atmospheric Response to South Atlantic Sea Surface Temperature Anomalies

Abstract The sensitivity of the atmospheric circulation to sea surface temperature (SST) anomalies in the tropical and subtropical South Atlantic Ocean is studied by means of simulations with an atmospheric general circulation model (GCM). Two types of prescribed SST anomalies are used, motivated by previous analyses of data. The first occurs during austral summers in association with a strengthening of the South Atlantic convergence zone (SACZ) and consists of cold SST anomalies over the subtropical South Atlantic. The second is the leading seasonally varying empirical orthogonal function of SST, consisting of warm basin-scale anomalies with maximum amplitude in the subtropics during January–March and at the equator in June. An ensemble of about 10 seasonal simulations is made using each type of anomaly, focusing on the January–March period in the first case and the January–June seasonal evolution in the second. During January–March both experiments yield a statistically significant baroclinic response o...

Deciphering the paleoceanographic significance of Early Oligocene Braarudosphaera chalks in the South Atlantic

The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic models within the context of stable isotope (δ18O, δ13C) data measured from bulk-sediment samples and well-preserved foraminifera. Two closely-spaced Braarudosphaera layers from a lower Oligocene (foram Subzone P21a, 29.4–28.5 Ma) section drilled in the southeastern Atlantic (DSDP Site 363) are investigated. Maximum durations for the blooms that deposited the lower and upper Braarudosphaera layers are estimated to be 1.1 and 2.2 k.y., respectively. Bulk-sediment samples enriched in braarudosphaerid carbonate exhibit pronounced δ18O increases on the order of 0.6–1.0‰ which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal δ18O values, suggesting that these blooms may recur on glacial/interglacial timescales. This same pair of braarudosphaerid layers also occurs as a couplet bundled with prolonged (∼6.7 k.y.) thermocline cooling, evidence that these stratigraphically distinct deposits may represent a ‘split signal’ for a single paleoceanographic/paleoclimatic event. Subsumed within this episode of subsurface cooling are two short-lived, negative excursions (∼0.5‰) in the δ13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline δ18O and δ13C gradients were reduced during the braarudosphaerid blooms, a hallmark signature for strengthened upwelling. Both braarudosphaerid layers are marked by transient divergences in the stable isotopic signals of two shallow-dwelling species of planktonic foraminifera. These transient δ18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer species. If so, then braarudosphaerid depositional events may represent ‘subsurface blooms’ that took place within the lower parts of the euphotic zone. Alternatively, these transient δ18O offsets may reflect periods of pronounced seasonality, with braarudosphaerid blooms occurring during spring upwelling. The recurrence of Braarudosphaera blooms on both sides of the South Atlantic is believed to reflect rhythmic changes in the vigor and configuration of gyre circulation. We speculate that the termination of Braarudosphaera blooms in the South Atlantic near the end of the Early Oligocene may be related to paleoceanographic change caused by the crossing of a critical threshold in the tectonic opening of the Drake Passage and the development of the Antarctic Circum-Polar Current.

Sources of the Equatorial Undercurrent in the Atlantic in a High-Resolution Ocean Model

Abstract The ventilation of the Equatorial Undercurrent (EUC) in the Atlantic is investigated using data from a high-resolution ocean model. Overturning streamfunctions, subduction patterns, and pathways are determined from Eulerian and Lagrangian mean transports. The role of high-frequency variability is highlighted. The meridional overturning circulation shows that the EUC is mainly ventilated from the south. This is seen because transports induced by high-frequency variability compensate for tropical cells that are associated with downwelling at 5° poleward of the equator and upwelling at the equator. The impact of high-frequency variability is large, especially to the north of the equator. Lagrangian trajectory analysis shows that the main subduction sites that ventilate the EUC are located along the South Equatorial Current: one region in the southwest and one in the central subtropical South Atlantic. From these sites water masses transfer toward the western boundary. Following the boundary they fin...

Rossby wave and ocean color: The cells uplifting hypothesis in the South Atlantic Subtropical Convergence Zone

Rossby waves signatures on filtered ocean color data are detected in the subtropical convergence zone of the South Atlantic ocean. We investigate whether these chlorophyll anomalies can be accounted for by the uplifting mechanism of phytoplanktonic cells associated with the passage of a Rossby wave. We consider vertical chlorophyll profiles exhibiting a subsurface chlorophyll maximum typical of the South Atlantic Subtropical Convergence Province. Chlorophyll remotely‐sensed concentrations resulting from an academic uplifting of the three chlorophyll profiles are reconstructed with a radiative transfer model. Amplitude of the chlorophyll enrichments found with this hypothesis (0.06 mg chl.m−3) compares well with propagative ocean color anomalies detected in the SeaWiFS data. However, other processes such as the Rototiller effect [Siegel, 2001] or advection of chlorophyll gradients could also play a non negligible role.

High variability of primary production in oligotrophic waters of the Atlantic Ocean: uncoupling from phytoplankton biomass and size structure

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 257:1-11 (2003) - doi:10.3354/meps257001 High variability of primary production in oligotrophic waters of the Atlantic Ocean: uncoupling from phytoplankton biomass and size structure Emilio Marañón1,*, Michael J. Behrenfeld2, Natalia González3, Beatriz Mouriño1, Mikhail V. Zubkov4 1Departamento de Ecología y Biología Animal, Facultad de Ciencias, Universidad de Vigo, 36200 Vigo, Spain 2National Aeronautics and Space Administration, Goddard Space Flight Center, Code 971, Building 33, Greenbelt, Maryland 20771, USA 3Departamento de Matemáticas, Física Aplicada y Ciencias de la Naturaleza, Universidad Rey Juan Carlos, 28933 Móstoles, Madrid, Spain 4George Deacon Division for Ocean Processes, Southampton Oceanography Centre, University of Southampton, Southampton SO14 3ZH, United Kingdom *Email: em@uvigo.es ABSTRACT: The oligotrophic waters of the Subtropical Gyres cover >60% of the total ocean surface and contribute >30% of the global marine carbon fixation. Despite apparently uniform growth conditions over broad areas, primary production in these regions exhibits a remarkable degree of variability. In this study of 34 stations in the North and South Atlantic Subtropical Gyres, we found a 20 fold variation (from 18 to 362 mgC m-2 d-1) in water-column-integrated primary production rate (∨PP), while chlorophyll biomass only varied by a factor of 3. The changes in productivity were not associated with variations in incident surface irradiance, chlorophyll concentration, phytoplankton C biomass or phytoplankton size structure. The rate of nutrient supply to the euphotic layer, as estimated from variations in the depth of nitracline, appeared as the most relevant environmental factor in explaining the observed variability in ∨PP. We found significant changes in the composition of the picophytoplankton community across the range of measured productivities. The relative biomass contribution of Synechococcus spp. and the picoeukaryotes tended to increase with increasing ∨PP, whereas the opposite was true for Prochlorococcus spp. Across the wide range of measured primary productivity rates, the persistent dominance of picophytoplankton indicates that the microbial loop and the microbial food web continued to be the most important trophic pathways. Our observations of the oligotrophic ocean reflect a dynamic ecosystem where the microbial community responds to environmental forcing with significant changes in biological rates rather than trophic organization. KEY WORDS: Primary production · Chlorophyll · Picoplankton · Size structure · Subtropical Gyres · Atlantic Ocean Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 257. Online publication date: August 07, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Monitoring Termination II by environmental magnetic records from subtropical South Atlantic sediments

Termination II, the transition from oxygen isotope substages 6.2 to 5.5 at around 130 ka, is the penultimate drastic change from extreme glacial to interglacial climatic conditions, invoking shifts in terrigenous and biogenous influx to the deep ocean. Environmental magnetic methods are applied to reveal these shifts in an ocean‐spanning transect of the subtropical South Atlantic. Magnetic mineralogy, grain size, and coercivity reflect intensified eolian inflow of volcanogenic material from the Andes due to enhanced continental aridity and wind intensity during substage 6.2. An African dust supply is only observable in continental vicinity. On the Santos Plateau, magnetic attributes indicate fluvial influx of eroded flood basalts. Layers of reductive magnetic mineral diagenesis were identified only at the westernmost sites and appear to reflect paleoredox boundaries related to enhanced Corg deposition at the termination and during stage 5.5. In all unaltered sections, stage 5.5 is magnetically similar to the Holocene.

Antarctic Bottom Water Flow through the Hunter Channel*

The Rio Grande Rise acts as a natural barrier for the equatorward flow of Antarctic Bottom Water in the subtropical South Atlantic. In addition to the Vema Channel, the Hunter Channel cuts through this obstacle and offers a separate route for bottom-water import into the southern Brazil Basin. On the occasion of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment (WOCE), the expected deep flow through the Hunter Channel was directly observed for the first time by an array of moored current meters and thermistor chains from December 1992 to May 1994. Main results are (i) the Hunter Channel is, in fact, a conduit for bottom-water flow into the Brazil Basin. Our new mean transport from moored current meters [2.92 (±1.24) × 106 m3 s−1] is significantly higher than earlier estimates that were based on geostrophic calculations. (ii) During the WOCE observational period a tendency toward increased bottom-water temperatures was observed. This observation from the Hunter Channel is consistent with findings from the Vema Channel. (iii) The overflow through the Hunter Channel is highly variable and puts in perspective earlier synoptic geostrophic transport estimates.

Behaviour of the centres of action above the Atlantic since 1881. Part I: Characteristics of seasonal and interannual variability

Based on generated time series of the central pressure, latitudinal and longitudinal position of the Iceland Low, Azores High, Inter Tropical Convergence Zone and South Atlantic Subtropical High, the behaviour of these atmospheric centres of action since 1881 is examined with regard to the intra- and interannual variations, persistence of anomalies, linear trends and interactions. The critical analysis of these time series as well as those of the pressure gradients between the neighbouring centres suggests that the existing non-periodic fluctuations of the surface atmospheric circulation on the time-scale of one or more decades suppress the long-term changes. These fluctuations are similar to the ‘internal’ variations of the climate system, recognized in the climate simulations of coupled ocean–atmosphere models. © 1998 Royal Meteorological Society.

Agulhas eddies invade the south Atlantic: Evidence From Geosat altimeter and shipboard conductivity‐temperature‐depth survey

There is mounting evidence that Indian Ocean water penetrates into the South Atlantic. A sequence of Geosat altimeter data from November 1986 to November 1987 discloses numerous transient anticyclonic mesoscale features within the South Atlantic subtropical gyre. They migrate from the Agulhas Retroflection towards the northwest, then after stalling, cross the Walvis Ridge, to continue drifting to the west, at 5 to 8 cm s−1. The “family” of these features can be traced to the western South Atlantic in the latitude range of 20°–30°S. The 1987 population of eddies in the South Atlantic and their mean drift velocity suggests a production rate of about five eddies per year. As much as 15×106 m3 s−1 of Indian Ocean water may pass into the subtropical South Atlantic by this process. One of the larger eddies disclosed by the Geosat altimeter was crossed by the Discovery during a conductivity‐temperature‐depth survey of the Benguela Current in May 1987. The dynamic height of the sea surface relative to 3000 dbar reveals a 30 dyn cm crest, in excellent agreement with the Geosat data. The eddy displays a distinct sign of Indian Ocean stratification: an isohaline layer in the 15°–19°C interval. The colder South Atlantic water cannot achieve these characteristics by local interaction with the atmosphere. However, there is evidence for some surface modification: surface salinity values are higher than those found within the Agulhas Current. This is explained by enhanced evaporation driven by the warm temperatures of the eddy below the colder atmosphere. This process would eventually remove the Indian Ocean signature, converting the water within the eddy to typical South Atlantic thermocline water.

The General Circulation and Meridional Heat Transport of the Subtropical South Atlantic Determined by Inverse Methods

The circulation and meridional heat transport of the subtropical South Atlantic Ocean are determined through the application of the inverse method of Wunsch (1978) to hydrographic data from the IGY and METEOR expeditions. Meridional circulation results of the two data sets agree on a northward mass transport of about 20 million metric tons/sec for waters above the North Atlantic Deep Water (NADW), and a comparable southward transport of deep waters. Additional gross features held in common are the Benguela, South Equatorial and North Brazilian Coastal currents' northward transport of the Surface Water, and the deflection of the southward-flowing NADW from the South American Coast into the mid ocean by a seamount chain near 20 deg S. Total heat transport is equatorward, with a magnitude of 0.8 X 10 to the 15th W near 30 deg S and indistinguishable from zero near 8 deg S.