Changes in the South Atlantic Subtropical Gyre circulation from the 20th into the 21st century

Interocean waters that are carried northward through South Atlantic surface boundary currents get meridionally split between two large-scale systems when meeting the South American coast at the western subtropical portion of the basin. This distribution of the zonal flow along the coast is investigated during the Last Millennium, when natural forcing was key to establish climate variability. Of particular interest are the changes between the contrasting periods of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The investigation is conducted with the simulation results from the Community Earth System Model Last Millennium Ensemble (CESM-LME). It is found that the subtropical South Atlantic circulation pattern differs substantially between these natural climatic extremes, especially at the northern boundary of the subtropical gyre, where the westward-flowing southern branch of the South Equatorial Current (sSEC) bifurcates off the South American coast, originating the equatorward-flowing North Brazil Undercurrent (NBUC) and the poleward Brazil Current (BC). It is shown that during the MCA, a weaker anti-cyclonic subtropical gyre circulation took place (inferred from decreased southern sSEC and BC transports), while the equatorward transport of the Meridional Overturning Circulation return flow was increased (intensified northern sSEC and NBUC). The opposite scenario occurs during the LIA: a more vigorous subtropical gyre circulation with decreased northward transport.

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Meridional changes in the South Atlantic Subtropical Gyre during Heinrich Stadials

Scientific Reports ◽

10.1038/s41598-021-88817-0 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Tainã M. L. Pinho ◽

Cristiano M. Chiessi ◽

Rodrigo C. Portilho-Ramos ◽

Marília C. Campos ◽

Stefano Crivellari ◽

...

Keyword(s):

Global Climate ◽

South Atlantic ◽

Subtropical Gyre ◽

Climate System ◽

The South ◽

Foraminiferal Species ◽

Ocean Gyres ◽

Long Term Monitoring ◽

Term Monitoring

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

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Analysis of the position and strength of westerlies and trades with implications for Agulhas leakage and South Benguela upwelling

Earth System Dynamics ◽

10.5194/esd-10-847-2019 ◽

2019 ◽

Vol 10 (4) ◽

pp. 847-858 ◽

Cited By ~ 1

Author(s):

Nele Tim ◽

Eduardo Zorita ◽

Kay-Christian Emeis ◽

Franziska U. Schwarzkopf ◽

Arne Biastoch ◽

...

Keyword(s):

Indian Ocean ◽

21St Century ◽

Emission Scenario ◽

Saline Water ◽

South Atlantic ◽

Data Sets ◽

The South ◽

Trade Winds ◽

Benguela Upwelling ◽

Poleward Shift

Abstract. The westerlies and trade winds over the South Atlantic and Indian Ocean are important drivers of the regional oceanography around southern Africa, including features such as the Agulhas Current, the Agulhas leakage, and the Benguela upwelling. Agulhas leakage constitutes a fraction of warm and saline water transport from the Indian Ocean into the South Atlantic. The leakage is stronger during intensified westerlies. Here, we analyze the wind stress of different observational and modeled atmospheric data sets (covering the last 2 millennia, the recent decades, and the 21st century) with regard to the intensity and position of the southeasterly trades and the westerlies. The analysis reveals that variations of both wind systems go hand in hand and that a poleward shift of the westerlies and trades and an intensification of westerlies took place during the recent decades. Furthermore, upwelling in South Benguela is slightly intensified when trades are shifted poleward. Projections for strength and position of the westerlies in the 21st century depend on assumed CO2 emissions and on their effect relative to the ozone forcing. In the strongest emission scenario (RCP8.5) the simulations show a further southward displacement, whereas in the weakest emission scenario (RCP2.6) a northward shift is modeled, possibly due to the effect of ozone recovery dominating the effect of anthropogenic greenhouse forcing. We conclude that the Agulhas leakage has intensified during the last decades and is projected to increase if greenhouse gas emissions are not reduced. This will have a small impact on Benguela upwelling strength and may also have consequences for water mass characteristics in the upwelling region. An increased contribution of Agulhas water to the upwelling water masses will import more preformed nutrients and oxygen into the upwelling region.

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Variability and trends of the South Atlantic subtropical gyre

Journal of Geophysical Research Oceans ◽

10.1029/2020jc016405 ◽

2020 ◽

Author(s):

K. L. Drouin ◽

M. S. Lozier ◽

W. E. Johns

Keyword(s):

South Atlantic ◽

Subtropical Gyre ◽

The South

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Sea level variability and semiannual Rossby waves in the South Atlantic subtropical gyre

Journal of Geophysical Research Atmospheres ◽

10.1029/93jc00456 ◽

1993 ◽

Vol 98 (C7) ◽

pp. 12315 ◽

Cited By ~ 21

Author(s):

P.-Y. Le Traon ◽

J.-F. Minster

Keyword(s):

Sea Level ◽

Rossby Waves ◽

South Atlantic ◽

Subtropical Gyre ◽

The South ◽

Sea Level Variability

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Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system

Biogeosciences ◽

10.5194/bg-11-885-2014 ◽

2014 ◽

Vol 11 (3) ◽

pp. 885-897 ◽

Cited By ~ 14

Author(s):

A. Flohr ◽

A. K. van der Plas ◽

K.-C. Emeis ◽

V. Mohrholz ◽

T. Rixen

Keyword(s):

N2 Fixation ◽

South Atlantic ◽

Subtropical Gyre ◽

Total Alkalinity ◽

Nutrient Concentrations ◽

The South ◽

Upwelling System ◽

Benguela Upwelling ◽

Bottom Waters ◽

The Impact

Abstract. On a global scale the ratio of fixed nitrogen (N) and phosphate (P) is characterized by a deficit of N with regard to the classical Redfield ratio of N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanic oxygen minimum zones. The northern Benguela upwelling system (NBUS) is known for losses of N and the accumulation of P in sub- and anoxic bottom waters and sediments of the Namibian shelf resulting in low N : P ratios in the water column. To study the impact of the N : P anomalies on the regional carbon cycle and their consequences for the export of nutrients from the NBUS into the oligotrophic subtropical gyre of the South Atlantic, we measured dissolved inorganic carbon (CT), total alkalinity (AT), oxygen (O2) and nutrient concentrations in February 2011. The results indicate increased P concentrations over the Namibian shelf due to P efflux from sediments resulting in a C : N : P : -O2 ratio of 106 : 16 : 1.6 : 138. N reduction further increase C : N and reduce N : P ratios in those regions where O2 concentrations in bottom waters are < 20 μmol kg−1. However, off the shelf along the continental margin, the mean C : N : P : -O2 ratio is again close to the Redfield stoichiometry. Additional nutrient data measured during two cruises in 2008 and 2009 imply that the amount of excess P, which is created in the bottom waters on the shelf, and its export into the subtropical gyre after upwelling varies through time. The results further reveal an inter-annual variability of excess N within the South Atlantic Central Water (SACW) that flows from the north into the NBUS, with highest N values observed in 2008. It is postulated that the N excess in SACW occurred due to the impact of remineralized organic matter produced by N2 fixation and that the magnitude of excess P formation and its export is governed by inputs of excess N along with SACW flowing into the NBUS. Factors controlling N2 fixation north of the BUS need to be addressed in future studies to better understand the role of the NBUS as a P source and N sink in the coupled C : N : P cycles.

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A Lagrangian view of the transfer of Southern waters to the South Atlantic Ocean

10.5194/egusphere-egu21-9448 ◽

2021 ◽

Author(s):

Anna Olivé Abelló ◽

Josep L. Pelegrí ◽

Ignasi Vallès-Casanova

Keyword(s):

South Africa ◽

Surface Water ◽

Drake Passage ◽

South Atlantic ◽

Subtropical Gyre ◽

The South ◽

The North ◽

Brazil Current ◽

North Brazil ◽

North Brazil Current

<p>The Atlantic Meridional Overturning Circulation (AMOC), a key component of the Earth's climate system, is sustained through the northward transport of Southern Ocean waters to high latitudes. This returning limb of the AMOC consists largely of relatively cold waters entering from the Pacific Ocean through the Drake Passage, what is commonly referred to as cold-water route. Here, we explore the pathways and transit times of these Antarctic waters that are incorporated to the South Atlantic, with special attention to their recirculation in the subtropical gyre and their escape northward through the North Brazil Current. For this purpose, we use daily values of the climatological GLORYS12v1 velocity field, as obtained using data for 2002-2018 and track the trajectories with the help of the OceanParcels software. We trace the particles transiting through four sections in the Southern and South Atlantic Oceans: 64&#176;W and 27&#176;E, crossing entire Antarctic Circumpolar Current (ACC) through the Drake Passage and off South Africa, respectively; 32&#176;S, from the African coast out to 5&#176;S, sampling the eastern boundary current system; and 21&#176;S, from the American coast out to 30&#176;W, sampling the North Brazil Current.</p><p>Particles are released daily in the Drake Passage down to 1800 m during one full year, its spatial distribution and number being proportional to the transport crossing each vertical portion of the section. This represents an annual-mean of 116.3 Sv entering the Atlantic sector through the Drake Passage, split into 13.3 Sv for surface (Subantarctic Surface Water, SASW, and Subantarctic Mode Water, SAMW), 40.2 Sv for intermediate (Antarctic Surface Water, AASW, and Antarctic Intermediate Water, AAIW) and 62.8 Sv for deep (Upper Circumpolar Deep Water, UCDW) water masses. The particles are then tracked forward, with a one-day resolution, during 20 years. The simulation shows that about 83% of the SASW/SAMW transport follow the ACC past South Africa while the remaining 17% are incorporated to the subtropical gyre. Among the latter, only 13% veer northward and cross the 21&#176;S section. Regarding the intermediate waters, AASW/AAIW, 93% of transport follows the ACC, and 7% join the subtropical gyre. Finally, for the UCDW transport, which remains part of ACC, about 97% follow eastward as the ACC and only 3% drift cross the 32&#176;S section, and only 4% of the latter reach through the 21&#176;S section. The median times for the Drake Passage water particles to get to the 27&#176;E, 32&#176;S and 21&#176;S sections are: 1.7, 2.1 and 5.7 yr for the SASW/SAMW; 2.3, 5.3 and 6.5 yr for the AASW/AAIW; and 3.3, 6.0 and 11.7 yr for the UCDW, respectively. Long tails in the age distributions reflect a high degree of recirculation, being remarkable the high presence of mesoscale eddies around 32&#176;S over Cape Basin.</p>

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