scholarly journals Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

Ocean Science ◽  
2021 ◽  
Vol 17 (4) ◽  
pp. 1067-1080
Author(s):  
Christina Schmidt ◽  
Franziska U. Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
Global Climate ◽  
Horizontal Resolution ◽  
South Atlantic ◽  
Comparison Study ◽  
The South ◽  
Agulhas Current ◽  
Cape Basin ◽  
Total Transport ◽  
The Indian Ocean ◽  
Ice Model

Abstract. The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 (1/20∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.

scholarly journals Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

2021 ◽  
Author(s):  
Christina Schmidt ◽  
Franziska U. Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
Global Climate ◽  
Horizontal Resolution ◽  
South Atlantic ◽  
Comparison Study ◽  
The South ◽  
Agulhas Current ◽  
Cape Basin ◽  
Total Transport ◽  
The Indian Ocean ◽  
Ice Model

Abstract. The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean-sea ice model INALT20 (1/20° horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasising the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport is similar across these estimates. During the transit from the Agulhas Current at 32° S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air-sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.

scholarly journals Agulhas ring injection into the South Atlantic during glacials and interglacials

Ocean Science ◽  
2008 ◽  
Vol 4 (3) ◽  
pp. 223-237 ◽  
Author(s):  
V. Zharkov ◽  
D. Nof
Keyword(s):  
Critical Angle ◽  
South Atlantic ◽  
African Continent ◽  
The South ◽  
Wind Stress Curl ◽  
Agulhas Current ◽  
The North ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
The Last Glacial

Abstract. Recent proxies suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. Using nonlinear analytical solutions for eddy shedding, we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in satisfactory agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical.

scholarly journals Characteristics and robustness of Agulhas Leakage estimates: an inter-comparison study of Lagrangian methods

2021 ◽  
Author(s):  
Christina Schmidt ◽  
Franziska Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Clear Trend ◽  
Agulhas Current ◽  
Ocean General Circulation Models ◽  
Cape Basin ◽  
Meridional Overturning ◽  
The Indian Ocean

<p>The exchange of water between the Indian Ocean and South Atlantic with their different thermohaline properties via Agulhas Leakage is important for the meridional overturning circulation. Agulhas Leakage as well as the output of ocean general circulation models in general can be analysed using a Lagrangian approach with a variety of different tools available. Here, Agulhas Leakage is estimated with both the newly developed tool Parcels and the well established tool Ariane, and different designs of the Lagrangian experiment are analysed. In a hindcast simulation with the eddy-rich ocean sea-ice model INALT20 (1/20° horizontal resolution) under the new JRA55-do forcing, Agulhas Leakage increases from the early 1960s to mid 1980s, but there is no clear trend afterwards, which is in contrast to earlier studies using hindcast simulations under the CORE forcing. During the transit from the Agulhas Current at 32°S to the Cape Basin, a cooling and freshening of Agulhas Leakage waters occurs especially in the western part of the Retroflection, resulting in a density increase as the thermal effect dominates. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas Leakage is simulated equally with the Lagrangian tools Ariane and Parcels, emphasising the robustness of our method.</p>

scholarly journals Agulhas ring injection into the South Atlantic during glacials and interglacials

2008 ◽  
Vol 5 (1) ◽  
pp. 39-75 ◽  
Author(s):  
V. Zharkov ◽  
D. Nof
Keyword(s):  
Critical Angle ◽  
South Atlantic ◽  
The South ◽  
Wind Stress Curl ◽  
Agulhas Current ◽  
The North ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Good Agreement ◽  
The Last Glacial

Abstract. Recent proxies analysis suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. Using nonlinear analytical solutions for eddy shedding we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in very good agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical.

scholarly journals Meridional changes in the South Atlantic Subtropical Gyre during Heinrich Stadials

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.

New species of Neotanais Beddard, 1886 (Crustacea, Tanaidacea) from the deep sea of the tropical and southern East Atlantic Ocean

Zootaxa ◽  
2009 ◽  
Vol 1992 (1) ◽  
pp. 20-36 ◽  
Author(s):  
SIMON WEIGMANN ◽  
JÜRGEN GUERRERO-KOMMRITZ
Keyword(s):  
New Species ◽  
Atlantic Ocean ◽  
Deep Sea ◽  
South Atlantic ◽  
South Atlantic Ocean ◽  
Full Description ◽  
The South ◽  
East Atlantic ◽  
Cape Basin

As part of the sampling efforts during the DIVA-II expedition several Tanaidacea of the genus Neotanais were captured in the Guinea and the Cape Basin in the tropical and southern East Atlantic Ocean. Two different species were sampled, Neotanais rotermundiae sp. n. from the Guinea and Neotanais guskei sp. n. from the Cape Basin. The distribution of both species is limited to these basins. A full description for both species is presented. Neotanais guskei sp. n. is the largest Neotanais reported for the South Atlantic Ocean.

scholarly journals The Role of Interocean Exchanges on Decadal Variations of the Meridional Heat Transport in the South Atlantic

2011 ◽  
Vol 41 (8) ◽  
pp. 1498-1511 ◽  
Author(s):  
Shenfu Dong ◽  
Silvia Garzoli ◽  
Molly Baringer
Keyword(s):  
Indian Ocean ◽  
Heat Transport ◽  
South Atlantic ◽  
Similar Response ◽  
The South ◽  
Meridional Heat Transport ◽  
Interior Region ◽  
The Pacific ◽  
The Indian Ocean

Abstract The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 106 m3 s−1) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.

scholarly journals Radium-228-derived ocean mixing and trace element inputs in the South Atlantic

2020 ◽  
Author(s):  
Yu-Te Hsieh ◽  
Walter Geibert ◽  
E. Malcolm S. Woodward ◽  
Neil J. Wyatt ◽  
Maeve C. Lohan ◽  
...  
Keyword(s):  
Trace Elements ◽  
Continental Shelf ◽  
Trace Element ◽  
Vertical Mixing ◽  
South Atlantic ◽  
Open Ocean ◽  
The South ◽  
Horizontal Mixing ◽  
Cape Basin ◽  
Argentine Basin

Abstract. Trace elements play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40° S is a prominent region of high productivity and a transition zone between the nitrate-depleted Subtropical Gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40° S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx = 1.7 ± 1.4 (106 cm2 s−1) and an integrated advection velocity w = 0.6 ± 0.3 cm s−1. In the Cape Basin, horizontal mixing is Kx = 2.7 ± 0.8 (107 cm2 s−1) and vertical mixing Kz = 1.0–1.5 cm2 s−1 in the upper 600 m layer. Three different approaches (228Ra-diffusion, 228Ra-advection and 228Ra/TE-ratio) have been applied to estimate the dissolved trace-element fluxes from shelf to open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine margin to be: 3.8–22 (× 103) nmol Co m−2 d−1, 7.9–20 (× 104) nmol Fe m−2 d−1 and 2.7–6.5 (× 104) nmol Zn m−2 d−1. Off-shelf fluxes from the Cape margin are: 4.3–6.2 (× 103) nmol Co m−2 d−1, 1.2–3.1 (× 104) nmol Fe m−2 d−1 and 0.9–1.2 (× 104) nmol Zn m−2 d−1. On average, at 40° S in the Atlantic, vertical mixing supplies 0.4–1.2 nmol Co m−2 d−1, 3.6–11 nmol Fe m−2 d−1, and 13–16 nmol Zn m−2 d−1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40° S Atlantic. It is insufficient, however, to provide the trace elements removed by biological uptake. Other inputs (e.g. particulate, or from winter deep-mixing) are required to balance the trace element budgets in this region.

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