scholarly journals Is Oligotrophy an Equalizing Factor Driving Microplankton Species Functional Diversity Within Agulhas Rings?

2020 ◽  
Vol 7 ◽  
Author(s):  
Caio Cesar-Ribeiro ◽  
Fernanda R. Piedras ◽  
Leticia C. da Cunha ◽  
Domênica T. de Lima ◽  
Luana Q. Pinho ◽  
...  
Keyword(s):  
Functional Diversity ◽  
Functional Traits ◽  
South Atlantic ◽  
The South ◽  
Niche Complementarity ◽  
Agulhas Current ◽  
Scarce Resources ◽  
Functional Strategy ◽  
Anticyclonic Eddies

From the southwestern termination of the Agulhas current, anticyclonic eddies are emitted and drift across the South Atlantic Ocean. This study is based on a FORSA (Following Ocean Rings in the South Atlantic) oceanographic cruise, from Cape Town (South AFRica) to Arraial do Cabo (Brazil) in June 2015, during which three eddies of different ages (E1, 7 months; E3, 11 months; E5, 24 months) from the Agulhas current were sampled for microplankton identification and determination of functional traits. The stations where sampling occurred at each eddy included a control outside the eddy and three stations inside the eddy—border, midway (between the border and center), and center (identified through satellite images of sea level anomaly—SLA). Functional traits were determined based on microscopic observations and consultation of the literature. An evident decay in the Agulhas eddies toward the west was observed, and each eddy proved to be different. E1 represented a younger and more robust structure. At the same time, the other eddies, E3 and E5, were more alike with similar physical, chemical, and ecological characteristics and almost the same indices values of functional diversity, demonstrating that although their species compositions were different, the strategies used by the species were the same. The most crucial ecological trait for microplankton was nutrition mode. The microplankton contained mainly mixotrophic dinoflagellates and cyanobacteria adapted to oligotrophic conditions. The functional strategy of microplankton did not differ among the eddies and stations, with mixotrophy being the most striking trait. Therefore, the older eddies’ microplankton community fits the neutrality theory, whereby species perform similar ecological functions, and the younger eddy fits in the niche complementarity. Even with the species composition being different in each eddy and/or within the same eddy, the functional strategy was the same, with scarce resources and species selected that best use any source of nutrients or use evolutionary advantages to live in an oligotrophic environment.

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 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 Eddy characteristics in the South Indian Ocean as inferred from surface drifter

2014 ◽  
Vol 11 (6) ◽  
pp. 2879-2905
Author(s):  
Shaojun Zheng ◽  
Yan Du ◽  
Jiaxun Li ◽  
Xuhua Cheng
Keyword(s):  
Indian Ocean ◽  
The South ◽  
South Indian Ocean ◽  
Agulhas Current ◽  
South Indian ◽  
Surface Drifter ◽  
Eddy Characteristics ◽  
Large Eddy ◽  
Surface Drifters ◽  
Anticyclonic Eddies

Abstract. Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. 19252 eddies are identified with 60% anticyclonic eddies. For the submesoscale eddies (radius r < 10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, number of anticyclonic and cyclonic eddies with radius r ≥ 10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distribution. Eddies with radius r ≥ 100 km mainly appear in a band along 25° S, in Mozambique Channel, and Agulhas Current, characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and then minimum in February.

scholarly journals Eddy characteristics in the South Indian Ocean as inferred from surface drifters

Ocean Science ◽  
2015 ◽  
Vol 11 (3) ◽  
pp. 361-371 ◽  
Author(s):  
S. Zheng ◽  
Y. Du ◽  
J. Li ◽  
X. Cheng
Keyword(s):  
Indian Ocean ◽  
The South ◽  
South Indian Ocean ◽  
Agulhas Current ◽  
Leeuwin Current ◽  
South Indian ◽  
Eddy Characteristics ◽  
Large Eddy ◽  
Surface Drifters ◽  
Anticyclonic Eddies

Abstract. Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. A total of 19 252 eddies are identified, 60% of them anticyclonic eddies. For the submesoscale eddies (radius r<10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, the number of anticyclonic and cyclonic eddies with radius r≥10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distributions. Eddies with radius r≥100 km mainly appear in the Leeuwin Current, a band along 25° S, Mozambique Channel, and Agulhas Current, areas characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and minimum in February.

scholarly journals Eddy-Train Encounters with a Continental Boundary: A South Atlantic Case Study

2012 ◽  
Vol 42 (9) ◽  
pp. 1548-1565 ◽  
Author(s):  
José L. L. Azevedo ◽  
Doron Nof ◽  
Mauricio M. Mata
Keyword(s):  
Vertical Wall ◽  
South Atlantic ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Agulhas Current ◽  
The North ◽  
Anticyclonic Eddies ◽  
Good Agreement

Abstract Satellite altimetry suggests that large anticyclonic eddies (rings) originating from the Agulhas Current retroflection occasionally make their way across the entire South Atlantic Ocean. What happens when these rings encounter a western boundary current? In this work, interactions between a “train” of nonlinear lens-like eddies and a Southern Hemisphere continental boundary are investigated analytically and numerically on a β plane. The train of eddies is modeled as a steady double-frontal zonal current with the same vorticity and transport as the eddies themselves. The continental boundary is represented by a vertical wall, which is purely meridional in one case and is tilted with respect to the north in another case. It is demonstrated analytically that the eddy–wall encounter produces an equatorward flow parallel to the continental wall, thus suggesting a weakening of the transport of the associated (poleward flowing) western boundary current upstream of the encounter zone and unchanged transport downstream. A large stationary eddy is established in the contact zone because its β-induced force is necessary to balance the other forces along the wall. The size of this eddy is directly proportional to the transport of the eddy train and the meridional tilt of the wall. These scenarios are in good agreement with results obtained numerically using an isopycnal Bleck and Boudra model.

scholarly journals Determination of the South Atlantic Anomaly from DAMPE data

2017 ◽  
Author(s):  
Wei Jiang ◽  
Xiang Li ◽  
Jingjing Zang ◽  
Chuan Yue ◽  
Yuanpeng Wang ◽  
...  
Keyword(s):  
South Atlantic ◽  
The South ◽  
South Atlantic Anomaly

scholarly journals Retroflection from a double-slanted coastline: a model for the Agulhas leakage variability

Ocean Science ◽  
2010 ◽  
Vol 6 (4) ◽  
pp. 997-1011 ◽  
Author(s):  
V. Zharkov ◽  
D. Nof ◽  
W. Weijer
Keyword(s):  
South African ◽  
Mass Flux ◽  
South Atlantic ◽  
Eastern Coast ◽  
The South ◽  
Slant Angle ◽  
Agulhas Current ◽  
Low Leakage ◽  
Nonlinear Analytical Model ◽  
Kink Model

Abstract. The Agulhas leakage to the South Atlantic exhibits a strong anti-correlation with the mass flux of the Agulhas Current. When the Agulhas retroflection is in its normal position near Cape Agulhas, leakage is relatively high and the nearby South African coastal slant (angle of derivation from zonal) is very small and relatively invariant alongshore. During periods of strong incoming flux (low leakage), the retroflection shifts upstream to Port Elizabeth or East London, where the coastline shape has a "kink", i.e., the slant changes abruptly from small on the west side, to large (about 55°) on the east side. Here, we show that the variability of rings shedding and anti-correlation between Agulhas mass flux and leakage to the South Atlantic may be attributed to this kink. To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, a zonal western section and a strongly slanted eastern section. The principal difference between this and the model of a straight slanted coast (discussed in our earlier papers) is that, here, free purely westward propagation of eddies along the zonal coastline section is allowed. This introduces an interesting situation in which strong slant of the coast east of the kink prohibits the formation and shedding of rings, while the almost zonal coastal orientation west of the kink encourages shedding. Therefore, the kink "locks" the position of the retroflection, forcing it to occur just downstream of the kink. Rings are necessarily shed from the retroflection area in our kinked model, regardless of the degree of eastern coast slant. In contrast, a no-kink model with a coastline of intermediate slant indicates that shedding is almost completely arrested by that slant. We suggest that the observed difference in ring-shedding intensity during times of normal retroflection position and times when the retroflection is shifted eastward is due to the change in the retroflection location with respect to the kink. When the incoming flux detaches from the coast north of the kink, ring transport is small; when the flux detaches south of the kink, transport is large. Simple process-oriented numerical simulations are in fair agreement with our analytical results.

Latitudinal gradient of copepod functional diversity in the South Atlantic Ocean

2021 ◽  
pp. 102710
Author(s):  
Érica Caroline Becker ◽  
Maria Grazia Mazzocchi ◽  
Luis Carlos Pinto de Macedo-Soares ◽  
Manoela Costa Brandão ◽  
Andrea Santarosa Freire
Keyword(s):  
Atlantic Ocean ◽  
Functional Diversity ◽  
Latitudinal Gradient ◽  
South Atlantic ◽  
South Atlantic Ocean ◽  
The South

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.

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