scholarly journals A study of the variability in the Benguela Current volume transport

Ocean Science ◽  
2018 ◽  
Vol 14 (2) ◽  
pp. 273-283 ◽  
Author(s):  
Sudip Majumder ◽  
Claudia Schmid
Keyword(s):  
Energy Density ◽  
Three Dimensional ◽  
Atlantic Water ◽  
Volume Transport ◽  
South Atlantic ◽  
The South ◽  
Meridional Transport ◽  
Data Set ◽  
Argo Floats ◽  
Benguela Current

Abstract. The Benguela Current forms the eastern limb of the subtropical gyre in the South Atlantic and transports a blend of relatively fresh and cool Atlantic water and relatively warm and salty Indian Ocean water northwestward. Therefore, it plays an important role not only for the local freshwater and heat budgets but for the overall meridional heat and freshwater transport in the South Atlantic. Historically, the Benguela Current region is relatively data sparse, especially with respect to long-term velocity observations. A new three-dimensional data set of the horizontal velocity in the upper 2000 m that covers the years 1993 to 2015 is used to analyze the variability in the Benguela Current. This data set was derived using observations from Argo floats, satellite sea surface height, and wind fields. Since Argo floats do not cover regions shallower than 1000 m, the data set has gaps inshore. The main features of the horizontal circulation observed in this data set are in good agreement with those from earlier studies based on limited observations. Therefore, it can be used for a more detailed study of the flow pattern as well as the variability in the circulation in this region. It is found that the mean meridional transport in the upper 800 m between the continental shelf of Africa and 3∘ E, decreases from 23 ± 3 Sv (1 Sv = 106 m3 s−1) at 31∘ S to 11 ± 3 Sv at 28∘ S. In terms of variability, the 23-year long time series at 30 and 35∘ S reveals phases with large energy densities at periods of 3 to 7 months, which can be attributed to the occurrence of Agulhas rings in this region. The prevalence of Agulhas rings is also behind the fact that the energy density at 35∘ S at the annual period is smaller than at 30∘ S because the former latitude is closer to Agulhas Retroflection and therefore more likely to be impacted by the Agulhas rings. In agreement with this, the energy density associated with mesoscale variability at 30∘ S is weaker than at 35∘ S. With respect to the forcing, the Sverdrup balance and the observed transport at 30∘ S exhibit a strong correlation of 0.7. No significant correlation between these parameters is found at 35∘ S.

scholarly journals A Study of the Variability of the Benguela Current

2017 ◽  
Author(s):  
Sudip Majumder ◽  
Claudia Schmid
Keyword(s):  
Energy Density ◽  
Three Dimensional ◽  
Atlantic Water ◽  
South Atlantic ◽  
The South ◽  
Wind Fields ◽  
Meridional Transport ◽  
Data Set ◽  
Benguela Current ◽  
Heat Budgets

Abstract. The Benguela Current forms the eastern limb of the subtropical gyre in the South Atlantic and transports a blend of relatively fresh and cool Atlantic water as well as relatively warm and salty Indian Ocean water northward. Therefore, it plays an important role not only for the local freshwater and heat budgets but for the overall meridional heat and freshwater transports in the South Atlantic. Historically, the Benguela Current region is relatively data sparse, especially with respect to long-term observations. A new three dimensional data set of the horizontal velocity in the upper 2000 m that covers the years 1993 to 2015 is used to analyze the variability of the Benguela Current. This data set was derived using observations from Argo floats, satellite sea surface height and wind fields. The main features of the horizontal circulation observed in this data set are in good agreement with those from earlier observations based on more limited data sets. Therefore, it can be used for a more detailed study the flow pattern as well as the variability of the circulation in this region. It is found that the mean meridional transport in the upper 800 m between the continental shelf of Africa and 3° E, decreases from 23 ± 3 Sv at 31° S to 11 ± 3 Sv at 28° S. In terms of variability, the 23-year long timeseries at 30° S and 35° S reveal phases with large energy densities at periods of 3 to 7 months, which can be attributed to the occurrence of Agulhas rings in this region. The prevalence of these rings is also behind the fact that the energy density at 35° S at the annual period is smaller than at 30° S, because the former latitude is closer to Agulhas retroflection and therefore more likely to be impacted by the Agulhas rings. In agreement with this, the energy density associated with mesoscale variability at 30° S is weaker than at 35° S. With respect to the forcing, the significant correlation between the Sverdrup balance derived from the wind stress and the observed transports at 30° S is 0.7. No significant correlation between these parameters was found at 35° S.

Temporal Variability of the Meridional Overturning Cells in the South Atlantic

2020 ◽  
Author(s):  
Marion Kersalé ◽  
Christopher Meinen ◽  
Renellys Perez ◽  
Matthieu Le Hénaff ◽  
Daniel Valla ◽  
...  
Keyword(s):  
Spatial Scales ◽  
Global Scale ◽  
Volume Transport ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
The South ◽  
Data Set ◽  
Air Temperatures ◽  
Coastal Sea ◽  
Meridional Overturning

<p>Variations in the Meridional Overturning Circulation (MOC) are known to have important impacts on global scale climate phenomena including precipitation patterns, surface air temperatures, coastal sea level, and extreme weather. The MOC flow structure in the South Atlantic is thought to control the stability of the entire global MOC system. Given this importance, significant resources have been invested on observing the MOC in the South Atlantic over the past decade. Multiple years of full-depth daily observations from moored instruments at 34.5°S are used to calculate the meridional transports near the western and eastern boundaries, as well as the basin-wide interior transports, via geostrophic methods. These transport estimates are combined with Ekman transports derived from satellite wind products to yield daily estimates of the total meridional transports. Analysis of the MOC volume transport using all available moored instruments from 2013 to 2017 allows us to quantify for the first time the daily volume transport of both the upper and abyssal overturning cells at 34.5°S. The structure of these flows is characterized in unprecedented detail; no statistically significant trend is detectable in either cell. Abyssal-cell transport variability is largely independent of the transport variability in the upper-cell. Analysis of this new data set is crucial for improving our understanding of the temporal and spatial scales of variability that governs MOC related flows, and for disentangling their respective roles in modulating its overall variability.</p>

scholarly journals Transport variability of the Brazil Current from observations and a data assimilation model

Ocean Science ◽  
2018 ◽  
Vol 14 (3) ◽  
pp. 417-436 ◽  
Author(s):  
Claudia Schmid ◽  
Sudip Majumder
Keyword(s):  
Interannual Variability ◽  
Southern Oscillation ◽  
Three Dimensional ◽  
Austral Summer ◽  
Southern Annular Mode ◽  
South Atlantic ◽  
The South ◽  
Meridional Transport ◽  
Large Power ◽  
Brazil Current

Abstract. The Brazil Current transports from observations and the Hybrid Coordinate Model (HYCOM) model are analyzed to improve our understanding of the current's structure and variability. A time series of the observed transport is derived from a three-dimensional field of the velocity in the South Atlantic covering the years 1993 to 2015 (hereinafter called Argo & SSH). The mean transports of the Brazil Current increases from 3.8 ± 2.2 Sv (1 Sv is 106 m3 s−1) at 25∘ S to 13.9 ± 2.6 Sv at 32∘ S, which corresponds to a mean slope of 1.4 ± 0.4 Sv per degree. Transport estimates derived from HYCOM fields are somewhat higher (5.2 ± 2.7 and 18.7 ± 7.1 Sv at 25 and 32∘ S, respectively) than those from Argo & SSH, but these differences are small when compared with the standard deviations. Overall, the observed latitude dependence of the transport of the Brazil Current is in agreement with the wind-driven circulation in the super gyre of the subtropical South Atlantic. A mean annual cycle with highest (lowest) transports in austral summer (winter) is found to exist at selected latitudes (24, 35, and 38∘ S). The significance of this signal shrinks with increasing latitude (both in Argo & SSH and HYCOM), mainly due to mesoscale and interannual variability. Both Argo & SSH, as well as HYCOM, reveal interannual variability at 24 and 35∘ S that results in relatively large power at periods of 2 years or more in wavelet spectra. It is found that the interannual variability at 24∘ S is correlated with the South Atlantic Subtropical Dipole Mode (SASD), the Southern Annular Mode (SAM), and the Niño 3.4 index. Similarly, correlations between SAM and the Brazil Current transport are also found at 35∘ S. Further investigation of the variability reveals that the first and second mode of a coupled empirical orthogonal function of the meridional transport and the sea level pressure explain 36 and 15 % of the covariance, respectively. Overall, the results indicate that SAM, SASD, and El Niño–Southern Oscillation have an influence on the transport of the Brazil Current.

scholarly journals Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

2018 ◽  
Author(s):  
Svein Østerhus ◽  
Rebecca Woodgate ◽  
Héðinn Valdimarsson ◽  
Bill Turrell ◽  
Laura de Steur ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Global Ocean ◽  
Bering Strait ◽  
Data Set ◽  
Considerable Uncertainty ◽  
Denmark Strait

Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Into this region, water enters through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow) and then modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat, salt, and other substances into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, has the observational coverage become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993–2015. The total AM-import (oceanic inflows plus freshwater) is found to be 9.1 ± 0.7 Sv (1 Sv = 106 m3 s−1) and has a seasonal variation of amplitude close to 1 Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remaining two thirds leaving as overflow. The overflow is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater, but is still ~ 2/3rds from modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s and this allows us to estimate trends for the AM-exchanges as a whole. At the 95 % level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM-inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM-exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the Meridional Overturning Circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM-exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflows through the Denmark Strait, the overflow across the Iceland-Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

scholarly journals Variability of carbon monoxide and carbon dioxide apparent quantum yield spectra in three coastal estuaries of the South Atlantic Bight

2012 ◽  
Vol 9 (6) ◽  
pp. 6947-6985 ◽  
Author(s):  
H. E. Reader ◽  
W. L. Miller
Keyword(s):  
Carbon Dioxide ◽  
Carbon Monoxide ◽  
Quantum Yield ◽  
South Atlantic ◽  
South Atlantic Bight ◽  
The South ◽  
Apparent Quantum Yield ◽  
Data Set ◽  
Direct Production ◽  
Coastal System

Abstract. The photochemical oxidation of oceanic dissolved organic carbon (DOC) to carbon monoxide (CO) and carbon dioxide (CO2) has been estimated to be a significant process with global photoproduction transforming petagrams of DOC to inorganic carbon annually. To further quantify the importance of these two photoproducts in coastal DOC cycling, 38 paired apparent quantum yield (AQY) spectra for CO and CO2 were determined at three locations along the coast of Georgia, USA over the course of one year. The AQY spectra for CO2 were considerably more varied than CO. CO AQY spectra exhibited a seasonal shift in spectrally integrated (260 nm–490 nm) AQY from higher efficiencies in the fall to less efficient photoproduction in the summer. While full-spectrum photoproduction rates for both products showed positive correlation with pre-irradiation UV-B sample absorption (i.e. chromophoric dissolved organic matter, CDOM) as expected, we found no correlation between AQY and CDOM for either product at any site. Molecular size, approximated with pre-irradiation spectral slope coefficients, and aromatic content, approximated by the specific ultraviolet absorption of the pre-irradiated samples, were also not correlated with AQY in either data set. The ratios of CO2 to CO photoproduction determined using both an AQY model and direct production comparisons were 23.2 ± 12.5 and 22.5 ± 9.0, respectively. Combined, both products represent a loss of 2.2 to 2.6 % of the DOC delivered to the estuaries and inner shelf of the South Atlantic Bight yearly, and 5 to 6 % of the total annual degassing of CO2 to the atmosphere. This result suggests that direct photochemical production of CO and CO2 is a small, yet significant contributor to both DOC cycling and CO2 gas exchange in this coastal system.

scholarly journals Morphological and morphometric comparisons of the first zoea of four species of purse crabs (Decapoda: Brachyura: Leucosiidae) from the Brazilian South Atlantic

Zootaxa ◽  
2012 ◽  
Vol 3167 (1) ◽  
pp. 17 ◽  
Author(s):  
GUSTAVO L. HIROSE ◽  
RAFAEL A. GREGATI ◽  
ADILSON FRANSOZO ◽  
MARIA L. NEGREIROS-FRANSOZO
Keyword(s):  
Discriminant Function ◽  
South Atlantic ◽  
Species Level ◽  
Ecological Studies ◽  
The South ◽  
Morphometric Measurements ◽  
Data Set ◽  
Larval Stages ◽  
Identification Of Species

Morphological descriptions of the first larval stages of Persephona mediterranea, P. lichtensteinii and P. punctata (Leu-cosiidae: Eballinae) from the South Atlantic are provided, with comparisons among the morphometry of the species. Adetailed description and eight morphometric measurements were made for each larva, and a discriminant function analysiswas used to interpret the data set. The species differed significantly in certain morphological and morphometric features.These characteristics may help in the identification of species of Leucosioidea found in the South Atlantic, and can be an important tool in ecological studies, supporting the identification of planktonic morphotypes at the species level.

scholarly journals Highly variable upper and abyssal overturning cells in the South Atlantic

2020 ◽  
Vol 6 (32) ◽  
pp. eaba7573
Author(s):  
M. Kersalé ◽  
C. S. Meinen ◽  
R. C. Perez ◽  
M. Le Hénaff ◽  
D. Valla ◽  
...  
Keyword(s):  
Time Scales ◽  
Vertical Structure ◽  
Volume Transport ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
The South ◽  
Primary Mechanism ◽  
Meridional Overturning ◽  
Earth’S Climate ◽  
High Degree

The Meridional Overturning Circulation (MOC) is a primary mechanism driving oceanic heat redistribution on Earth, thereby affecting Earth’s climate and weather. However, the full-depth structure and variability of the MOC are still poorly understood, particularly in the South Atlantic. This study presents unique multiyear records of the oceanic volume transport of both the upper (<~3100 meters) and abyssal (>~3100 meters) overturning cells based on daily moored measurements in the South Atlantic at 34.5°S. The vertical structure of the time-mean flows is consistent with the limited historical observations. Both the upper and abyssal cells exhibit a high degree of variability relative to the temporal means at time scales, ranging from a few days to a few weeks. Observed variations in the abyssal flow appear to be largely independent of the flow in the overlying upper cell. No meaningful trends are detected in either cell.

Assessing "Drake Passage Leakage" in an eddy resolving ocean model

2021 ◽  
Author(s):  
Jonathan Wiskandt ◽  
Siren Ruehs ◽  
Franziska Schwarzkopf ◽  
Arne Biastoch
Keyword(s):  
Upper Limb ◽  
Drake Passage ◽  
Ocean Model ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Return Flow ◽  
The South ◽  
The North ◽  
Transit Times ◽  
Benguela Current

&lt;p&gt;The upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is supplied in the South Atlantic from Drake Passage (DP) and Agulhas Leakage (AL). The relative contributions from DP and AL influence the stratification as well as the properties of the upper limb return flow and potentially impact the formation of deep water in the North Atlantic. &lt;br&gt;While early studies suggested a clear dominance of the AL contribution, recent studies indicate that the DP contribution is not negligible. Here, we use a set of Lagrangian experiments in the eddy-resolving (1/20 degree) ocean model INALT20 to analyze the inflow from DP into the South Atlantic in more detail. We find that the majority of water, that enters the subtropical South Atlantic across 30&amp;#176; S from DP, originates from the upper 2000 m of the northern branch of the ACC that follows the Sub Antarctic Front (SAF). Before&amp;#160; entering the South Atlantic, the majority of theses particles turn northward east of DP and follow the SAF through the Brazil Malvinas Confluence, where the SAF meets the Sub Tropical Front. In or parallel to the South Atlantic Current, particles cross the basin and become part of the subtropical gyre to follow the Benguela Current northward. We further compare pathways, volume transports, transit times and thermohaline properties of particles entering through DP and leaking into the South Atlantic to those from particles not leaking into the South Atlantic. These analyses help exploring potential recipes for building a timeseries of &amp;#8220;Drake Passage leakage&amp;#8221;, complementary to the already established Agulhas Leakage timeseries.&lt;/p&gt;

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