scholarly journals South Atlantic meridional transports from NEMO-based simulations and reanalyses

2017 ◽  
Author(s):  
Davi Mignac ◽  
David Ferreira ◽  
Keith Haines
Keyword(s):  
Data Assimilation ◽  
Large Scale ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Western Boundary ◽  
Boundary Current ◽  
The South ◽  
Western Boundary Currents ◽  
Boundary Currents

Abstract. The South Atlantic meridional transports are evaluated for four state-of-the-art global Ocean Reanalyses (ORAs) and two Free-Running Models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans model, and the ORAs share very similar configurations. The ORA basin interior transports are consistently modified relative to the FRMs, especially in the Argo period, with an improved representation of the south equatorial currents. The ORAs also exhibit systematically higher meridional transports than the FRMs, in closer agreement with large-scale observational estimates at 35° S and western boundary measurements at 11° S. However, the transport impacts by data assimilation still greatly vary between the ORAs, leading to differences up to ~ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the Meridional Heat Transports (MHTs), respectively. Large inter-product discrepancies arise in the ORA western boundary currents at both upper and deep levels explaining up to ~ 85 % of the inter-product differences in their total MHTs, and meridional velocity differences, rather than temperatures differences, drive ~ 83 % of this spread. Further analysis shows that only very confined temperature differences right against the western boundary geostrophically explain the large boundary current velocity differences. These findings suggest that the current data assimilation schemes, even with Argo data, can consistently constrain the basin interior circulation in the ORAs, but not the overturning transport component dominated by the narrow western boundary currents as in the South Atlantic.

scholarly journals South Atlantic meridional transports from NEMO-based simulations and reanalyses

Ocean Science ◽  
2018 ◽  
Vol 14 (1) ◽  
pp. 53-68 ◽  
Author(s):  
Davi Mignac ◽  
David Ferreira ◽  
Keith Haines
Keyword(s):  
Data Assimilation ◽  
Heat Transport ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Western Boundary ◽  
The South ◽  
Western Boundary Currents ◽  
Boundary Currents ◽  
Basin Scale

Abstract. The meridional heat transport (MHT) of the South Atlantic plays a key role in the global heat budget: it is the only equatorward basin-scale ocean heat transport and it sets the northward direction of the global cross-equatorial transport. Its strength and variability, however, are not well known. The South Atlantic transports are evaluated for four state-of-the-art global ocean reanalyses (ORAs) and two free-running models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans (NEMO) model, and the ORAs share very similar configurations. Very few previous works have looked at ocean circulation patterns in reanalysis products, but here we show that the ORA basin interior transports are consistently improved by the assimilated in situ and satellite observations relative to the FRMs, especially in the Argo period. The ORAs also exhibit systematically higher meridional transports than the FRMs, which is in closer agreement with observational estimates at 35 and 11∘ S. However, the data assimilation impact on the meridional transports still greatly varies among the ORAs, leading to differences up to ∼ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the MHTs, respectively. We narrow this down to large inter-product discrepancies in the western boundary currents (WBCs) at both upper and deep levels explaining up to ∼ 85 % of the inter-product differences in MHT. We show that meridional velocity differences, rather than temperature differences, in the WBCs drive ∼ 83 % of this MHT spread. These findings show that the present ocean observation network and data assimilation schemes can be used to consistently constrain the South Atlantic interior circulation but not the overturning component, which is dominated by the narrow western boundary currents. This will likely limit the effectiveness of ORA products for climate or decadal prediction studies.

scholarly journals Circulation and Transport in the Western Boundary Currents at Cape Farewell, Greenland

2009 ◽  
Vol 39 (8) ◽  
pp. 1854-1870 ◽  
Author(s):  
N. P. Holliday ◽  
S. Bacon ◽  
J. Allen ◽  
E. L. McDonagh
Keyword(s):  
Water Transport ◽  
Deep Water ◽  
Flow Direction ◽  
Labrador Sea ◽  
Western Boundary ◽  
Boundary Current ◽  
The South ◽  
Western Boundary Currents ◽  
Boundary Currents ◽  
Irminger Sea

Abstract The circulation and volume transports in the western boundary currents around Cape Farewell, Greenland, are derived from full-depth hydrographic and velocity measurements from August–September 2005. The western boundary currents from surface to seafloor transport 40.5 ± 8.1 Sv (Sv ≡ 106 m3 s−1) southward in the Irminger Sea, and 53.8 ± 10.8 Sv northward in the Labrador Sea. The Deep Western Boundary Current (DWBC, defined as water with potential density greater than 27.80 kg m−3) transports 12.3 ± 2.5 Sv southward in the Irminger Sea. The deep water transport is reduced south of Cape Farewell, where it changes flow direction from southward to northward (the south corner). At a section over the Eirik Ridge, a bathymetric feature extending southwest of Cape Farewell, the DWBC transports 8.7 ± 1.7 Sv westward. The reduction in transport at the south corner is associated with decreased velocities within the deepest layers and the volumetric loss of the most saline deep water types. The observations suggest that the paths of the shallow and deep western boundary currents diverge at the south corner. Downstream in the eastern Labrador Sea the deep water transport is increased to 19.7 ± 3.9 Sv northward, with the addition of recirculating denser deep waters. The representativeness of the results from the semisynoptic survey is discussed with reference to companion current meter measurements of the DWBC.

scholarly journals Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S

2012 ◽  
Vol 9 (2) ◽  
pp. 977-1008 ◽  
Author(s):  
C. S. Meinen ◽  
A. R. Piola ◽  
R. C. Perez ◽  
S. L. Garzoli
Keyword(s):  
Annual Cycle ◽  
Statistical Significance ◽  
South Atlantic ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Model Output ◽  
Boundary Current ◽  
The South ◽  
Deep Western Boundary Current

Abstract. The first direct estimates of the temporal variability of the absolute transport of the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation, based on neutral density, temperature, salinity, and oxygen values. Both observations confirm that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed at 26.5° N, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ~40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observation record is too short to be definitive, and the dominant time scale during the first year of the experiment was about 9–10 days. A "Monte Carlo-style" analysis using 27 yr of model output from the same location as the observations indicates that another 48–60 months of data will be required to encompass a fairly complete span of deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however the statistical significance of the annual cycle with even 27 yr of model output is low, suggesting that annual period variations in the model are weak as well.

scholarly journals Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S

Ocean Science ◽  
2012 ◽  
Vol 8 (6) ◽  
pp. 1041-1054 ◽  
Author(s):  
C. S. Meinen ◽  
A. R. Piola ◽  
R. C. Perez ◽  
S. L. Garzoli
Keyword(s):  
Annual Cycle ◽  
Statistical Significance ◽  
South Atlantic ◽  
Western Boundary Current ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Model Output ◽  
Boundary Current ◽  
The South ◽  
Deep Western Boundary Current

Abstract. The first direct estimates of the temporal variability of the absolute transport in the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm, based on neutral density, temperature, salinity, and oxygen values, the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation. Both data sets indicate that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed in the North Atlantic, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observational records are too short to be definitive. A Monte Carlo-style analysis using 27 yr of model output from the same location as the observations indicates that about 48–60 months of data will be required to fully assess the deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however its statistical significance with even 27 yr of model output is low, suggesting that seasonal variations in the model are weak.

scholarly journals Radiating Instability of a Meridional Boundary Current

2008 ◽  
Vol 38 (10) ◽  
pp. 2294-2307 ◽  
Author(s):  
Hristina G. Hristova ◽  
Joseph Pedlosky ◽  
Michael A. Spall
Keyword(s):  
Western Boundary ◽  
Far Field ◽  
Boundary Current ◽  
Important Conclusion ◽  
Western Boundary Currents ◽  
Horizontal Structure ◽  
Boundary Currents ◽  
Zonal Jets ◽  
Eastern Boundary ◽  
Eastern Boundary Current

Abstract A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.

scholarly journals THE BIFURCATION OF THE WESTERN BOUNDARY CURRENT SYSTEM OF THE SOUTH ATLANTIC OCEAN

2014 ◽  
Vol 32 (2) ◽  
pp. 241 ◽  
Author(s):  
Janini Pereira ◽  
Mariela Gabioux ◽  
Martinho Marta Almeida ◽  
Mauro Cirano ◽  
Afonso M. Paiva ◽  
...  
Keyword(s):  
High Resolution ◽  
Atlantic Ocean ◽  
Ocean Circulation ◽  
South Atlantic ◽  
Western Boundary Current ◽  
South Atlantic Ocean ◽  
Western Boundary ◽  
Intermediate Water ◽  
Boundary Current ◽  
The South

ABSTRACT. The results of two high-resolution ocean global circulation models – OGCMs (Hybrid Coordinate Ocean Model – HYCOM and Ocean Circulation andClimate Advanced Modeling Project – OCCAM) are analyzed with a focus on the Western Boundary Current (WBC) system of the South Atlantic Ocean. The volumetransports are calculated for different isopycnal ranges, which represent the most important water masses present in this region. The latitude of bifurcation of the zonalflows reaching the coast, which leads to the formation of southward or northward WBC flow at different depths (or isopycnal levels) is evaluated. For the Tropical Water,bifurcation of the South Equatorial Current occurs at 13◦-15◦S, giving rise to the Brazil Current, for the South Atlantic Central Water this process occurs at 22◦S.For the Antarctic Intermediate Water, bifurcation occurs near 28◦-30◦S, giving rise to a baroclinic unstable WBC at lower latitudes with a very strong vertical shearat mid-depths. Both models give similar results that are also consistent with previous observational studies. Observations of the South Atlantic WBC system havepreviously been sparse, consequently these two independent simulations which are based on realistic high-resolution OGCMs, add confidence to the values presentedin the literature regarding flow bifurcations at the Brazilian coast.Keywords: Southwestern Atlantic circulation, water mass, OCCAM, HYCOM. RESUMO. Resultados de dois modelos globais de alta resolução (HYCOM e OCCAM) são analisados focando o sistema de Corrente de Contorno Oeste do Oceano Atlântico Sul. Os transportes de volume são calculados para diferentes níveis isopicnais que representam as principais massas de água da região. É apresentada a avaliação da latitude de bifurcação do fluxo zonal que atinge a costa, permitindo a formação dos fluxos da Corrente de Contorno Oeste para o sul e para o norte emdiferentes níveis de profundidades (ou isopicnal). Para a Água Tropical, a bifurcação da Corrente Sul Equatorial ocorre entre 13◦-15◦S, originando a Corrente do Brasil, e para a Água Central do Atlântico Sul ocorre em 22◦S. A bifurcação daÁgua Intermediária Antártica ocorre próximo de 28◦-30◦S, dando um aumento na instabilidade baroclínica da Corrente de Contorno Oeste em baixas latitudes e com um forte cisalhamento vertical em profundidades intermediárias. Ambos os modelos apresentamresultados similares e consistentes com estudos observacionais prévios. Considerando que as observações do sistema de Corrente de Contorno Oeste do Atlântico Sul são escassas, essas duas simulações independentes com modelos globais de alta resolução adicionam confiança aos valores apresentados na literatura, relacionadosaos fluxos das bifurcações na costa do Brasil.Palavras-chave: circulação do Atlântico Sudoeste, massas de água, OCCAM, HYCOM.

scholarly journals The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

2019 ◽  
Vol 12 (7) ◽  
pp. 3329-3355 ◽  
Author(s):  
Franziska U. Schwarzkopf ◽  
Arne Biastoch ◽  
Claus W. Böning ◽  
Jérôme Chanut ◽  
Jonathan V. Durgadoo ◽  
...  
Keyword(s):  
High Resolution ◽  
Boundary Conditions ◽  
Current System ◽  
Western Boundary Current ◽  
Global Ocean ◽  
Western Boundary ◽  
Boundary Current ◽  
The South ◽  
Agulhas Current ◽  
Mesoscale Dynamics

Abstract. The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.

scholarly journals Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans

2005 ◽  
Vol 18 (15) ◽  
pp. 2864-2882 ◽  
Author(s):  
J. C. Hermes ◽  
C. J. C. Reason
Keyword(s):  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Large Scale ◽  
Ocean Model ◽  
South Atlantic ◽  
Heat Fluxes ◽  
Global Ocean ◽  
The South ◽  
Sst Variability ◽  
South Indian

Abstract A global ocean model (ORCA2) forced with 50 yr of NCEP–NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics. The large-scale forcing of these events appears to consist of a coherent modulation of the wavenumber-3 or -4 pattern in the Southern Hemisphere atmospheric circulation such that the semipermanent subtropical anticyclone in each basin is shifted from its summer mean position and its strength is modulated. A relationship to the Antarctic Oscillation is also apparent, and seems to strengthen after the mid-1970s. The modulated subtropical anticyclones lead to changes in the tropical easterlies and midlatitude westerlies in the South Atlantic and South Indian Oceans that result in anomalies in latent heat fluxes, upwelling, and Ekman heat transports, all of which contribute to the SST variability. In addition, there are significant modulations to the strong Rossby wave signals in the South Indian Ocean. The results of this study confirm the ability of the ORCA2 model to represent these dipole patterns and indicate connections between large-scale modulations of the Southern Hemisphere midlatitude atmospheric circulation and coevolving SST variability in the South Atlantic and South Indian Oceans.

scholarly journals Wind-driven and buoyancy-driven circulation in the subtropical North Atlantic Ocean

2021 ◽  
Vol 477 (2256) ◽  
Author(s):  
Harry L. Bryden
Keyword(s):  
Atlantic Ocean ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Gulf Stream ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Global Ocean ◽  
Western Boundary ◽  
Boundary Current ◽  
Deep Waters

Continuous observations of ocean circulation at 26°N in the subtropical Atlantic Ocean have been made since April 2004 to quantify the strength and variability in the Atlantic Meridional overturning circulation (AMOC), in which warm, upper waters flow northward and colder deep waters below 1100 m depth return southward. The principal components of the AMOC are northward western boundary current transport in the Gulf Stream and Antilles Current, northward surface Ekman transport and southward thermocline recirculation, all of which are generally considered to be part of the wind-driven circulation. Southward flowing deep waters below 1100 m depth are usually considered to represent the buoyancy-driven circulation. We argue that the Gulf Stream is partially wind-driven but also partially buoyancy-driven as it returns upper waters upwelled in the global ocean back to water mass formation regions in the northern Atlantic. Seasonal to interannual variations in the circulation at 26°N are principally wind-driven. Variability in the buoyancy-driven circulation occurred in a sharp reduction in 2009 in the southward flow of Lower North Atlantic Deep Water when its transport decreased by 30% from pre-2009 values. Over the 14-year observational period from 2004 to 2018, the AMOC declined by 2.4 Sv from 18.3 to 15.9 Sv.

scholarly journals MODELING THE SOUTH ATLANTIC OCEAN FROM MEDIUM TO HIGH-RESOLUTION

2013 ◽  
Vol 31 (2) ◽  
pp. 229 ◽  
Author(s):  
Mariela Gabioux Gabioux ◽  
Vladimir Santos Da Costa ◽  
João Marcos Azevedo Correia de Souza ◽  
Bruna Faria de Oliveira ◽  
Afonso De Moraes Paiva
Keyword(s):  
High Resolution ◽  
Atlantic Ocean ◽  
Large Scale ◽  
South Atlantic ◽  
Surface Relaxation ◽  
Brazilian Coast ◽  
Western Boundary ◽  
The South ◽  
Basic Set ◽  
Set Up

ABSTRACT. The standard REMO (a Brazilian approach towards operational oceanography) model configuration is tested, and results of two numerical simulations with HYCOM are presented and discussed. This configuration consists basically of a high-resolution eddy-resolving, 1/12 degree model for the Metarea V (latitudes from 7◦N to 35◦50’S, and longitudes between 20◦W and the Brazilian coast), nested in a medium-resolution eddy-permitting, 1/4 degree model of the Atlantic Ocean. These simulations aim for: a) creating a basic set-up for implementation of assimilation techniques leading to ocean prediction; b) the development of hydrodynamic bases for environmental studies; and c) providing boundary conditions for regional domains with increased resolution. This is the first time HYCOM is applied in high-resolution and particularly tailored for this region of the ocean. The 1/4 degree simulation was able to simulate realistic Equatorial and South Atlantic large scale circulation, both the wind-driven and the thermohaline components. The high-resolution introduces realistic mesoscale activity, in particular that associated with the dynamics of western boundary currents, and captures also both the continental shelf and the upper-ocean modes of variability associated with atmospheric synoptic forcing. Important issues for the simulation of the South Atlantic with high-resolution are discussed, like the ideal place for boundaries, improvements in the bathymetric representation, and the control of SST bias by the introduction of surface relaxation. In order to make a preliminary assessment of the model behavior when submitted to data assimilation, the Cooper & Haines (1996) method was used to extrapolate SSH anomalies fields to deeper layers every 7 days, with encouraging results.Keywords: numerical simulation, nesting, southwest Atlantic, Brazil Current. RESUMO. Neste trabalho são apresentados e discutidos resultados de duas simulações numéricas realizadas com o model HYCOM e que representam a configuração padrão do projeto REMO (Rede de Modelagem e Observação Oceanográfica), uma abordagem brasileira para a oceanografia operacional. Esta configuração consiste em um modelo em alta resolução (1/12 de grau, que resolve a mesoescala) da região denominada de Metarea V (latitudes de 7◦N a 35◦50’S e longitudes desde 20◦W até a costa brasileira), aninhado em um modelo em média-resolução (1/4 de grau, que resolve apenas parcialmente a mesoescala) do oceano Atlântico. Estas simulações tem como objetivos: a) a geração de um set-up básico para implementação de técnicas de assimilação visando a previsão oceânica; b) o desenvolvimento de bases hidrodinâmicas para estudos ambientais; e c) a geração de condições de contorno para domínios regionais com maior resolução. Esta é a primeira vez que o HYCOM é aplicado em alta resolução e especialmente configurado para esta região do oceano. A simulação em 1/4 de grau simulou de forma realista a circulação de larga escala no Atlântico Sul e Equatorial, tanto a componente eólica quanto a termohalina. A simulação em alta resolução foi capaz de introduzir também de forma realista a mesoescala, em particular aquela associada à dinâmica das correntes de contorno oeste, e de capturar a variabilidade da porção superior do oceano e da plataforma continental associada à forçante atmosférica em escala sinótica. Aspectos importantes para a simulação do Atlântico Sul em alta resolução são discutidos, como o posicionamento dos contornos, a representação da batimetria e o controle de possíveis tendências na TSM pela introdução de um termo de relaxamento para climatologia em superfície. Uma avaliação preliminar do comportamento do modelo submetido à assimilação de dados foi realizada com o método de Cooper & Haines (1996), capaz de extrapolar campos de anomalias de elevação da superfície para camadas mais profundas a cada 7 dias, com resultados promissores.Palavras-chave: simulação numérica, aninhamento, Atlântico sudoeste, Corrente do Brasil.

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