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 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.

Intense Subsurface Upwelling Associated with Major Western Boundary Currents

2020 ◽  
Author(s):  
Fanglou Liao ◽  
Xinfeng Liang ◽  
Yun Li ◽  
Andreas Thurnherr
Keyword(s):  
Southern Oscillation ◽  
Global Climate ◽  
Vertical Transport ◽  
Global Ocean ◽  
Western Boundary ◽  
Atmospheric Conditions ◽  
Western Boundary Currents ◽  
Boundary Currents ◽  
Starting Point ◽  
Basin Scale

<p>Western boundary currents (WBC), fast flowing currents on the western side of ocean basins, transport a huge amount of warm water poleward, affect the atmospheric conditions along their paths, take up a large amount of carbon dioxide, and regulate the global climate (Minobe et al. 2008; Takahashi et al. 2009; Wu et al. 2012). In contrast to their widely examined horizontal motions, much less attention has been paid to the vertical motions associated with the WBC systems. Here, we examined the spatial and temporal characteristics of vertical motions associated with the major WBC systems by analyzing vertical velocity estimates from five ocean synthesis products and one eddy-permitting ocean simulation over an overlapping period from Jan 1992 to Dec 2009. Robust and intense subsurface upwelling occurs in the five major subtropical WBC systems. These upwelling systems together with the vast downwelling inside subtropical ocean basins form basin-scale zonal overturning circulations and play a crucial role in the vertical transport of ocean properties and tracers inside the global ocean. Also, the vertical motions in the Kuroshio Current and the Eastern Australian Current regions display robust interannual and decadal oscillations, which are well correlated with El Niño–Southern Oscillation and Pacific Decadal Oscillation, respectively. This study unveils an overlooked role of the WBCs in the subsurface oceanic vertical transport and is expected to be a starting point for more in-depth investigations on their dynamics and roles in the climate system.</p>

Observed seasonal regimes of North-South Atlantic Ocean interbasin exchange

2019 ◽  
Author(s):  
Hamed D. Ibrahim
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Volume Flux ◽  
The South ◽  
The North ◽  
Basin Scale ◽  
Interbasin Exchange ◽  
North And South

North and South Atlantic lateral volume exchange is a key component of the Atlantic Meridional Overturning Circulation (AMOC) embedded in Earth’s climate. Northward AMOC heat transport within this exchange mitigates the large heat loss to the atmosphere in the northern North Atlantic. Because of inadequate climate data, observational basin-scale studies of net interbasin exchange between the North and South Atlantic have been limited. Here ten independent climate datasets, five satellite-derived and five analyses, are synthesized to show that North and South Atlantic climatological net lateral volume exchange is partitioned into two seasonal regimes. From late-May to late-November, net lateral volume flux is from the North to the South Atlantic; whereas from late-November to late-May, net lateral volume flux is from the South to the North Atlantic. This climatological characterization offers a framework for assessing seasonal variations in these basins and provides a constraint for climate models that simulate AMOC dynamics.

Forced and chaotic variability of interannual variability of regional sea level and its causes scale over 1993-2015

2020 ◽  
Author(s):  
Alice Carret ◽  
William Llovel ◽  
Thierry Penduff ◽  
Jean-Marc Molines ◽  
Benoît Meyssignac
Keyword(s):  
Interannual Variability ◽  
Sea Level ◽  
Satellite Altimetry ◽  
Global Ocean ◽  
Western Boundary ◽  
Western Boundary Currents ◽  
Steric Sea Level ◽  
Boundary Currents ◽  
Ocean Variability ◽  
Regional Sea Level

<p>Since the early 1990s, satellite altimetry has become the main observing system for continuously measuring the sea level variations with a near global coverage. Satellite altimetry has revealed a global mean sea level rise of 3.3 mm/yr since 1993 with large regional sea level variability that differs from the mean estimate. These measurements highlight complex structures especially for the western boundary currents or the Antarctic Circumpolar Current. A recent study shows that the chaotic ocean variability may mask atmospherically-forced regional sea level trends over 38% of the global ocean area from 1993 to 2015. The chaotic variability is large for the western boundary currents and in the Southern Ocean. The present study aims to complement this previous work in focusing on the interannual variability of regional sea level. A global ¼° ocean/sea-ice 50-member ensemble simulation is considered to disentangle the imprints of the atmospheric forcing and the chaotic ocean variability on the interannual variability of regional sea level over 1993-2015. We investigate the forced (i.e., ensemble mean) versus the chaotic variability (i.e., ensemble standard deviation) for the interannual variability of regional sea level and its causes (i.e., steric sea level and manometric sea level contribution). We complement our investigations by partitioning the steric component into thermosteric sea level (i.e., temperature change only) and halosteric sea level (i.e., salinity change only). One of the goals of the study is to highlight the hot spots region of large chaotic variability for regional sea level and its different components.</p>

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 Paleo Agulhas rings enter the subtropical gyre during the penultimate deglaciation

2013 ◽  
Vol 9 (2) ◽  
pp. 2095-2114
Author(s):  
P. Scussolini ◽  
E. van Sebille
Keyword(s):  
Ocean Circulation ◽  
Circulation Model ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
The South ◽  
Cape Basin ◽  
Globorotalia Truncatulinoides ◽  
Density Perturbations ◽  
Global Ocean Circulation

Abstract. A maximum in the strength of Agulhas Leakage has been registered at the interface between Indian and South Atlantic oceans during glacial Termination II (T II), presumably transporting the salt and heat necessary to maintain the Atlantic Meridional Overturning Circulation (AMOC) at rates similar to the present day. However, it was never shown whether these were effectively incorporated in the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern Oceans. To solve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic basin than previously studied. Analysis of a 20 yr dataset from a global ocean circulation model allows us to relate density perturbations, at the depth of the thermocline, to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of δ18O variability of Globorotalia truncatulinoides single specimens, revealing high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic gyre around and before T II. Our record closely follows the published quantifications of Agulhas Leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support to the view of a prominent role of the Agulhas Leakage in the shift from a glacial to an interglacial mode of AMOC.

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 On the Importance of High-Resolution in Large-Scale Ocean Models

2021 ◽  
Author(s):  
Eric P. Chassignet ◽  
Xiaobiao Xu
Keyword(s):  
High Resolution ◽  
Computational Cost ◽  
Value Added ◽  
Global Ocean ◽  
Western Boundary ◽  
Western Boundary Currents ◽  
Boundary Currents ◽  
Ocean Variability ◽  
Ocean Models ◽  
The Impact

AbstractEddying global ocean models are now routinely used for ocean prediction, and the value-added of a better representation of the observed ocean variability and western boundary currents at that resolution is currently being evaluated in climate models. This overview article begins with a brief summary of the impact on ocean model biases of resolving eddies in several global ocean-sea ice numerical simulations. Then, a series of North and Equatorial Atlantic configurations are used to show that an increase of the horizontal resolution from eddy-resolving to submesoscale-enabled together with the inclusion of high-resolution bathymetry and tides significantly improve the models’ abilities to represent the observed ocean variability and western boundary currents. However, the computational cost of these simulations is extremely large, and for these simulations to become routine, close collaborations with computer scientists are essential to ensure that numerical codes can take full advantage of the latest computing architecture.

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.

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