Global Energy-saving Map of Strong Ocean Currents

This study provides a global, detailed, and complete energy-saving map of strong ocean currents from the absolute geostrophic velocities calculated from satellite altimetry data, with the focus on the strong Western Boundary Currents (WBCs) in the global ocean. Theoretically, the WBCs with speeds of 2–3 knots can reduce fuel consumption by 25–50% for vessels at a sailing speed of 6 knots. The fuel savings are greater for a lower sailing speed than for a higher sailing speed. For about 1·8 million motorised fishing vessels with a lower ship speed, strong currents can evidently save fuel, time and money. Since global fishing vessels generate roughly 130 million tonnes of CO2 per annum (FAO, 2012), effective utilisation of the energy-saving map could significantly reduce CO2 emissions from ship operations.

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

10.5194/egusphere-egu2020-5689 ◽

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 &#188;&#176; 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>

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Intense Subsurface Upwelling Associated with Major Western Boundary Currents

10.5194/egusphere-egu2020-6214 ◽

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 Ni&#241;o&#8211;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>

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Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models

10.5194/os-2017-24 ◽

2017 ◽

Cited By ~ 3

Author(s):

Rafael Abel ◽

Claus W. Böning ◽

Richard J. Greatbatch ◽

Helene T. Hewitt ◽

Malcolm J. Roberts

Keyword(s):

Climate Model ◽

Global Climate Model ◽

Surface Wind ◽

Ocean Currents ◽

Global Ocean ◽

Ocean Current ◽

Western Boundary ◽

Coupling Coefficients ◽

Coupled Models ◽

Near Surface

Abstract. The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations.

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

Advances in Atmospheric Sciences ◽

10.1007/s00376-021-0385-7 ◽

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.

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South Atlantic meridional transports from NEMO-based simulations and reanalyses

Ocean Science ◽

10.5194/os-14-53-2018 ◽

2018 ◽

Vol 14 (1) ◽

pp. 53-68 ◽

Cited By ~ 9

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.

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South Atlantic meridional transports from NEMO-based simulations and reanalyses

10.5194/os-2017-69 ◽

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.

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Radiating Instability of a Meridional Boundary Current

Journal of Physical Oceanography ◽

10.1175/2008jpo3853.1 ◽

2008 ◽

Vol 38 (10) ◽

pp. 2294-2307 ◽

Cited By ~ 26

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.

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