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  • 1
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    Quantitative estimate of the paleo-Agulhas leakage (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 (4). pp. 1238-1246.
    Details
    Publication Date: 2017-04-10
    Description: The Indian-Atlantic water exchange south of Africa (Agulhas leakage) is a key component of the global ocean circulation. No quantitative estimation of the paleo-Agulhas leakage exists. We quantify the variability in interocean exchange over the past 640,000 years, using planktic foraminiferal assemblage data from two marine sediment records to define an Agulhas leakage efficiency index. We confirm the validity of our new approach with a numerical ocean model that realistically simulates the modern Agulhas leakage changes. Our results suggest that, during the past several glacial-interglacial cycles, the Agulhas leakage varied by ~10 sverdrup and more during major climatic transitions. This lends strong credence to the hypothesis that modifications in the leakage played a key role in changing the overturning circulation to full strength mode. Our results are instrumental for validating and quantifying the contribution of the Indian-Atlantic water leakage to the global climate changes.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2014GL059278
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    An observed 20-year time series of Agulhas leakage (2014)
    Copernicus Publications (EGU)
    In:  Ocean Science, 10 (4). pp. 601-609.
    Details
    Publication Date: 2019-09-23
    Description: We provide a time series of Agulhas leakage anomalies over the last 20-years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between the Agulhas Current and Agulhas Return Current, which allows us to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows us to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow us to determine the mean leakage but that leakage anomalies can be accurately computed.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.5194/os-10-601-2014
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Nano-tags for neonates and ocean-mediated swimming behaviours linked to rapid dispersal of hatchling sea turtles (2014)
    Royal Society of London
    In:  Proceedings of the Royal Society B: Biological Sciences, 281 (1796). p. 20141209.
    Details
    Publication Date: 2019-09-23
    Description: Dispersal during juvenile life stages drives the life-history evolution and dynamics of many marine vertebrate populations. However, the movements of juvenile organisms, too small to track using conventional satellite telemetry devices, remain enigmatic. For sea turtles, this led to the paradigm of the ‘lost years' since hatchlings disperse widely with ocean currents. Recently, advances in the miniaturization of tracking technology have permitted the application of nano-tags to track cryptic organisms. Here, the novel use of acoustic nano-tags on neonate loggerhead turtle hatchlings enabled us to witness first-hand their dispersal and behaviour during their first day at sea. We tracked hatchlings distances of up to 15 km and documented their rapid transport (up to 60 m min−1) with surface current flows passing their natal areas. Tracking was complemented with laboratory observations to monitor swimming behaviours over longer periods which highlighted (i) a positive correlation between swimming activity levels and body size and (ii) population-specific swimming behaviours (e.g. nocturnal inactivity) suggesting local oceanic conditions drive the evolution of innate swimming behaviours. Knowledge of the swimming behaviours of small organisms is crucial to improve the accuracy of ocean model simulations used to predict the fate of these organisms and determine resultant population-level implications into adulthood.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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    Format: other
    DOI: 10.1098/rspb.2014.1209
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Oceanic hindcast simulations at high resolution suggest that the Atlantic MOC is bistable (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (12). pp. 3069-3073.
    Details
    Publication Date: 2020-02-18
    Description: All climate models predict a freshening of the North Atlantic at high latitude that may induce an abrupt change of the Atlantic Meridional Overturning Circulation (hereafter AMOC) if it resides in the bistable regime, where both a strong and a weak state coexist. The latter remains uncertain as there is no consensus among observations and ocean reanalyses, where the AMOC is bistable, versus most climate models that reproduce a mono-stable strong AMOC. A series of four hindcast simulations of the global ocean at 1/12° resolution, which is presently unique, are used to diagnose freshwater transport by the AMOC in the South Atlantic, an indicator of AMOC bistability. In all simulations, the AMOC resides in the bistable regime: it exports freshwater southward in the South Atlantic, implying a positive salt advection feedback that would act to amplify a decreasing trend in subarctic deep water formation as projected in climate scenarios.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/grl.50534
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Taking action against ocean acidification: A review of management and policy options (2013)
    Springer
    In:  Environmental Management, 52 (4). pp. 761-779.
    Details
    Publication Date: 2018-09-12
    Description: Ocean acidification has emerged over the last two decades as one of the largest threats to marine organisms and ecosystems. However, most research efforts on ocean acidification have so far neglected management and related policy issues to focus instead on understanding its ecological and biogeochemical implications. This shortfall is addressed here with a systematic, international and critical review of management and policy options. In particular, we investigate the assumption that fighting acidification is mainly, but not only, about reducing CO2 emissions, and explore the leeway that this emerging problem may open in old environmental issues. We review nine types of management responses, initially grouped under four categories: preventing ocean acidification; strengthening ecosystem resilience; adapting human activities; and repairing damages. Connecting and comparing options leads to classifying them, in a qualitative way, according to their potential and feasibility. While reducing CO2 emissions is confirmed as the key action that must be taken against acidification, some of the other options appear to have the potential to buy time, e.g. by relieving the pressure of other stressors, and help marine life face unavoidable acidification. Although the existing legal basis to take action shows few gaps, policy challenges are significant: tackling them will mean succeeding in various areas of environmental management where we failed to a large extent so far.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1007/s00267-013-0132-7
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Decadal fingerprints of freshwater discharge around Greenland in a multi-model ensemble (2013)
    Springer
    In:  Climate Dynamics, 41 (3-4). pp. 695-720.
    Details
    Publication Date: 2019-09-23
    Description: The recent increase in the rate of the Greenland ice sheet melting has raised with urgency the question of the impact of such a melting on the climate. As former model projections, based on a coarse representation of the melting, show very different sensitivity to this melting, it seems necessary to consider a multi-model ensemble to tackle this question. Here we use five coupled climate models and one ocean-only model to evaluate the impact of 0.1 Sv (1 Sv = 106 m3/s) of freshwater equally distributed around the coast of Greenland during the historical era 1965–2004. The ocean-only model helps to discriminate between oceanic and coupled responses. In this idealized framework, we find similar fingerprints in the fourth decade of hosing among the models, with a general weakening of the Atlantic Meridional Overturning Circulation (AMOC). Initially, the additional freshwater spreads along the main currents of the subpolar gyre. Part of the anomaly crosses the Atlantic eastward and enters into the Canary Current constituting a freshwater leakage tapping the subpolar gyre system. As a consequence, we show that the AMOC weakening is smaller if the leakage is larger. We argue that the magnitude of the freshwater leakage is related to the asymmetry between the subpolar-subtropical gyres in the control simulations, which may ultimately be a primary cause for the diversity of AMOC responses to the hosing in the multi-model ensemble. Another important fingerprint concerns a warming in the Nordic Seas in response to the re-emergence of Atlantic subsurface waters capped by the freshwater in the subpolar gyre. This subsurface heat anomaly reaches the Arctic where it emerges and induces a positive upper ocean salinity anomaly by introducing more Atlantic waters. We found similar climatic impacts in all the coupled ocean–atmosphere models with an atmospheric cooling of the North Atlantic except in the region around the Nordic Seas and a slight warming south of the equator in the Atlantic. This meridional gradient of temperature is associated with a southward shift of the tropical rains. The free surface models also show similar sea-level fingerprints notably with a comma-shape of high sea-level rise following the Canary Current.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1007/s00382-012-1479-9
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Reconciling tracer and float observations of the export pathways of Labrador Sea Water (2012)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 39 (24). L24606.
    Details
    Publication Date: 2019-09-23
    Description: For more than fifty years, it has been generally accepted by oceanographers that the Deep Western Boundary Current (DWBC) is the principal conduit of recently-convected Labrador Sea Water (LSW) exported from the high-latitude North Atlantic to the equator. Supporting this supposition is observational evidence that the waters of the DWBC have consistently greater equatorward velocities, higher concentrations of passive tracers, and younger ages compared to ocean interior waters. However, recent observations and simulations of floats launched in the DWBC in the Labrador Sea show that most water parcels are quickly ejected from the DWBC and follow instead interior pathways to the subtropics. Here, we show that tracer observations from the last three decades are compatible with the existence of both DWBC and basin-interior export pathways. From analyses of observational data and model output, we find that equatorward transport in the basin interior is consistent with the large-scale vorticity balance at mid-depth. Furthermore, from the modeling analysis we show that despite higher, localized concentrations of tracer and particles in the DWBC, only 5% of particles released in the Labrador Sea are transported from the subpolar to subtropical gyre via a continuous DWBC pathway. Thus, the interior pathway is a significant contributor to LSW export. Highlights: - Lagrangian observations of Labrador Sea Water match Eulerian observations - There is deep equatorward flow in the basin interior - This interior pathway is significant compared to the pathway along the boundary
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2012GL053978
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Anthropogenic impact on Agulhas leakage (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (6). pp. 1138-1143.
    Details
    Publication Date: 2020-02-18
    Description: Recent work suggests that changes of the Southern Hemisphere (SH) winds led to an increase in Agulhas leakage and a corresponding salinification of the Atlantic. Climate model projections for the 21st century predict a progressive southward migration and intensification of the SH westerlies. The potential effects on the ocean circulation of such an anthropogenic trend in wind stress are studied here with a high-resolution ocean model forced by a step-function change in SH wind stress that involves a 7% increase in westerlies strength and a 2° shift in the zero wind stress curl. The model simulation suggests a rapid dynamic adjustment of Agulhas leakage by 4.5 Sv, about a third of its original value, after a few years. The change in leakage is reflected in a concomitant change in the transport of the South Atlantic subtropical gyre, but leads only to a small increase in the Atlantic Meridional Overturning Circulation (AMOC) of O(1 Sv) after three decades. A main effect of the increasing inflow of Indian Ocean waters with potential long-term ramifications for the AMOC is the salinification and densification of upper-thermocline waters in the South Atlantic, which extends into the North Atlantic within the first three decades.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/grl.50243
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Spurious AMOC trends in global ocean sea-ice models related to subarctic freshwater forcing (2013)
    Elsevier
    In:  Ocean Modelling, 69 . pp. 39-49.
    Details
    Publication Date: 2019-09-23
    Description: Highlights: • OGCM simulations of the AMOC are highly sensitive to the subarctic freshwater forcing. • Trends in the simulated AMOC are linked to the salinity of the DSOW. • DSOW salinity trends can be traced back to the freshwater transport by the NAC. • The NAC freshwater budget is highly affected by the salinity restoring used in OGCMs. • Modifications in the subarctic precipitation can help to minimize the restoring flux. Global ocean sea-ice models with an atmospheric forcing based on bulk formulations of the air-sea fluxes exhibit spurious trends in key flow indices like the Atlantic Meridional Overturning Circulation (AMOC), constraining their use in investigations of multi-decadal ocean variability. To identify the critical model factors affecting the temporal evolution of the AMOC on time scales of up to 60 years, a series of experiments with both eddy-permitting (0.25°) and non-eddying (0.5°) ocean-ice models has been performed, focusing on the influence of artificial choices for the freshwater forcing, in particular the restoring of sea surface salinity towards climatological values. The atmospheric forcing builds on the proposal for Coordinated Ocean-ice Reference Experiments (CORE), utilizing the refined atmospheric reanalysis products for 1948–2006 compiled by Large and Yeager. Sensitivity experiments with small variations in precipitation (within the observational uncertainty) and sea surface salinity restoring in the subarctic Atlantic produce a wide range of AMOC transports, between upward drifts to more than 22 Sv and nearly-collapsed states with less than 7 Sv, reflecting the excessive role of the salinity feedback in such simulations. In all cases the AMOC is tightly related to the density of the Denmark Strait overflow; changes in that density are governed by the salinity in the Nordic Seas; and in turn, that salinity is strongly affected by the properties of the inflowing North Atlantic water.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1016/j.ocemod.2013.05.004
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Prevalence of strong bottom currents in the greater Agulhas system (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (9). pp. 1772-1776.
    Details
    Publication Date: 2017-05-24
    Description: Deep current meter data and output from two high-resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater Agulhas Current system. The two models and current meter data are remarkably consistent, showing that benthic storms, with bottom currents greater than 0.2 m s(-1), occur throughout the Agulhas retroflection region south of Africa more than 20% of the time. Furthermore, beneath the mean Agulhas Current core and the retroflection front, bottom currents exceed 0.2 m s(-1) more than 50% of the time, while away from strong surface currents, bottom currents rarely exceed 0.2 m s(-1). Implications for sediment transport are discussed and the results are compared to atmospheric storms. Benthic storms of this strength (0.2 m s(-1)) are comparable to a 9 m s(-1) (Beaufort 5) windstorm, but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/grl.50400
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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