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  • AGU (American Geophysical Union)  (49)
  • AMS (American Meteorological Society)  (20)
  • Kiel : Universitätsbibliothek Kiel  (2)
  • 11
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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
  • 12
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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
  • 13
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    Advective timescales and pathways of Agulhas leakage (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 40 (15). pp. 3997-4000.
    Details
    Publication Date: 2017-06-20
    Description: Current research indicates an increase in Agulhas leakage for the past and coming decades. This change potentially alters the strength of the Atlantic meridional overturning circulation, in particular, through advection of positive density anomalies into the North Atlantic. To explore the fate of Agulhas leakage, results from a Lagrangian analysis were evaluated, with virtual floats advected within an eddy-permitting ocean model (ORCA025). A considerable fraction of Agulhas leakage reached the subtropical North Atlantic: of a mean Agulhas leakage transport of 15.3 Sv entering the South Atlantic, 9.7, 7.7, and 6.1 Sv crossed sections at 6 degrees S, 6 degrees N, and 26 degrees N, respectively. The most probable transit time of leakage to reach the respective latitudes is one to two decades. We suggest that changes in Agulhas leakage could manifest in the Gulf Stream regime most probably within two decades. These results were supported by an eddy-resolving implementation of the ocean model (INALT01)
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/grl.50782
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 14
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    Agulhas Leakage Predominantly Responds to the Southern Hemisphere Westerlies (2013)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 43 (10). pp. 2113-2131.
    Details
    Publication Date: 2020-08-04
    Description: The Agulhas Current plays a crucial role in the thermohaline circulation through its leakage into the South Atlantic. Under both past and present climates, the trade winds and westerlies could have the ability to modulate the amount of Indian-Atlantic inflow. Compelling arguments have been put forward suggesting that trade winds alone have little impact on the magnitude of Agulhas leakage. Here, employing three ocean models for robust analysis – a global coarse resolution, a regional eddy-permitting and a nested high-resolution eddy-resolving configuration – and systematically altering the position and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown that the westerlies, in particular their intensity, control the leakage. Leakage responds proportionally to the westerlies intensity up to a certain point. Beyond this, through the adjustment of the large-scale circulation, energetic interactions occur between the Agulhas Return Current and the Antarctic Circumpolar Current that result in a state where leakage no longer increases. This adjustment takes place within 1 to 2 decades. Contrary to previous assertions, our results further show that an equatorward (poleward) shift in westerlies increases (decreases) leakage. This occurs due to the redistribution of momentum input by the winds. It is concluded that the reported present-day leakage increase could therefore reflect an unadjusted oceanic response mainly to the strengthening westerlies over the last few decades.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
    DOI: 10.1175/JPO-D-13-047.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 15
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    Pacific-to-Indian Ocean connectivity: Tasman leakage, Indonesian Throughflow, and the role of ENSO (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 119 (2). pp. 1365-1382.
    Details
    Publication Date: 2019-09-23
    Description: The upper ocean circulation of the Pacific and Indian Oceans is connected through both the Indonesian Throughflow north of Australia and the Tasman leakage around its south. The relative importance of these two pathways is examined using virtual Lagrangian particles in a high-resolution nested ocean model. The unprecedented combination of a long integration time within an eddy-permitting ocean model simulation allows the first assessment of the interannual variability of these pathways in a realistic setting. The mean Indonesian Throughflow, as diagnosed by the particles, is 14.3 Sv, considerably higher than the diagnosed average Tasman leakage of 4.2 Sv. The time series of Indonesian Throughflow agrees well with the Eulerian transport through the major Indonesian Passages, validating the Lagrangian approach using transport-tagged particles. While the Indonesian Throughflow is mainly associated with upper ocean pathways, the Tasman leakage is concentrated in the 400–900 m depth range at subtropical latitudes. Over the effective period considered (1968–1994), no apparent relationship is found between the Tasman leakage and Indonesian Throughflow. However, the Indonesian Throughflow transport correlates with ENSO. During strong La Niñas, more water of Southern Hemisphere origin flows through Makassar, Moluccas, Ombai, and Timor Straits, but less through Moluccas Strait. In general, each strait responds differently to ENSO, highlighting the complex nature of the ENSO-ITF interaction.
    Type: Article , PeerReviewed
    Format: text
    Format: video
    DOI: 10.1002/2013JC009525
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 16
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    Sea surface slope as a proxy for Agulhas Current strength (2010)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 37 (9). L09610.
    Details
    Publication Date: 2017-06-20
    Description: The linear relation between the strength of the Agulhas Current at nominal latitude 34°S and the gradient in sea level height anomaly across the current is investigated in a 1/10° resolution regional numerical ocean model. Our results show that the strength of the current can be estimated with reasonable accuracy using altimeter data, once it has been calibrated using in-situ transport measurements. Three years of transport measurements provide a calibration with worst-case correlation R = 0.78. In that case the errors in proxy transport have a standard deviation of 9.8 Sv, compared to a 20.2 Sv standard deviation of the transport time series itself. From these results we conclude that the design of the Agulhas Current Timeseries (ACT) experiment, a three-year deployment of moorings across the Agulhas Current and along a TOPEX/Jason altimeter ground track, will likely produce a good quality multi-decadal time series of Agulhas Current strength.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2010GL042847
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 17
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    On the sub-decadal variability of South Atlantic Antarctic Intermediate Water (2012)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 39 (10). L10605.
    Details
    Publication Date: 2017-06-20
    Description: Key Points: - New observations show westward propagating salinity anomalies in S.Atlantic AAIW - They are the dominant form of variability on subdecadal timescales in a model - They have a speed close to the speed of second baroclinic mode Rossby waves Variability of Antarctic Intermediate Water salinity in the South Atlantic is investigated on interannual and intradecadal timescales. Novel observations of slow, westward propagating salinity anomalies in Argo data are presented. The features have no corresponding signal in temperature and are anomalous in density. Analysis of 40 years of model output supports the existence of these westward propagating salinity anomalies and indicates that they are typical occurrences in time. The features are intensified in a latitude band around 30°S associated with the propagation of Agulhas rings. However, the features are much larger than Agulhas rings and occur on decadal timescales in the model. They propagate westward with speeds of 2.3 cm/s in observations, and 1.7 cm/s in model data. They are more consistent with planetary waves than with the advection of large-scale salinity anomalies. The observation of these features has implications for the interpretation of salinity anomalies, such as the linking of hydrological cycle changes to salinity changes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2012GL051270
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 18
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    The role of mesoscale eddies in the source regions of the Agulhas Current (1999)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 29 . pp. 2303-2317.
    Details
    Publication Date: 2020-08-04
    Description: A primitive equation model to study the dynamics of the Agulhas system has been developed. The model domain covers the South Atlantic and the south Indian Ocean with a resolution of ⅓° in the Agulhas region while coarser outside. It is driven by a climatology of the European Centre for Medium-Range Weather Forecasts. It is shown that the model simulates the Agulhas Current, its retroflection, and the ring shedding successfully. The model results show baroclinic anticyclonic eddies in the Mozambique Channel and east of Madagascar, which travel toward the northern Agulhas Current. After the eddies reach the current they are advected southward with the mean flow. Due to the limited numerical resolution only a few eddies reach the retroflection region without much modification. These eddies are responsible for drastic enhancement of the heat transfer from the Indian Ocean to the South Atlantic and lead to periodicities in the interoceanic heat transport of about 50 days superimposed on the seasonal variability. Combined satellite data from TOPEX/Poseidon and ERS-1 show that the observed vortices in the Mozambique Channel are comparable to those seen in the model. In contrast to this the simulated eddies east of Madagascar seem not to be well reproduced. Analyses of the energy conversion terms between the mean flow and the eddies suggest that barotropic instability plays an important role in the generation of Mozambique Channel eddies. For the generation of Agulhas rings and other eddy structures in the model the barotropic instability mechanism seems to be minor, and baroclinic instability mechanisms are more likely.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(1999)029〈2303:TROMEI〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 19
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    Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation (2010)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 23 (15). pp. 4243-4254.
    Details
    Publication Date: 2020-08-04
    Description: Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    Format: text
    DOI: 10.1175/2010JCLI3341.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 20
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    Decoupling of the Agulhas leakage from the Agulhas Current (2014)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 44 (7). pp. 1776-1797.
    Details
    Publication Date: 2018-04-12
    Description: The relationship between the Agulhas Current and the Agulhas leakage is not well understood. Here, this is investigated using two basin-scale and two global ocean models, of incrementally increasing resolution. The response of the Agulhas Current is evaluated under a series of sensitivity experiments, in which idealised anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean basin. The imposed wind stress changes exceed ±2 standard deviations from the annual mean trade winds and, in the case of intensification, are partially representative of recently observed trends. The Agulhas leakage is quantified using complimentary techniques based on Lagrangian virtual floats and Eulerian passive tracer flux. As resolution increases, model behavior converges and the sensitivity of the leakage to Agulhas Current transport anomalies is reduced. In the two eddy-resolving configurations tested, the leakage is insensitive to changes in Agulhas Current transport at 32°S, though substantial eddy kinetic energy anomalies are evident. Consistent with observations, the position of the retroflection remains stable. The decoupling of Agulhas Current variability from the Agulhas leakage suggests that, while correlations between the two may exist, they may not have a clear dynamical basis. It is suggested that present and future Agulhas leakage proxies be considered in the context of potentially transient forcing regimes.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/JPO-D-13-093.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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