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  • OceanRep  (10)
  • AMS (American Meteorological Society)  (8)
  • Association for Computing Machinery  (2)
  • 2020-2024  (6)
  • 2010-2014  (4)
Document type
  • OceanRep  (10)
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  • 1
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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
  • 2
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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
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    DOI: 10.1175/2010JCLI3341.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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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
  • 4
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    The Submesoscale Kinetic Energy Cascade: Mesoscale Absorption of Submesoscale Mixed-Layer Eddies and Frontal Downscale Fluxes (2020)
    AMS (American Meteorological Society) | Wiley
    Details
    Publication Date: 2023-02-08
    Description: Mesoscale eddies can be strengthened by the absorption of submesoscale eddies resulting from mixed-layer baroclinic instabilities. This is shown for mesoscale eddies in the Agulhas Current system by investigating the kinetic energy cascade with a spectral and a coarse-graining approach in two model simulations of the Agulhas region. One simulation resolves mixed-layer baroclinic instabilities and one does not. When mixed-layer baroclinic instabilities are included, the largest submesoscale near-surface fluxes occur in winter-time in regions of strong mesoscale activity for upscale as well as downscale directions. The forward cascade at the smallest resolved scales occurs mainly in frontogenetic regions in the upper 30 m of the water column. In the Agulhas ring path, the forward cascade changes to an inverse cascade at a typical scale of mixed-layer eddies (15 km). At the same scale, the largest sources of the upscale flux occur. After the winter, the maximum of the upscale flux shifts to larger scales. Depending on the region, the kinetic energy reaches the mesoscales in spring or early summer aligned with the maximum of mesoscale kinetic energy. This indicates the importance of submesoscale flows for the mesoscale seasonal cycle. A case study shows that the underlying process is the mesoscale absorption of mixed-layer eddies. When mixed-layer baroclinic instabilities are not included in the simulation, the open-ocean upscale cascade in the Agulhas ring path is almost absent. This contributes to a 20 %-reduction of surface kinetic energy at mesoscales larger than 100 km when submesoscale dynamics are not resolved by the model.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Pacific Ocean Contribution to the Asymmetry in Eastern Indian Ocean Variability (2013)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2022-03-10
    Description: Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean Dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    VoCC: Vortex Correlation Clustering Based on Masked Hough Transformation in Spatial Databases (2023)
    Association for Computing Machinery
    Details
    Publication Date: 2023-09-28
    Description: A special focus in data mining is to identify agglomerations of data points in spatial or spatio-temporal databases. Multiple applications have been presented to make use of such clustering algorithms. However, applications exist, where not only dense areas have to be identified, but also requirements regarding the correlation of the cluster to a specific shape must be met, i.e. circles. This is the case for eddy detection in marine science, where eddies are not only specified by their density, but also their circular-shaped rotation. Traditional clustering algorithms lack the ability to take such aspects into account. In this paper, we introduce Vortex Correlation Clustering which aims to identify those correlated groups of objects oriented along a vortex. This can be achieved by adapting the Circle Hough Transformation, already known from image analysis. The presented adaptations not only allow to cluster objects depending on their location next to each other, but also allows to take the orientation of individual objects into considerations. This allows for a more precise clustering of objects. A multi-step approach allows to analyze and aggregate cluster candidates, to also include final clusters, which do not perfectly satisfy the shape condition. We evaluate our approach upon a real world application, to cluster particle simulations composing such shapes. Our approach outperforms comparable methods of clustering for this application both in terms of effectiveness and efficiency.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Conference paper
  • 7
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    Depth structure of Ningaloo Niño/Niña events and associated drivers (2021)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2024-08-16
    Description: Marine heatwaves along the coast ofWestern Australia, referred to as Ningaloo Niño, have had dramatic impacts on the ecosystem in the recent decade. A number of local and remote forcing mechanisms have been put forward, however little is known about the depth structure of such temperature extremes. Utilizing an eddy-active global Ocean General Circulation Model, Ningaloo Niño and the corresponding cold Ningaloo Niña events are investigated between 1958-2016, with focus on their depth structure. The relative roles of buoyancy and wind forcing are inferred from sensitivity experiments. Composites reveal a strong symmetry between cold and warm events in their vertical structure and associated large-scale spatial patterns. Temperature anomalies are largest at the surface, where buoyancy forcing is dominant and extend down to 300m depth (or deeper), with wind forcing being the main driver. Large-scale subsurface anomalies arise from a vertical modulation of the thermocline, extending from the western Pacific into the tropical eastern Indian Ocean. The strongest Ningaloo Niños in 2000 and 2011 are unprecedented compound events, where long-lasting high temperatures are accompanied by extreme freshening, which emerges in association with La Niñas, more common and persistent during the negative phase of the Interdecadal Pacific Oscillation. It is shown that Ningaloo Niños during La Nina phases have a distinctively deeper reach and are associated with a strengthening of the Leeuwin Current, while events during El Niño are limited to the surface layer temperatures, likely driven by local atmosphere-ocean feedbacks, without a clear imprint on salinity and velocity.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Atlantic meridional overturning response to increased Southern Ocean wind stress in a climate model with an eddy-rich ocean (2024)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2024-08-20
    Description: The Southern Hemisphere westerly winds experienced significant changes over recent decades and are projected to further strengthen, altering ocean hydrography and dynamics. While anomalies of Southern Hemisphere origin are hypothesised to impact the Atlantic Meridional Overturning Circulation (AMOC), many details of this relationship remain unknown as previous modelling studies are limited by the application of forced, coarse resolution ocean models, or a short integration length. Here a coupled, nested climate model configuration, covering the Atlantic Ocean at an eddy-rich 1/10° resolution is applied to study the adjustment of the large-scale circulation to a 30% increase of the Southern Ocean wind stress. The AMOC responds to the stronger wind stress with a strengthening of 0.6 to 1.4 Sv across the entire Atlantic after approximately 80 years. At that time, anomalous watermass transformation mainly occurs at the entry into the Nordic Seas. A density anomaly in the overflow water then induces anomalous sinking in the eastern subpolar gyre, providing a link between AMOC changes in density and depth coordinates. Our study suggests that these watermass changes are caused by northward propagating anomalies and provides a detailed hypothesis for the link between the Indian Ocean inflow via Agulhas leakage and the AMOC. Nevertheless, due to coupled ocean-atmosphere adjustments, anomalies do not simply follow the main volume transport pathways along the western boundary. Mixing between advectively transported and locally forced anomalies leads to an increasingly complex evolution of watermass anomalies and less certainty in the relative contributions of involved mechanisms towards subpolar latitudes.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    SUSTeR: Sparse Unstructured Spatio Temporal Reconstruction on Traffic Prediction (2023)
    Association for Computing Machinery
    Details
    Publication Date: 2024-02-05
    Description: Mining spatio-temporal correlation patterns for traffic prediction is a well-studied field. However, most approaches are based on the assumption of the availability of and accessibility to a sufficiently dense data source, which is rather the rare case in reality. Traffic sensors in road networks are generally highly sparse in their distribution: fleet-based traffic sensing is sparse in space but also sparse in time. There are also other traffic application, besides road traffic, like moving objects in the marine space, where observations are sparsely and arbitrarily distributed in space. In this paper, we tackle the problem of traffic prediction on sparse and spatially irregular and non-deterministic traffic observations. We draw a border between imputations and this work as we consider high sparsity rates and no fixed sensor locations. We advance correlation mining methods with a Sparse Unstructured Spatio Temporal Reconstruction (SUSTeR) framework that reconstructs traffic states from sparse non-stationary observations. For the prediction the framework creates a hidden context traffic state which is enriched in a residual fashion with each observation. Such an assimilated hidden traffic state can be used by existing traffic prediction methods to predict future traffic states. We query these states with query locations from the spatial domain.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Conference paper
  • 10
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    Vertical Velocity Dynamics in the North Atlantic and Implications for AMOC (2024)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2024-09-16
    Description: The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally integrated transport either in depth space or in density space. While this view has the advantage of simplicity, it obscures the rich and complex three-dimensional structure, so that the exact physics of the downwelling and upwelling branch remains poorly understood. The near-equivalence of the depth- and density-space MOC in the subtropics suggests that vertical and diapycnal volumes transports are intimately coupled, whereas the divergence of these two metrics at higher latitudes indicates that any such coupling is neither instantaneous nor local. Previous work has characterized the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we develop a new analytical decomposition of vertical volume transport based on the vorticity budget. We show that most terms can be estimated from observations and provide additional insights from a high-resolution numerical simulation of the North Atlantic. Our analysis highlights the roles of 1) relative vorticity advection for the sinking of overflow water at the northern subpolar North Atlantic boundaries and 2) the geostrophic β effect for the sinking of dense waters in the intergyre region. These results provide insights into the coupling between density- and depth-space overturning circulations. Significance Statement The purpose of this study is to better understand where and why dense water sinks in the North Atlantic. This is important because dense water sinking in the North Atlantic is a crucial component of the global thermohaline circulation. Our results reveal the primary controls on dense water sinking at a regional level and highlight the importance of mesoscale processes at high latitudes in shaping the circulation and heat distribution throughout the Atlantic Ocean.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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