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  • 1
    Publication Date: 2018-02-19
    Description: Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high-resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s-decline, of about 15% of the long-term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.
    Type: Article , PeerReviewed
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  • 2
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    AGU
    In:  Geophysical Research Letters, 33 (L21S08).
    Publication Date: 2018-02-19
    Description: Direct observations at the Grand Banks have raised a quandary concerning the pathways of the lower branch of the meridional overturning circulation: In contrast to moored current meters that depict an intense, narrow Deep Western Boundary Current (DWBC), observations using different float types failed to show this continuous export path. Here, this issue is addressed by a Lagrangian analysis of synthetic particles in an eddy-resolving circulation model. Due to intense eddy activity around the Grand Banks, about 40% of the deep water in the DWBC is diverted into the interior, spreading southward along the western flank of the Mid-Atlantic Ridge or with the eddying flow field in the basin interior. Imposing constraints on the vertical displacements of particles similar to those experienced by observational floats further reduces their adherence to the DWBC, particularly near the southern tip of the Grand Banks.
    Type: Article , PeerReviewed
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  • 3
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    AGU
    In:  Geophysical Research Letters, 32 . L09602.
    Publication Date: 2018-03-28
    Description: Changes of the meridional overturning circulation (MOC) due to surface heat flux variability related to the North Atlantic Oscillation (NAO) are analyzed in various ocean models, i.e., eddying and non‐eddying cases. A prime signature of the forcing is variability of the winter‐time convection in the Labrador Sea. The associated changes in the strength of the MOC near the subpolar front (45°N) are closely related to the NAO‐index, leading MOC anomalies by about 2–3 years in both the eddying and non‐eddying simulation. Further south the speed of the meridional signal propagation depends on model resolution. With lower resolution (non‐eddying case, 4/3° resolution) the MOC signal propagates equatorward with a mean speed of about 0.6 cm/s, similar as spreading rates of passive tracer anomalies. Eddy‐permitting experiments (1/3°) show a significantly faster propagation, with speeds corresponding to boundary waves, thus leading to an almost in‐phase variation of the MOC transport over the subtropical to subpolar North Atlantic.
    Type: Article , PeerReviewed
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  • 4
    Publication Date: 2017-11-08
    Description: The quantification of inter-ocean leakage from the South Indian to the South Atlantic Ocean is an important measure for the role of the Agulhas system in the global thermohaline circulation. To explore the specific role of mesoscale variability (such as Agulhas rings and Mozambique eddies) in this process a high-resolution model (based on NEMO-ORCA) for the Agulhas region has been set up. It is nested into a global coarse-resolution model. The high-resolution nest captures all salient features of the greater Agulhas region, including the upstream perturbations of the Agulhas Current and Natal Pulses along the African coast. A comparison of the inter-ocean exchange in the high-resolution nest with its coarse resolution counterpart reveals that the latter significantly over-estimates the amount of water flowing into the Atlantic Ocean, demonstrating the need to explicitly simulate the mesoscale features. A sensitivity experiment that excludes the upstream perturbations revealed no difference in the amount of inter-ocean exchange. However, the realistic representation of Agulhas rings and their drift path into the South Atlantic depends on the simulation of those upstream perturbations.
    Type: Article , PeerReviewed
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  • 5
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    AGU
    In:  In: Ocean Circulation: Mechanisms and Impacts - Past and Future Changes of the Ocean's Meridional Overturning. , ed. by Schmittner, A., Chiang, J. and Hemming, S. AGU Monograph, 173 . AGU, Washington D.C., pp. 149-166. ISBN 978-0-87590-438-2
    Publication Date: 2019-09-23
    Type: Book chapter , PeerReviewed
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  • 6
    Publication Date: 2019-09-23
    Description: The Leeuwin Current, a warm, poleward flowing eastern boundary current, dominates the surface circulation off the west coast of Australia and has profound influence on regional marine ecosystem and fisheries recruitment. In this study, the seasonal and interannual variations of upper ocean heat balance in the Leeuwin Current region are analyzed by using an eddy-resolving numerical model simulation, as a first step to quantify the climate impacts on regional ocean thermodynamics and marine ecosystem. The volume transport and heat advection of the Leeuwin Current are stronger during the austral winter on the seasonal cycle and are stronger during a La Nina event on the interannual scale. On both seasonal and interannual timescales, the mixed layer heat budget off the west coast of Australia is predominantly balanced between the variations of the Leeuwin Current heat advection and heat flux across the air-sea interface. On the interannual timescale, the variation of the Leeuwin Current heat advection tends to lead that of the air-sea (latent) heat flux by two months, which is likely a reflection of advection timescales of the Leeuwin Current and its eddy field. The interannual variation of the average February–April sea surface temperature off the west coast of Australia, which is crucial for the larval settlement of western rock lobster, is mostly influenced by the Leeuwin Current heat advection as well as the ocean memory from the previous austral winter, with the air-sea heat exchange playing a buffering role.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2019-09-23
    Description: We compare total dissolved inorganic carbon (DIC) and chlorofluorocarbon (CFC) measurements in the northwest Atlantic made during the Transient Tracers in the Ocean, North Atlantic Study (TTO‐NAS) in 1981 with modern measurements from a cruise in 2004. The observed changes in the DIC and CFC fields are compared to those predicted from an eddy‐permitting ocean circulation model. The rapid, but time‐variable, atmospheric CFC increase in relation to the relatively steady anthropogenic CO2 increase influences the relationship between the observed uptake of DIC and CFC. We demonstrate the importance of ocean mixing in the calculation of anthropogenic CO2 (Cant) based on transient tracer data by comparing our observations to a “no‐mixing” scenario. We further find that the Cant is in transient steady state in the North Atlantic; that is, the Cant concentration increases proportionally over time through the whole water column in a manner that is directly related to the time‐dependent surface concentration.
    Type: Article , PeerReviewed
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