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  • 1
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    Composition and variability of the Denmark Strait Overflow Water in a high-resolution numerical model hindcast simulation (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 122 (4). 2830-2846 .
    Details
    Publication Date: 2020-02-06
    Description: The upstream sources and pathways of the Denmark Strait Overflow Water and their variability have been investigated using a high-resolution model hindcast. This global simulation covers the period from 1948 to 2009 and uses a fine model mesh (1/20°) to resolve mesoscale features and the complex current structure north of Iceland explicitly. The three sources of the Denmark Strait Overflow, the shelfbreak East Greenland Current (EGC), the separated EGC, and the North Icelandic Jet, have been analyzed using Eulerian and Lagrangian diagnostics. The shelfbreak EGC contributes the largest fraction in terms of volume and freshwater transport to the Denmark Strait Overflow and is the main driver of the overflow variability. The North Icelandic Jet contributes the densest water to the Denmark Strait Overflow and shows only small temporal transport variations. During summer, the net volume and freshwater transports to the south are reduced. On interannual time scales, these transports are highly correlated with the large-scale wind stress curl around Iceland and, to some extent, influenced by the North Atlantic Oscillation, with enhanced southward transports during positive phases. The Lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of Iceland, where water carried by the North Icelandic Irminger Current is transformed and feeds the North Icelandic Jet. Monitoring these two currents and the region north of the Iceland shelfbreak could provide the potential to track long-term changes in the Denmark Strait Overflow and thus also the AMOC.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 2
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    Flow paths and variability of the North Atlantic Current: A comparison of observations and a high-resolution model (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 122 (4). 2686-2708 .
    Details
    Publication Date: 2020-02-06
    Description: The North Atlantic Current (NAC) is subject to variability on multiannual to decadal time scales, influencing the transport of volume, heat, and freshwater from the subtropical to the eastern subpolar North Atlantic (NA). Current observational time series are either too short or too episodic to study the processes involved. Here we compare the observed continuous NAC transport time series at the western flank of the Mid-Atlantic Ridge (MAR) and repeat hydrographic measurements at the OVIDE line in the eastern Atlantic with the NAC transport and circulation in the high-resolution (1/20°) ocean model configuration VIKING20 (1960–2008). The modeled baroclinic NAC transport relative to 3400 m (24.5 ± 7.1 Sv) at the MAR is only slightly lower than the observed baroclinic mean of 27.4 ± 4.7 Sv from 1993 to 2008, and extends further north by about 0.5°. In the eastern Atlantic, the western NAC (WNAC) carries the bulk of the transport in the model, while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern NAC (ENAC). The model is able to simulate the main features of the subpolar NA, providing confidence to use the model output to analyze the influence of the North Atlantic Oscillation (NAO). Model based velocity composites reveal an enhanced NAC transport across the MAR of up to 6.7 Sv during positive NAO phases. Most of that signal (5.4 Sv) is added to the ENAC transport, while the transport of the WNAC was independent of the NAO.
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  • 3
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    Indian Ocean sources of Agulhas leakage (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 122 (4). pp. 3481-3499.
    Details
    Publication Date: 2020-02-06
    Description: We examine the mean pathways, transit timescales, and transformation of waters flowing from the Pacific and the marginal seas through the Indian Ocean (IO) on their way toward the South Atlantic within a high-resolution ocean/sea-ice model. The model fields are analyzed from a Lagrangian perspective where water volumes are tracked as they enter the IO. The IO contributes 12.6 Sv to Agulhas leakage, which within the model is 14.1 ± 2.2 Sv, the rest originates from the South Atlantic. The Indonesian Through-flow constitutes about half of the IO contribution, is surface bound, cools and salinificates as it leaves the basin within 10–30 years. Waters entering the IO south of Australia are at intermediate depths and maintain their temperature-salinity properties as they exit the basin within 15–35 years. Of these waters, the contribution from Tasman leakage is 1.4 Sv. The rest stem from recirculation from the frontal regions of the Southern Ocean. The marginal seas export 1.0 Sv into the Atlantic within 15–40 years, and the waters cool and freshen on-route. However, the model's simulation of waters from the Gulfs of Aden and Oman are too light and hence overly influenced by upper ocean circulations. In the Cape Basin, Agulhas leakage is well mixed. On-route, temperature-salinity transformations occur predominantly in the Arabian Sea and within the greater Agulhas Current region. Overall, the IO exports at least 7.9 Sv from the Pacific to the Atlantic, thereby quantifying the strength of the upper cell of the global conveyor belt.
    Type: Article , PeerReviewed
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  • 4
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    Seasonal Cycles of Oceanic Transports in the Eastern Subpolar North Atlantic (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 123 (2). pp. 1471-1484.
    Details
    Publication Date: 2021-02-08
    Description: The variability of the Atlantic Meridional Overturning Circulation (AMOC) may play a role in sea surface temperature predictions on seasonal to decadal time scales. Therefore, AMOC seasonal cycles are a potential baseline for interpreting predictions. Here we present estimates for the seasonal cycle of transports of volume, temperature, and freshwater associated with the upper limb of the AMOC in the eastern subpolar North Atlantic on the Extended Ellett Line hydrographic section between Scotland and Iceland. Due to weather, ship‐based observations are primarily in summer. Recent glider observations during other seasons present an opportunity to investigate the seasonal variability in the upper layer of the AMOC. First, we document a new method to quality control and merge ship, float, and glider hydrographic observations. This method accounts for the different spatial sampling rates of the three platforms. The merged observations are used to compute seasonal cycles of volume, temperature, and freshwater transports in the Rockall Trough. These estimates are similar to the seasonal cycles in two eddy‐resolving ocean models. Volume transport appears to be the primary factor modulating other Rockall Trough transports. Finally, we show that the weakest transports occur in summer, consistent with seasonal changes in the regional‐scale wind stress curl. Although the seasonal cycle is weak compared to other variability in this region, the amplitude of the seasonal cycle in the Rockall Trough, roughly 0.5–1 Sv about a mean of 3.4 Sv, may account for up to 7–14% of the heat flux between Scotland and Greenland.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 5
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    Seasonal variability of eddy kinetic energy in a global high-resolution ocean model (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (21). pp. 9379-9386.
    Details
    Publication Date: 2020-11-04
    Description: A global ocean model with 1/12∘ horizontal resolution is used to assess the seasonal cycle of surface Eddy Kinetic Energy (EKE). The model reproduces the salient features of the observed mean surface EKE, including amplitude and phase of its seasonal cycle in most parts of the ocean. In all subtropical gyres of the Pacific and Atlantic, EKE peaks in summer down to a depth of ∼350 m, below which the seasonal cycle is weak. Investigation of the possible driving mechanisms reveals the seasonal changes in the thermal interactions with the atmosphere to be the most likely cause of the summer maximum of EKE. The development of the seasonal thermocline in spring and summer is accompanied by stronger mesoscale variations in the horizontal temperature gradients near the surface which corresponds, by thermal wind balance, to an intensification of mesoscale velocity anomalies towards the surface.
    Type: Article , PeerReviewed
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  • 6
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    Variability and trends in Southern Ocean eddy activity in 1/12° ocean model simulations (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 43 . pp. 4517-4523.
    Details
    Publication Date: 2019-02-26
    Description: The response of eddy kinetic energy (EKE) to the strengthening of Southern Hemisphere winds occurring since the 1950s is investigated with a global ocean model having a resolution of 1/12° in the Antarctic Circumpolar Current domain. The simulations expose regional differences in the relative importance of stochastic and wind-related contributions to inter-annual EKE changes. In the Pacific and Indian sectors the model captures the EKE variability observed since 1993 and confirms previous hypotheses of a lagged response to regional wind stress anomalies. Here, the multi-decadal trend in wind stress is reflected in an increase in EKE typically exceeding 5 cm2 sec-2 decade-1. In the western Atlantic EKE variability is mostly stochastic, is weakly correlated with wind fluctuations, and its multi-decadal trends are close to zero. The non-uniform distribution of wind-related changes in the eddy activity could affect the regional patterns of ocean circulation and biogeochemical responses to future climate change.
    Type: Article , PeerReviewed
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  • 7
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    The Atlantic Subtropical Cells inferred from observations (2019)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2022-02-18
    Description: The Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres they are characterized by equatorward transport at thermocline level, upwelling at the equator and poleward Ekman transport in the surface layer. This study uses recent data from Argo oats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward ow and the thermocline equatorward ow. The latter is bounded by the 26.0 kg m‐3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ±1.1 Sv from the southern hemisphere compared to 2.9 ±1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ±3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ±0.8 Sv at 10°S and 6.4 ±1.4 Sv at 10°N). A diapycnal transport of about 4 Sv across the 26.0 kg m‐3 isopycnal is required in order to maintain the mass balance in the STC circulation.
    Type: Article , PeerReviewed
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  • 8
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    The Deep Western Boundary Current in the Labrador Sea From Observations and a High-Resolution Model (2018)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2022-03-09
    Description: Long‐term observations from a 17 year long mooring array at the exit of the Labrador Sea at 53°N are compared to the output of a high‐resolution model (VIKING20). Both are analyzed to define robust integral properties on basin and regional scale, which can be determined and evaluated equally well. While both, the observations and the model, show a narrow DWBC cyclonically engulfing the Labrador Sea, the model's boundary current system is more barotropic than in the observations and spectral analysis indicates stronger monthly to interannual transport variability. Compared to the model, the observations show a stronger density gradient, hence a stronger baroclinicity, from center to boundary. Despite this, the observed temporal evolution of the temperature in the central Labrador Sea is reproduced. The model results yield a mean export of North Atlantic Deep Water (NADW) (33.0 +/‐ 5.7 Sv), which is comparable to the observed transport (31.2 +/‐ 5.5 Sv) at 53°N. The results also include a comparable spatial pattern and March mixed layer depth in the central Labrador Sea (maximum depth ∼ 2000 m). During periods containing enhanced deep convection (1990's) our analyses show increased correlation between LSW and LNADW model transport at 53°N. Our results indicate that the transport variability in LSW and LNADW at 53°N is a result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 9
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    On the suitability of North Brazil Current transport estimates for monitoring basin-scale AMOC changes (2015)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-11-08
    Description: The North Brazil Current (NBC) constitutes a bottleneck for the mean northward return flow of the Atlantic Meridional Overturning Circulation (AMOC) in the tropical South Atlantic. Previous studies suggested a link between interannual to multidecadal NBC and AMOC transport variability and proposed to use NBC observations as an index for the AMOC. Here we use a set of hindcast, sensitivity, and perturbation experiments performed within a hierarchy of ocean general circulation models to show that decadal to multidecadal buoyancy-forced changes in the basin-scale AMOC transport indeed manifest themselves in the NBC. The relation is, however, masked by a strong interannual to decadal wind-driven gyre variability of the NBC. While questioning the NBC transport as a direct index for the AMOC, the results support its potential merit for an AMOC monitoring system, provided that the wind-driven circulation variability is properly accounted for.
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