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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
    Format: text
    Format: text
    DOI: 10.1002/2016JC012158
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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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
    Format: text
    DOI: 10.1002/2017JC013350
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    The impact of Agulhas leakage on the central water masses in the Benguela upwelling system from a high‐resolution ocean simulation (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 123 . pp. 9416-9428.
    Details
    Publication Date: 2021-02-08
    Description: We analyze the contribution of the Agulhas Current on the central water masses of the Benguela upwelling system (BUS) over the last decades in a high-resolution ocean simulation driven by atmospheric reanalysis. The BUS is an Eastern Boundary Upwelling System where upwelling of cold nutrient-rich water favors biomass growth. The two distinct subregions, North and South Benguela, differ in nutrient and oxygen properties of the upwelling water mass. Our analysis indicates that the contribution of Agulhas water to the upwelling is very strong in both subregions. Although the water masses feeding the upwelling have a common origin, their pathways are distinct in both regions. Whereas for the central waters of South Benguela the path is rather direct from where it is formed, the central waters of North Benguela takes a longer route through the equatorial current system. Not only the travel time from the Agulhas Current to the BUS is longer but also the central water mass is twice as old for the northern part when compared to the southern. Our analysis traces the pathways, history, and origin of the central water masses feeding upwelling in the BUS and emphasizes the direct impact of the Agulhas Current on the upwelling region. The variability of that link between the Indian Ocean and the South Atlantic is likely to change the nutrient and oxygen content, as well as temperature and salinity of the water masses in the upwelling region.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2018JC014218
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Ocean currents generate large footprints in marine palaeoclimate proxies (2015)
    Nature Publishing Group
    In:  Nature Communications, 6 (6521).
    Details
    Publication Date: 2019-02-25
    Description: Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones of palaeoclimatological studies. It is often assumed that the proxies derived from their shells represent ocean conditions above the location where they were deposited. Planktic foraminifera, however, are carried by ocean currents and, depending on the life traits of the species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean models to assess the footprint of planktic foraminifera and validate our method with proxy analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic conditions, may originate from areas up to several thousands of kilometres away, reflecting an ocean state significantly different from the core site. In the eastern equatorial regions and the western boundary current extensions, the offset may reach 1.5 °C for species living for a month and 3.0 °C for longer-living species. Oceanic transport hence appears to be a crucial aspect in the interpretation of proxy signals.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1038/ncomms7521
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation (2008)
    Nature Publishing Group
    In:  Nature, 456 . pp. 489-492.
    Details
    Publication Date: 2019-09-23
    Description: Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC)1. Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic2, 3 (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades4, 5. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of plusminus1.5–3 Sv (1 Sv = 106 m3 s-1) on decadal timescales in the subtropics6. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water7. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1038/nature07426
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Water column methanotrophy controlled by a rapid oceanographic switch (2015)
    Nature Publishing Group
    In:  Nature Geoscience, 8 (5). pp. 378-382.
    Details
    Publication Date: 2020-11-23
    Description: Large amounts of the greenhouse gas methane are released from the seabed to the water column1, where it may be consumed by aerobic methanotrophic bacteria2. The size and activity of methanotrophic communities, which determine the amount of methane consumed in the water column, are thought to be mainly controlled by nutrient and redox dynamics3–7. Here, we report repeated measurements of methanotrophic activity and community size at methane seeps west of Svalbard, and relate them to physical water mass properties and modelled ocean currents. We show that cold bottom water, which contained a large number of aerobic methanotrophs, was displaced by warmer water with a considerably smaller methanotrophic community within days. Ocean current simulations using a global ocean/sea-ice model suggest that this water mass exchange is consistent with short-term variations in the meandering West Spitsbergen Current. We conclude that the shift from an offshore to a nearshore position of the current can rapidly and severely reduce methanotrophic activity in the water column. Strong fluctuating currents are common at many methane seep systems globally, and we suggest that they affect methane oxidation in the water column at other sites, too.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1038/ngeo2420
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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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
    Format: text
    Format: text
    Format: video
    Format: video
    DOI: 10.1002/2016GL069026
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Atlantic multi-decadal oscillation covaries with Agulhas leakage (2015)
    Nature Publishing Group
    In:  Nature Communications, 6 (10082).
    Details
    Publication Date: 2019-02-25
    Description: The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlantic
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1038/ncomms10082
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Designing variable ocean model resolution based on the observed ocean variability (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Advances in Modeling Earth Systems, 8 (2). pp. 904-916.
    Details
    Publication Date: 2019-07-17
    Description: If unstructured meshes are refined to locally represent eddy dynamics in ocean circulation models, a practical question arises on how to vary the resolution and where to deploy the refinement. We propose to use the observed sea surface height variability as the refinement criterion. We explore the utility of this method (i) in a suite of idealized experiments simulating a wind-driven double gyre flow in a stratified circular basin and (ii) in simulations of global ocean circulation performed with FESOM. Two practical approaches of mesh refinement are compared. In the first approach the uniform refinement is confined within the areas where the observed variability exceeds a given threshold. In the second one the refinement varies linearly following the observed variability. The resolution is fixed in time. For the double gyre case it is shown that the variability obtained in a high-resolution reference run can be well captured on variable-resolution meshes if they are refined where the variability is high and additionally upstream the jet separation point. The second approach of mesh refinement proves to be more beneficial in terms of improvement downstream the midlatitude jet. Similarly, in global ocean simulations the mesh refinement based on the observed variability helps the model to simulate high variability at correct locations. The refinement also leads to a reduced bias in the upper-ocean temperature
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2016MS000650
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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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
    Format: text
    DOI: 10.1002/2016JC012676
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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