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  • AGU (American Geophysical Union)  (22)
  • AMS (American Meteorological Society)  (10)
  • Nature Research  (1)
  • 2015-2019  (16)
  • 2005-2009  (17)
Document type
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Years
Year
  • 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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    Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean (2018)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 31 (10). pp. 4157-4174.
    Details
    Publication Date: 2021-02-08
    Description: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI-D-17-0654.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Decadal variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model (2018)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 48 (4). pp. 757-771.
    Details
    Publication Date: 2021-02-08
    Description: The Eddy Kinetic Energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production and the associated EKE, we investigate the decadal changes of EKE. We show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the Interdecadal Pacific Oscillation (IPO) lead to up- and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30–40 % of the density anomalies in the investigated region.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-17-0173.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 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
    Format: text
    DOI: 10.1002/2017JC013350
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin mode resonance (2016)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 46 (10). pp. 3011-3029.
    Details
    Publication Date: 2020-08-04
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semi-annual variability is also pronounced, despite weak forcing at that period. Here we use multi-year, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semi-annual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi-annual cycle by the 2nd mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semi-annual cycles of the respective dominant baroclinic modes are associated with characteristic basin-wide structures. Using an idealized linear reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e. square basin versus realistic coastlines) or forcing (i.e. spatially uniform versus spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/JPO-D-15-0248.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 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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    Format: video
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    DOI: 10.1002/2016GL069026
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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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
    Format: text
    Format: text
    DOI: 10.1002/2015GL066152
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Multidecadal Indian Ocean variability linked to the Pacific and implications for pre-conditioning Indian Ocean Dipole events (2017)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (5). pp. 1739-1751.
    Details
    Publication Date: 2020-02-06
    Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multi-decadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the 20th Century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multi-decadal variations associated with the Pacific Decadal Oscillation and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multi-decadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean Dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1175/JCLI-D-16-0200.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Uptake and spreading of anthropogenic trace gases in a eddy-permitting model of the Atlantic Ocean (2007)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 112 . C09017.
    Details
    Publication Date: 2018-04-19
    Description: An eddy-permitting circulation model of the Atlantic Ocean was used to study the effect of mesoscale processes on the uptake and spreading of anthropogenic CO2 and CFC-11. A comparison with a coarser-resolution model version shows anthropogenic tracer distributions with qualitatively similar patterns, but much more structure (e.g., stronger longitudinal gradients) in the eddy-permitting model, improving the agreement with observations. The better representation of the formation of water masses such as subpolar-mode water in the eddy-permitting model has an influence on the distribution of anthropogenic CO2 over density classes, but no influence on the total inventory taken up. In the subpolar Atlantic, the air-sea flux of CFC-11 is dominated by deep-water formation, while the air-sea flux of anthropogenic CO2 extends over a larger part of the subpolar gyre and has a clear association with North Atlantic surface currents. An in-depth analysis of the mechanisms shaping this distribution showed that the entrainment of water from below into the mixed layer determines the structure in the subpolar North Atlantic, whereas the temporal correlation between surface heat fluxes and mixed-layer depth is more important in the subtropical gyre. The northward, integrated heat and anthropogenic CO2 transports in midlatitudes are closely correlated on seasonal to interannual timescales. This has implications for using the ongoing monitoring arrays of the thermohaline circulation for estimation of the transport of anthropogenic CO2.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2006JC003966
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Lagrangian perspectives of deep water export from the subpolar North Atlantic (2006)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 33 (L21S08).
    Details
    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
    Format: text
    DOI: 10.1029/2006GL026470
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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