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  • 1
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    How predictable are the Arctic and North Atlantic Oscillations? Exploring the Variability and Predictability of the Northern Hemisphere (2018)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 31 . pp. 997-1014.
    Details
    Publication Date: 2021-02-08
    Description: The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Due to the strong connection of these patterns with surface climate, recent years have shown an increased interest and an increasing skill in forecasting them. However, it is unclear what the intrinsic limits of short-term predictability for the NAO and AO patterns are. This study compares the variability and predictability of both patterns, using a range of data and index computation methods for the daily NAO/AO indices. Small deviations from Gaussianity are found and characteristic decorrelation time scales of around one week. In the analysis of the Lyapunov spectrum it is found that predictability is not significantly different between the AO and NAO or between reanalysis products. Differences exist however between the indices based on EOF analysis, which exhibit predictability time scales around 12 - 16 days, and the station-based indices, exhibiting a longer predictability of 18 - 20 days. Both of these time scales indicate predictability beyond that currently obtained in ensemble prediction models for short-term predictability. Additional longer-term predictability for these patterns may be gained through local feedbacks and remote forcing mechanisms for particular atmospheric conditions.
    Type: Article , PeerReviewed
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    DOI: 10.1175/JCLI-D-17-0226.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Fates and Travel Times of Denmark Strait Overflow Water in the Irminger Basin* (2013)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 43 (12). pp. 2611-2628.
    Details
    Publication Date: 2021-06-17
    Description: The Denmark Strait Overflow (DSO) supplies about one-third of the North Atlantic Deep Water and is critical to global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete, however. The authors deploy over 10 000 Lagrangian particles at the Denmark Strait in a high-resolution ocean model to study these issues. First, the particle trajectories show that the mean position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger Basin to the Spill Jet section (65.25°N) in 5–7 days and to the Angmagssalik section (63.5°N) in 2–3 weeks. Second, the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the Angmagssalik section within 60 days (~25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing: to the southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, the overflow particles exhibit a wide range of densities.
    Type: Article , PeerReviewed
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    DOI: 10.1175/JPO-D-13-023.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Seasonal variability in warm-water inflow towards Kangerdlugssuaq Fjord (2017)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 47 (7). pp. 1685-1699.
    Details
    Publication Date: 2020-02-06
    Description: Seasonal variability in pathways of warm water masses toward the Kangerdlugssuaq Fjord-Glacier system (KF/KG), southeast Greenland, is investigated by backtracking Lagrangian particles seeded at the fjord mouth in a high-resolution regional ocean model simulation in the ice-free and the ice-covered seasons. The waters at KF are a mixture of Atlantic-origin water advected from the Irminger Basin (FF for Faxaflói), the deep waters from the Denmark Strait and the waters from the Arctic Ocean, both represented by the Kögur section (KO). Below 200m depth, the warm water is a mixture of FF and KO water masses, and is warmer in winter than in summer. We find that seasonal differences in pathways double the fraction of FF particles in winter, causing the seasonal warming and salinification. Seasonal temperature variations at the upstream sections (FF and KO) have a negligible impact on temperature variations near the fjord. Successful monitoring of heat flux to the fjord therefore needs to take place close to the fjord, and cannot be inferred from upstream conditions.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-16-0202.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model? (2019)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 49 (5). pp. 1141-1157.
    Details
    Publication Date: 2022-01-31
    Description: To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-18-0152.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Eddy diffusivity estimates from Lagrangian trajectories simulated with ocean models and surface drifter data - a case study for the greater Agulhas system (2018)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2023-01-31
    Description: The Lagrangian analysis of sets of particles advected with the flow fields of ocean models are used to study connectivity, i.e. exchange pathways, timescales and volume transports, between distinct oceanic regions. One important factor influencing the dispersion of fluid particles and hence connectivity is the Lagrangian eddy diffusivity, which quantifies the influence of turbulent processes on the rate of particle dispersal. Due to spatial and temporal discretization, turbulence is not fully resolved in modelled velocities, and the concept of eddy diffusivity is used to parametrize the impact of unresolved processes. However, the relations between observational- and model-based Lagrangian eddy diffusivity estimates as well as eddy parameterizations are not clear. This study presents an analysis of the spatially variable near-surface lateral eddy diffusivity estimates obtained from Lagrangian trajectories simulated with 5-day mean velocities from an eddy-resolving ocean model (INALT01) for the Agulhas system. INALT01 features diffusive regimes for dynamically different regions, some of which exhibit strong suppression of eddy mixing by mean flow, and is consistent with the pattern and magnitude of drifter-based eddy diffusivity estimates. Using monthly-mean velocities decreases the estimated diffusivities less than eddy kinetic energy, supporting the idea that large and persistent eddy features dominate eddy diffusivities. For a non-eddying ocean model (ORCA05), Lagrangian eddy diffusivities are greatly reduced, in particular when the Gent and McWilliams parameterization of mesoscale eddies is employed.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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