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  • AGU (American Geophysical Union)  (45)
  • Copernicus Publications (EGU)  (9)
  • BMBF / IFM-GEOMAR  (1)
  • 11
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    On the propagation and decay of North Brazil Current rings (2010)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 115 (C10). C10004.
    Details
    Publication Date: 2019-09-23
    Description: Near the western boundary of the tropical North Atlantic, where the North Brazil Current (NBC) retroflects into the North Equatorial Countercurrent, large anticyclonic rings are shed. After separating from the retroflection region, the so-called NBC rings travel northwestward along the Brazilian coast, until they reach the island chain of the Lesser Antilles and disintegrate. These rings contribute substantially to the upper limb return flow of the Atlantic Meridional Overturning Circulation by carrying South Atlantic Water into the northern subtropical gyre. Their relevance for the northward transport of South Atlantic Water depends on the frequency of their generation as well as on their horizontal and vertical structure. The ring shedding and propagation and the complex interaction of the rings with the Lesser Antilles are investigated in the inline equation Family of Linked Atlantic Model Experiments (FLAME) model. The ring properties simulated in FLAME reach the upper limit of the observed rings in diameter and agree with recent observations on seasonal variability, which indicates a maximum shedding during the first half of the year. When the rings reach the shallow topography of the Lesser Antilles, they are trapped by the island triangle of St. Lucia, Barbados and Tobago and interact with the island chain. The model provides a resolution that is capable of resolving the complex topographic conditions at the islands and illuminates various possible fates for the water contained in the rings. It also reproduces laboratory experiments that indicate that both cyclones and anticyclones are formed after a ring passes through a topographic gap. Trajectories of artificial floats, which were inserted into the modeled velocity field, are used to investigate the pathways of the ring cores and their fate after they encounter the Lesser Antilles. The majority of the floats entered the Caribbean, while the northward Atlantic pathway was found to be of minor importance. No prominent pathway was found east of Barbados, where a ring could avoid the interaction with the islands and migrate toward the northern Lesser Antilles undisturbed.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2009JC006042
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 12
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    Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study (2008)
    Copernicus Publications (EGU)
    In:  Ocean Science, 4 (1). pp. 73-88.
    Details
    Publication Date: 2015-02-26
    Description: A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.5194/os-4-73-2008
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 13
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    Variability in subtropical-tropical cells drives oxygen levels in the tropical Pacific Ocean (2014)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 41 (24). pp. 8926-8934.
    Details
    Publication Date: 2019-09-23
    Description: Previous studies found a negative trend in oxygen concentrations in tropical regions during the last decades. Employing a biogeochemical circulation model, we highlight the importance of wind driven ocean transport associated with the Subtropical-Tropical Cells (STCs) in setting the oxygen levels in the tropical ocean. The observed and simulated slowdown of the STCs by 30 percent from the 1960s to the 1990s caused a decrease in oxygen transport to the tropics. Transport of phosphate was similarly reduced, decreasing export production and respiration. The effects of physical transport and biological consumption partly compensate, damping oxygen interannual and decadal variability. Our results suggest that the observed residual oxygen trend in the tropical Pacific is mainly driven by changes in oxygen transport. Accordingly, the observed recent strengthening of the STCs leads us to expect a pause in the oxygen decrease or even an increase of tropical Pacific oxygen values in the near future.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1002/2014GL061774
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 14
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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
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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
  • 15
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    OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project (2016)
    Copernicus Publications (EGU)
    In:  Geoscientific Model Development, 9 (9). pp. 3231-3296.
    Details
    Publication Date: 2019-02-01
    Description: The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, sea-ice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, HighResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.5194/gmd-9-3231-2016
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 16
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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
  • 17
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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
  • 18
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    Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models (2017)
    Copernicus Publications (EGU)
    In:  Ocean Science Discussions .
    Details
    Publication Date: 2021-03-04
    Description: The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations.
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.5194/os-2017-24
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    OceanRep: Article in a Scientific Journal - without review
  • 19
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    The reversal of the multi-decadal trends of the equatorial Pacific easterly winds, and the Indonesian Throughflow and Leeuwin Current transports (2011)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 38 . L11604.
    Details
    Publication Date: 2017-06-20
    Description: Multi-decadal weakening trend of the equatorial Pacific easterly winds since 1960 has reversed after 1993. The trend reversal has induced cooling (shallow thermocline) trend in the equatorial western Pacific before 1993, followed by a warming (deep thermocline) trend from 1993 to the present. All available atmospheric reanalysis products corroborate the trend reversal during the two multi-decadal periods. The magnitudes of the multi-decadal trends of the easterly winds, however, differ among the reanalysis products. The trend reversals of regional ocean circulations are assessed using linear regressions between wind and transport anomalies in an eddy-permitting numerical model, suggesting that since 1993 the Indonesian Throughflow and the Leeuwin Current transports have also reversed their multi-decadal weakening trends. Key Points: - There have been reversals of the multi-decadal weakening trends of trade winds - Different reanalysis products capture different trends in trade winds
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1029/2011GL047291
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 20
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    Contribution of Pacific wind stress to multi-decadal variations in upper-ocean heat content and sea level in the tropical south Indian Ocean (2011)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 38 . L12602.
    Details
    Publication Date: 2019-09-23
    Description: Reconstructions of the spatial pattern of recent multi-decadal sea level trends in the Indian Ocean (IO) indicate a zonally-extended band in the southern tropics where sea level has substantially fallen between the 1960s and 1990s; the decline is consistent with the observed subsurface cooling associated with a shoaling thermocline in this region. Here the origin and spatio-temporal characteristics of these trends are elucidated by a sequence of ocean model simulations. Whereas interannual variability in the southwestern tropical IO appears mainly governed by IO atmospheric forcing, longer term changes in the south tropical IO involve a strong contribution from the western Pacific via wave transmission of thermocline anomalies through the Indonesian Archipelago, and their subsequent westward propagation by baroclinic Rossby waves. The late 20th-century IO subsurface cooling trend reversed in the 1990s, reflecting the major regime shift in the tropical Pacific easterlies associated with the Pacific Decadal Oscillation.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1029/2011GL047651
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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