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  • Other Sources  (28)
  • Articles (OceanRep)  (28)
  • 2015-2019  (28)
  • 2015  (28)
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  • Other Sources  (28)
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  • Articles (OceanRep)  (28)
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  • 2015-2019  (28)
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  • 1
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    In:  [Talk] In: PIRATA-PREFACE-CLIVAR Tropical Atlantic Variability Conference, 25.-27.08.2015, Cape Town, South Africa .
    Publication Date: 2016-04-18
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 2
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    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 45 . pp. 1709-1734.
    Publication Date: 2021-05-18
    Description: We perform eddy-resolving and high-vertical-resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper ocean currents. Two main simulations are discussed: Solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform; and Solution 2, with realistic coastlines and with an annual cycle of wind forcing varying zonally. Somewhat surprisingly, Solution 1 produces the more realistic DEC: The large-vertical-scale currents (Equatorial Intermediate Currents or EICs) are found over a large zonal portion of the basin, and the small-vertical-scale equatorial currents (Equatorial Deep Jets or EDJs) form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. We demonstrate that both types of currents arise from the rectification of DEIV, consistent with previous theories. We also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In Solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, we find that the main reason for this weaker DEIV is the use of realistic coastlines in Solution 2. It remains to be determined, what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 3
    Publication Date: 2021-05-19
    Description: An ocean circulation model is run using two different idealized equatorial basin configurations under steady wind forcing. Both model versions produce bands of vertically alternating zonal flow at depth, similar to observed Equatorial Deep Jets (EDJs) and with a time scale corresponding to that of the gravest equatorial basin mode for the dominant baroclinic vertical normal mode. Both model runs show evidence for enhanced variability in the surface signature of the North Equatorial Counter Current (NECC) with the same time scale. We also find the same link between the observed NECC and the EDJs in the Atlantic by comparing the signature of the EDJ in moored zonal velocity data at 23° W on the equator with the signature of the NECC in geostrophic velocities from altimeter data. We argue that the presence of a peak in variability in the NECC associated with the EDJ basin mode period is evidence that the influenceatthis time scale is upward, from the EDJ to the NECC
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 4
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    In:  [Talk] In: 26. IUGG General Assembly, 22.6. - 2.7.2015, Prague, Czech Republic .
    Publication Date: 2015-07-07
    Description: The North Atlantic cold bias, associated with a too zonal path of the North Atlantic Current and a missing “northwest corner”, is a major problem in coupled models. It affects the North Atlantic Sector climate mean state, variability and predictability, as this severe model error is located in the North Atlantic storm track region. In the standard model version of the Kiel Climate Model (KCM), like in many other climate models, the surface heat flux is reversed in the northwest corner; the ocean gains heat, instead of releasing heat to the atmosphere as observed. The use of a Flow Field Correction (FFC) to adjust the path of the North Atlantic Current is investigated as well as additional corrections to the surface heat and freshwater fluxes. The FFC can be regarded as a means to correct for model error, e.g. associated with the deep water mass pathways and their impact on the circulation, and to parameterize unresolved processes such as eddy momentum flux convergence. The FFC does not depend on the state of the coupled model. Results show that the FFC allows a northward flow into the northwest corner, largely eliminating the subsurface bias in the KCM. A cold bias remains at the surface but can be eliminated by additionally correcting the surface freshwater flux, without adjusting the surface heat flux seen by the ocean model. Sea ice and convection occurs in more realistic positions in the corrected model versions, connected to a more northward extension of the Atlantic Meridional Overturning Circulation (AMOC). Using the corrected model versions, we explore the North Atlantic region climate variability with a focus on the AMOC and basin-wide North Atlantic sea surface temperature variability known as the Atlantic Multidecadal Oscillation or Variability (AMO/V).
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
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    In:  [Poster] In: PIRATA-PREFACE-CLIVAR Tropical Atlantic Variability Conference, 24.-28.08.2015, Cape Town, South Africa .
    Publication Date: 2015-10-06
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    In:  [Poster] In: Joint MiKlip/SPECS Meeting on Decadal Climate Prediction, 23.-26.02.2015, Offenbach .
    Publication Date: 2015-03-04
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  • 7
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    In:  [Poster] In: EGU General Assembly 2015, 12.–17.04.2015 , Vienna, Austria .
    Publication Date: 2015-04-23
    Description: Variations in the global tropospheric zonal mean zonal wind ([U]) during boreal winter are investigated using Rotated Empirical Orthogonal Functions applied to monthly means. The first two modes correspond to the Northern and Southern Annular Mode and modes 3 and 4 represent variability in the tropics. One is related to El Nino Southern Oscillation and the other has variability that is highly correlated with the time series of [U] at 150 hPa between 5 ◦ N and 5 ◦ S ([U150]e) and is related to activity of the Madden-Julian Oscillation (MJO). High amplitude of the MJO with strong precipitation anomalies over the western tropical Pacific (late MJO phases) are associated with the westerly phase of [U150]e (and vice versa). The extratropical response to [U150]e is investigated using linear regressions of 500 hPa geopotential height onto the [U150]e time series. Use is made of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts (ECMWF) model in which the tropical atmosphere is relaxed towards reanalysis data. Both the 45- year ECMWF reanalysis (ERA-40) and the ERA-Interim reanalysis data sets are used for the relaxation experiment as well as for the regression analysis. Therefore the analysis is covering 52 boreal winters from 1960/61 to 2012/13. The regression analysis reveals a robust shift of the Aleutian low and a wave train across the North Atlantic associated with [U150]e. It is found that the subtropical Rossby waveguides and the link between the North Pacific and North Atlantic are stronger during the easterly phase of [U150]e. The wave train over the North Atlantic is associated with Rossby wave sources over the subtropical North Pacific and North America.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Poster] In: Workshop on Energy transfers in Atmosphere and Ocean, 20.-22.04.2015, Hamburg, Germany .
    Publication Date: 2019-09-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
    Publication Date: 2020-10-21
    Description: Classical theory concerning theEliassen–Palmrelation is extended in this study to allowfor a unified treatment of midlatitude inertia–gravity waves (MIGWs), midlatitude Rossby waves (MRWs), and equatorial waves (EQWs). A conservation equation for what the authors call the impulse-bolus (IB) pseudomomentum is useful, because it is applicable to ageostrophic waves, and the associated three-dimensional flux is parallel to the direction of the group velocity of MRWs. The equation has previously been derived in an isentropic coordinate system or a shallow-water model. The authors make an explicit comparison of prognostic equations for the IB pseudomomentum vector and the classical energy-based (CE) pseudomomentum vector, assuming inviscid linear waves in a sufficiently weak mean flow, to provide a basis for the former quantity to be used in an Eulerian time-mean (EM) framework. The authors investigate what makes the three-dimensional fluxes in the IB and CE pseudomomentum equations look in different directions. It is found that the two fluxes are linked by a gauge transformation, previously unmentioned, associated with a divergence-form wave-induced pressure L. The quantity L vanishes for MIGWs and becomes nonzero for MRWs and EQWs, and it may be estimated using the virial theorem. Concerning the effect of waves on the mean flow, L represents an additional effect in the pressure gradient term of both (the three-dimensional versions of) the transformed EM momentum equations and the merged form of the EMmomentumequations, the latter of which is associated with the nonacceleration theorem.
    Type: Article , PeerReviewed
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  • 10
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    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 . pp. 5540-5546.
    Publication Date: 2017-04-10
    Description: We examine the impact of sea surface temperature (SST) bias on inter-annual variability during boreal summer over the equatorial Atlantic using two suites of partially coupled model (PCM) experiments with and without surface heat flux correction. In the experiments, surface wind stress anomalies are specified from observations while the thermodynamic coupling between the atmospheric and oceanic components is still active as in the fully coupled model. The results show that the PCM can capture around 50% of the observed variability associated with the Atlantic Niño from 1958 to 2013, but only when the bias is substantially reduced using heat flux correction, with no skill otherwise. We further show that ocean dynamics explain a large part of the SST variability in the eastern equatorial Atlantic in both observations (50-60%) and the PCM experiments (50-70%) with heat flux correction, implying that the seasonal predictability potential may be higher than currently thought.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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