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  • Other Sources  (9)
  • Articles (OceanRep)  (9)
  • Springer  (6)
  • AMS (American Meteorological Society)  (3)
  • 2015-2019  (9)
  • 2015  (9)
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  • Other Sources  (9)
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  • Articles (OceanRep)  (9)
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  • 2015-2019  (9)
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  • 1
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    The deep equatorial ocean circulation in wind-forced numerical solutions (2015)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 45 . pp. 1709-1734.
    Details
    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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  • 2
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    Influence of the equatorial deep jets on the north equatorial countercurrent (2015)
    Springer
    In:  Ocean Dynamics, 65 . pp. 1095-1102.
    Details
    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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  • 3
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    A Divergence-Form Wave-Induced Pressure Inherent in the Extension of the Eliassen–Palm Theory to a Three-Dimensional Framework for All Waves at All Latitudes (2015)
    AMS (American Meteorological Society)
    In:  Journal of the Atmospheric Sciences, 72 (7). pp. 2822-2849.
    Details
    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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  • 4
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    Multiple timescales of stochastically forced North Atlantic Ocean variability: A model study (2015)
    Springer
    In:  Ocean Dynamics, 65 (9). pp. 1367-1381.
    Details
    Publication Date: 2019-01-23
    Description: The Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) are important elements in mechanisms for multidecadal variability in models in the North Atlantic Ocean. In this study, a 2000-year long global ocean model integration forced with the atmospheric patterns associated with a white noise North Atlantic Oscillation (NAO) index is shown to have three distinct timescales of North Atlantic Ocean variability. First, an interannual timescale with variability shorter than 15 years, that can be related to Ekman dynamics. Second, a multidecadal timescale, on the 15- to 65-year range, that is mainly concentrated in the SPG region and is controlled by constructive interference between density anomalies around the gyre and the changing NAO forcing. Finally, the centennial timescales, with variability longer than 65 years, that can be attributed to the ocean being in a series of quasi-equilibrium states. The relationship between the ocean’s response and the NAO index differs for each timescale; the 15-year and shorter timescales are directly related to the NAO of the same year, 15- to 65-year timescales are dependent on the NAO index in the last 25–30 years in a sinusoidal sense while the 65-year and longer timescales relate to a sum of the last 50–80 years of the NAO index.
    Type: Article , PeerReviewed
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  • 5
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    The East Asian Summer Monsoon in pacemaker experiments driven by ENSO (2015)
    Springer
    In:  Ocean Dynamics, 65 . pp. 385-393.
    Details
    Publication Date: 2017-04-13
    Description: The variability of the East Asian summer monsoon (EASM) is studied using a pacemaker technique driven by ENSO in an atmospheric general circulation model (AGCM) coupled to a slab mixed layer model. In the pacemaker experiments, sea surface temperature (SST) is constrained to observations in the eastern equatorial Pacific through a q-flux that measures the contribution of ocean dynamics to SST variability, while the AGCM is coupled to the slab model. An ensemble of pacemaker experiments is analyzed using a multivariate EOF analysis to identify the two major modes of variability of the EASM. The results show that the pacemaker experiments simulate a substantial amount (around 45 %) of the variability of the first mode (the Pacific-Japan pattern) in ERA40 from 1979 to 1999. Different from previous work, the pacemaker experiments also simulate a large part (25 %) of the variability of the second mode, related to rainfall variability over northern China. Furthermore, we find that the lower (850 hPa) and the upper (200 hPa) tropospheric circulation of the first mode display the same degree of reproducibility whereas only the lower part of the second mode is reproducible. The basis for the success of the pacemaker experiments is the ability of the experiments to reproduce the observed relationship between El Niño Southern Oscillation (ENSO) and the EASM.
    Type: Article , PeerReviewed
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  • 6
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    Interannual variability of wind power input to near-inertial motions in the North Atlantic (2015)
    Springer
    In:  Ocean Dynamics, 65 (6). pp. 859-875.
    Details
    Publication Date: 2019-09-24
    Description: Near-inertial oscillations are ubiquitous in the ocean and are believed to play an important role in the global climate system. Studies on wind power input to near-inertial motions (WPI) have so far focused primarily on estimating the time-mean WPI, with little attention being paid to its temporal variability. In this study, a combination of atmospheric reanalysis products, a high-resolution ocean model and linear regression models are used to investigate for the first time the relationship between interannual variability of WPI in the North Atlantic and the North Atlantic Oscillation (NAO), motivated by the idea that the NAO serves as a good indicator for storminess over the North Atlantic and that storms account for the majority of WPI. It is found that WPI at low and high latitudes of the North Atlantic is significantly correlated to the NAO, owing to its influence on the configuration of the storm track. Positive (negative) NAO conditions are associated with increased WPI in the subpolar (subtropical) ocean. Basin-wide WPI is found to be significantly enhanced under negative NAO conditions, but is not significantly different from the climatological average under positive NAO conditions. This indicates a weak inverse relationship between basin-wide WPI and the NAO, contradicting intuitive expectations. The asymmetric impact of the NAO on basin-wide WPI results from greater sensitivity of WPI to near-inertial wind forcing at lower latitudes due to the variation of the Coriolis parameter with latitude.
    Type: Article , PeerReviewed
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  • 7
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    On the extratropical influence of variations of the upper tropospheric equatorial zonal mean zonal wind during boreal winter (2015)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 28 (1). pp. 168-185.
    Details
    Publication Date: 2020-08-04
    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 Niño 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. The extratropical response to [U150]E is investigated using linear regressions of 500 hPa geopotential height onto the [U150]E time series. We make use of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts model in which the tropical atmosphere is relaxed towards reanalysis data. The regression analysis reveals that a shift of the Aleutian low and a wave train across the North Atlantic are associated with [U150]E. We find that the subtropical 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. Finally, we show that a linear combination of both [U150]E and the Quasi Biennial Oscillation in the lower stratosphere can explain the circulation anomalies of the anomalously cold European winter of 1962/63 when both were in an extreme easterly phase.
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  • 8
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    Tropical impact on the interannual variability and long-term trend of the Southern Annular Mode during austral summer from 1960/1961 to 2001/2002 (2015)
    Springer
    In:  Climate Dynamics, 44 (7-8). pp. 2215-2228.
    Details
    Publication Date: 2017-04-13
    Description: Tropical influence on the austral summer Southern Annular Mode (SAM) over the ERA-40 period 1960/1961–2001/2002 is investigated using (1) a partially coupled climate model (PCM) driven by observed wind stress and (2) a version of the ECMWF atmospheric model by means of a relaxation technique. We show that the tropical influence in the PCM is dominated by El Niño Southern Oscillation (ENSO) whereas the relaxation experiments suggest an additional influence independent of ENSO. In the observations, we find that the simultaneous influence of ENSO on the summer SAM was much stronger after 1979 than before, with the consequence that the ensemble mean of the PCM captures around 50 % of the interannual variance of the SAM after 1979 and less than 10 % before. Nevertheless, in the ensemble mean of the PCM, the relationship between ENSO and the summer SAM is stable throughout the whole period 1960/1961–2001/2002, and it is the individual ensemble members that exhibit a non-stationary relationship like that found in the observations. It follows that variability not related to the observed wind forcing used to drive the PCM is important for obscuring the ENSO/SAM relationship. The experiments using relaxation show that tropical forcing was important for both the interannual variability and the trend of the summer SAM, even before 1979. Adding the observed extratropical sea surface temperature and sea-ice (SSTSI) to the tropical relaxation runs improves the model performance, indicative of a positive feedback from extratropical SSTSI onto the SAM.
    Type: Article , PeerReviewed
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  • 9
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    The use of a flow field correction technique for alleviating the North Atlantic cold bias with application to the Kiel Climate Model (2015)
    Springer
    Details
    Publication Date: 2023-11-08
    Description: The North Atlantic cold bias, associated with the misplacement of the North Atlantic Current (NAC) and typically extending from the surface to 1000 m depth, is a common problem in coupled models that compromises model fidelity. We investigate the use of a flow field correction (FFC) to adjust the path of the NAC and alleviate the cold bias. The FFC consists of three steps. First, climatological potential temperature (T) and salinity (S) fields for use with the model are produced using a three-dimensional restoring technique. Second, these T, S fields are used to modify the momentum equations of the ocean model. In the third stage, the correction term is diagnosed to construct a flow-independent correction. Results using the Kiel Climate Model show that the FFC allows the establishment of a northwest corner, substantially alleviating the subsurface cold bias. 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. A model version in which only the surface fluxes of heat and freshwater are corrected continues to exhibit the incorrect path of the NAC and a strong subsurface bias. We also show that the bias in the atmospheric circulation is reduced in some corrected model versions. The FFC can be regarded as a way to correct for model error, e.g. associated with the deep water mass pathways and their impact on the large-scale ocean circulation, and unresolved processes such as eddy momentum flux convergence.
    Type: Article , PeerReviewed
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