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  • Other Sources  (5)
  • AMS (American Meteorological Society)  (5)
  • 2015-2019  (5)
  • 2017  (3)
  • 2016  (2)
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  • Other Sources  (5)
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  • 2015-2019  (5)
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  • 1
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    The relationship between northern hemisphere winter blocking and tropical modes of variability (2017)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (22). pp. 9321-9337.
    Details
    Publication Date: 2020-08-04
    Description: In the present study, the influence of some major tropical modes of variability on northern hemisphere regional blocking frequency variability during boreal winter is investigated. Reanalysis data and an ensemble experiment with the ECMWF model using relaxation towards the ERA-Interim reanalysis data inside the tropics are used. The tropical modes under investigation are El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO) and the upper tropospheric equatorial zonal-mean zonal wind . An early (late) MJO phase refers to the part of the MJO cycle when enhanced (suppressed) precipitation occurs over the western Indian Ocean and suppressed (enhanced) precipitation occurs over the Maritime Continent and the western tropical Pacific. Over the North Pacific sector, it is found that enhanced (suppressed) high latitude blocking occurs in association with El Niño (La Niña) events, late (early) MJO phases and westerly (easterly) . Over central to southern Europe and the east Atlantic, it is found that late MJO phases, as well as a suppressed MJO are leading to enhanced blocking frequency. Furthermore, early (late) MJO phases are followed by blocking anomalies over the western North Atlantic region, similar to those associated with a positive (negative) North Atlantic Oscillation. Over northern Europe, the easterly (westerly) phase of is associated with enhanced (suppressed) blocking. These results are largely confirmed by both the reanalysis and the model experiment.
    Type: Article , PeerReviewed
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  • 2
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    Forcing of the Atlantic Equatorial Deep Jets derived from Observations (2016)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 46 (12). pp. 3549-3562.
    Details
    Publication Date: 2021-05-18
    Description: The Equatorial Deep Jets (EDJs) are an ubiquitous feature of the equatorial oceans; in the Atlantic Ocean, they are the dominant mode of interannual variability of the zonal flow at intermediate depth. On the basis of more than 10 years of moored observations of zonal velocity at 23°W, the vertically propagating EDJs are best described as superimposed oscillations of the 13th to the 23th baroclinic modes with a dominant oscillation period for all modes of 1650 days. This period is close to the resonance period of the respective gravest equatorial basin mode for the dominant vertical modes 16 and 17. It is argued that since the equatorial basin mode is composed of linear equatorial waves, a linear reduced gravity model can be employed for each baroclinic mode, driven by spatially homogeneous zonal forcing oscillating with the EDJ period. The fit of the model solutions to observations at 23°W yields a basin wide reconstruction of the EDJs and the associated vertical structure of their forcing. From the resulting vertical profile of mean power input and vertical energy flux on the equator, it follows that the EDJs are locally maintained over a considerable depth range, from 500-2500 m, with the maximum power input and vertical energy flux at 1300 m. The strong dissipation closely ties the apparent vertical propagation of energy to the vertical distribution of power input and, together with the EDJs’ prevailing downward phase propagation, require the phase of the forcing of the EDJs to propagate downward.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 3
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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
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  • 4
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    State-Dependence of Atmospheric Response to Extratropical North Pacific SST Anomalies (2017)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (2). pp. 509-525.
    Details
    Publication Date: 2020-10-22
    Description: By performing two sets of high-resolution atmospheric general circulation model (AGCM) experiments, we find that the atmospheric response to a sea surface temperature (SST) anomaly in the extratropical North Pacific is sensitive to decadal variations of the background SST on which the SST anomaly is superimposed. The response in the first set of experiments, in which the SST anomaly is superimposed on the observed daily SST of 1981-1990, strongly differs from the response in the second experiment, in which the same SST anomaly is superimposed on the observed daily SST of 1991-2000. The atmospheric response over the North Pacific during 1981-1990 is eddy-mediated, equivalent barotropic and concentrated in the east. In contrast, the atmospheric response during 1991-2000 is weaker and strongest in the west. The results are discussed in terms of Rossby wave dynamics, with the proposed primary wave source switching from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981-1990 to mean flow divergence over the western North Pacific in 1991-2000. The wave source changes are linked to the decadal reduction of daily SST variability over the eastern North Pacific and strengthening of the Oyashio Extension front over the western North Pacific. Thus, both daily and frontal aspects of the background SST variability in determining the atmospheric response to extratropical North Pacific SST anomalies are emphasized by our AGCM experiments.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 5
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    Evolution of the Atlantic Multidecadal Variability in a model with an improved North Atlantic Current (2017)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2023-11-08
    Description: This article investigates the dynamics and temporal evolution of the Atlantic Multidecadal Variability (AMV) in a coupled climate model. The model contains a correction to the North Atlantic flow field to improve the path of the North Atlantic Current, thereby alleviating the surface cold bias, a common problem with climate models, and offering a unique opportunity to study the AMV in a model. Changes in greenhouse gas forcing or aerosol loading are not considered. A striking feature of our results is the contrast between the western and eastern sides of the subpolar gyre in the model. On the western side, heat supply from the ocean plays a major role, with most of this heat being given up to the atmosphere in the warm phase, largely symmetrically about the time of the AMV maximum. By contrast, on the eastern side, the ocean gains heat from the atmosphere, with relatively little role for ocean heat supply in the years before the AMV maximum. Thereafter, the balance changes with heat now being removed from the eastern side by the ocean leading to a reducing ocean heat content, behavior we associate with the establishment of an intergyre gyre at the time of the AMV maximum. In the warm phase, melting sea-ice leads to a freshening of surface waters northeast of Greenland which travel southward into the Irminger and Labrador Sea, shutting down convection and terminating the AMV warm phase.
    Type: Article , PeerReviewed
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