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  • 2015-2019
  • 2010-2014  (27)
  • 2012  (27)
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  • 2015-2019
  • 2010-2014  (27)
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  • 1
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    Factors influencing Northern Hemisphere winter mean atmospheric circulation anomalies during the period 1960/61 to 2001/02 (2012)
    Royal Meteorological Society
    In:  Quarterly Journal of the Royal Meteorological Society, 138 . pp. 1970-1982.
    Details
    Publication Date: 2019-09-23
    Description: Influences from the Tropics, the stratosphere and the specification of observed sea surface temperature and sea-ice (SSTSI) on Northern Hemisphere winter mean circulation anomalies during the period 1960/61 to 2001/02 are studied using a relaxation technique applied to the ECMWF model. On interannual time-scales, the Tropics strongly influence the Pacific sector but also the North Atlantic sector, although weakly. The stratosphere is found to be influential on the North Atlantic Oscillation (NAO) on interannual time-scales but is less important over the Pacific sector. Adding the observed SSTSI to the tropical relaxation runs generally improves the model performance on interannual time-scales but degrades/enhances the model’s ability to capture the 42-year trend over the Pacific/Atlantic sector. While relaxing the stratosphere to the reanalysis fails to capture the trend over the whole 42-year period, the stratosphere is shown to be influential on the upward trend of the NAO index from 1965 to 1995, but with reduced amplitude compared to previous studies. Influence from the Tropics is found to be important for the trend over both time periods and over both sectors although, across all experiments, we can account for only 30% of the amplitude of the hemispheric trend. Copyright c� 2012 Royal Meteorological Society
    Type: Article , PeerReviewed
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  • 2
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    On the Driving Mechanism of the Annual Cycle of the Florida Current Transport (2012)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 42 (5). pp. 824-839.
    Details
    Publication Date: 2018-04-12
    Description: The mechanisms involved in setting the annual cycle of the Florida Current transport are revisited using an adjoint model approach. Adjoint sensitivities of the Florida Current transport to wind stress reproduce a realistic seasonal cycle with an amplitude of ~1.2 Sv (1 Sv ≡ 106 m3 s−1). The annual cycle is predominantly determined by wind stress forcing and related coastal upwelling (downwelling) north of the Florida Strait along the shelf off the North American coast. Fast barotropic waves propagate these anomalies southward and reach the Florida Strait within a month, causing an amplitude of ~1 Sv. Long baroclinic planetary Rossby waves originating from the interior are responsible for an amplitude of ~0.8 Sv but have a different phase. The sensitivities corresponding to the first baroclinic mode propagate westward and are highly influenced by topography. Considerable sensitivities are only found west of the Mid-Atlantic Ridge, with maximum values at the western shelf edge. The second baroclinic mode also has an impact on the Florida Current variability, but only when a mean flow is present. A second-mode wave train propagates southwestward from the ocean bottom on the western side of the Mid-Atlantic Ridge between ~36° and 46°N and at Flemish Cap, where the mean flow interacts with topography, to the surface. Other processes such as baroclinic waves along the shelf and local forcing within the Florida Strait are of minor importance.
    Type: Article , PeerReviewed
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  • 3
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    Thickness-weighted mean theory for the effect of surface gravity waves on mean flows in the upper ocean (2012)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 42 . pp. 725-747.
    Details
    Publication Date: 2018-04-12
    Description: The residual effect of surface gravity waves on mean flows in the upper ocean is investigated using thickness weighted mean (TWM) theory applied in a vertically Lagrangian and horizontally Eulerian coordinate system. Depth-dependent equations for the conservation of volume, momentum, and energy are derived. These equations allow for (i) finite amplitude fluid motions, (ii) the horizontal divergence of currents and (iii) a concise treatment of both the kinematic and viscous boundary conditions at the sea surface. Under the assumptions of steady and monochromatic waves and a uniform turbulent viscosity, the TWM momentum equations are used to illustrate the pressure- and viscosity-induced momentum fluxes through the surface that are implicit in previous studies of the wave-induced modification of the classical Ekman spiral problem. The TWM approach clarifies, in particular, the surface momentum flux associated with the so-called virtual wave stress of Longuet-Higgins. Overall the TWM framework can be regarded as an alternative to the three-dimensional Lagrangian mean framework of Pierson. Moreover the TWM framework can be used to include the residual effect of surface waves in large-scale circulation models. In specific models that carry the TWM velocity appropriate for advecting tracers as their velocity variable, the turbulent viscosity term should be modified so that the viscosity acts only on the Eulerian mean velocity.
    Type: Article , PeerReviewed
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  • 4
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    Thickness-weighted mean theory for surface gravity waves (2012)
    In:  [Talk] In: Department of Mathematics, University of Bergen, 18.06.2012, Bergen, Norway .
    Details
    Publication Date: 2012-07-10
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
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    Tropical impact on the East Asian winter monsoon (2012)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 39 (17). L17801.
    Details
    Publication Date: 2017-06-20
    Description: The tropical impact on the East Asian winter monsoon (EAWM) is examined in an ensemble of atmospheric general circulation model runs that use relaxation towards the ERA-40 reanalysis in the tropics for winters between 1960/61 and 2001/02 and performed with a recent version of the European Centre for Medium-Range Weather Forecasts model. 25% of the interannual variance of the EAWM can be reproduced in the ensemble mean by the model experiments with relaxation, even though the influence from ENSO appears to be weak. The implication is that there is the possibility of enhanced predictability for the EAWM resulting from improved forecast skill in the tropics as a whole. Prescribing observed sea surface temperature and sea ice without using relaxation cannot reproduce the interannual variability of the EAWM in our experiments, questioning the usefulness of uncoupled atmosphere models in this region, consistent with previous studies. Key Points: - Tropical impact on interannual variability of the East Asian winter monsoon. - Tropical influence and extratropical SST and sea-ice matter for the trend. - AGCMs driven only by observed SST and sea-ice give poor results.
    Type: Article , PeerReviewed
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  • 6
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    An analysis of trends in the boreal winter mean tropospheric circulation during the second half of the 20th century (2012)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 39 (L13809).
    Details
    Publication Date: 2019-07-09
    Description: A relaxation technique applied to the ECMWF model is used to analyse 11, 21 and 31 year trends in the boreal winter mean 500 hPa North Atlantic Oscillation (NAO), Pacific North America pattern (PNA) and Southern Annular Mode (SAM) indices. For the PNA, the results indicate a strong influence from the tropics on all time scales, whereas for the NAO, the stratosphere is important on time scales of 11 and 21 but with an indication of feedback from extratropical sea surface temperature and sea-ice (SSTSI) anomalies on the 11 year time scale. For the SAM, the tropics emerge as the most important influence. We find an influence from the stratosphere consistent with expectations based on ozone depletion, although no clear role for stratospheric forcing of the SAM is found in these experiments.
    Type: Article , PeerReviewed
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  • 7
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    Model initialization by partially coupled spin-up (Modini) (2012)
    In:  [Talk] In: MiKlip Module A Workshop, 08.02.2012, Hamburg, Germany .
    Details
    Publication Date: 2012-07-10
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  • 8
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    The atmospheric response to decadal ocean variability and its implications for predictability (ATMOS) (2012)
    In:  [Talk] In: MiKlip Module B Workshop, 13.02.2012, Berlin, Germany .
    Details
    Publication Date: 2012-07-10
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  • 9
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    Factors influencing northern hemisphere winter mean atmospheric circulation anomalies, 1960/61-2001/02 (2012)
    In:  [Talk] In: FB1-Seminar, 30.04.2012, Kiel, Germany .
    Details
    Publication Date: 2012-07-10
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
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    On the dynamics of the Gulf Stream Extension region (2012)
    In:  [Talk] In: Key Laboratory of Physical Oceanography, Ocean University of China, 21.05.2012, Qingdao, China .
    Details
    Publication Date: 2012-07-10
    Type: Conference or Workshop Item , NonPeerReviewed
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