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  • 1
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 70 (7). pp. 2103-2118.
    Publication Date: 2018-04-16
    Description: The wintertime northern annular mode (NAM) at the surface is known to undergo slow intraseasonal variations in association with stratospheric variability, which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratosphere–troposphere coupling mechanisms are not yet fully understood. In this study the relative roles of (i) the downward effect of the zonal-mean secondary circulation induced by quasigeostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) wave drag local to the troposphere are estimated. For this purpose, a spectral tendency equation of the QG zonal-mean zonal wind is derived and used, in a first step, to obtain the external mechanical forcing that, in the QG framework, drives exactly the observed stratospheric and tropospheric daily NAM. In a second step, the equation is then integrated in time to reconstruct the daily NAM, but with the forcing restricted to either stratospheric or tropospheric levels, each case leaving a characteristic NAM surface signal. The relative roles of the above-mentioned mechanisms are found to be of similar quantitative importance, but to differ in a qualitative sense. The downward effect of stratospheric QG adjustment is responsible for the initiation of the NAM surface signal, whereas subsequently local tropospheric wave drag actively maintains and persists the signal over several weeks. Furthermore, the downward effect of QG adjustment to stratospheric radiative damping is shown to have only a minor impact, compared to that from stratospheric wave drag. The robustness of these conclusions is demonstrated by a sensitivity study with respect to various model parameters.
    Type: Article , PeerReviewed
    Format: text
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  • 2
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    American Meteorological Society
    In:  Journal of the Atmospheric Sciences, 71 (6). pp. 2264-2279.
    Publication Date: 2018-04-16
    Description: The dynamical origin of the spectral and autocorrelation structure of annular variability in the troposphere is investigated by a deductive approach. Specifically, the structure of the power spectrum and autocorrelation function of the zonal-mean geopotential is analyzed for the case of a quasigeostrophic spherical atmosphere subject to a white noise mechanical forcing applied in a single Hough mode and concentrated at a particular level in the vertical, with vertically uniform Newtonian cooling and Rayleigh drag concentrated at a rigid lower boundary. Analytic expressions for the power spectrum are presented together with expressions for an approximate red noise (i.e., a Lorentzian-shaped) power spectrum. It is found that for an infinitely deep atmosphere the power spectrum can be well approximated by a red noise process for the first few Hough modes (associated with large Rossby heights), provided the distance from the forcing is not larger than about one Rossby height. When a frictional rigid lower boundary is included, however, the approximation is generally bad. The high-frequency part of the power spectrum exhibits near-exponential behavior and the autocorrelation function shows a transition from a rapid decay at short lags to a much slower decay at longer lags, if the thermal and mechanical damping time scales are sufficiently well separated. Since observed annular variability exhibits the same characteristics, the above results lead to the hypothesis that these characteristics may, to some extent, be intrinsic to the linear zonal-mean response problem—although the need for an additional contribution from eddy feedbacks is also implied by the results.
    Type: Article , PeerReviewed
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  • 3
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    Royal Meteorological Society
    In:  Quarterly Journal of the Royal Meteorological Society, 141 . pp. 153-165.
    Publication Date: 2017-04-13
    Description: A set of relaxation experiments using the ECMWF atmospheric model is used to analyse the severe European winter of 1962/63. We argue that the severe winter weather was associated with a wave train that originated in the tropical Pacific sector (where weak La Nina conditions were present) and was redirected towards Europe, a process we suggest was influenced by the combined effect of the strong easterly phase of the Quasi-Biennial Oscillation (QBO ) and unusually strong easterly winds in the upper equatorial troposphere that winter. A weak tendency towards negative North Atlantic Oscillation (NAO) conditions in December, associated with extratropical sea surface temperature and sea-ice anomalies, might have acted as a favourable preconditioning. The redirection of the wave train towards Europe culminated in the stratospheric sudden warming at the end of January 1963. We argue that in February, the sudden warming event helped maintain the negative NAO regime, allowing the severe weather to persist for a further month. A possible influence from the Madden-Julian Oscillation, as well as a role for internal atmospheric variability, is noted.
    Type: Article , PeerReviewed
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  • 4
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    In:  [Talk] In: SPARC DynVar/SNAP workshop, 22.-24.04.2013, Reading, UK .
    Publication Date: 2014-01-13
    Description: The wintertime northern annular mode (NAM) at the surface is known to undergo slow intra-seasonal variations in association with stratospheric variability which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratospheretroposphere coupling mechanisms are not yet fully understood. In this study the relative roles, of (i) the downward effect of the zonal-mean secondary circulation induced by quasi-geostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) of wave drag local to the troposphere, are estimated. For this purpose, a spectral tendency equation of the QG zonal-mean zonal wind is derived, and used, in a first step, to obtain the external mechanical forcing which, in the QG framework, drives exactly the observed stratospheric and tropospheric daily NAM. In a second step, the equation is then integrated in time to reconstruct the daily NAM, but with the forcing restricted to either stratospheric or tropospheric levels, each case leaving a characteristic NAM surface signal. The relative roles of the above-mentioned mechanisms are found to be of similar quantitative importance, but to differ in a qualitative sense. The downward effect of stratospheric QG adjustment is responsible for the initiation of the NAM surface signal, whereas subsequently local tropospheric wave drag actively maintains and persists the signal over several weeks. Furthermore, the downward effect of QG adjustment to stratospheric radiative damping is shown to have only a minor impact, compared to that from stratospheric wave drag. The robustness of these conclusions is demonstrated by a sensitivity study with respect to various model parameters.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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  • 5
    Publication Date: 2014-01-13
    Description: The dynamical origin of the spectral and auto-correlation structure of annular variability in the troposphere is investigated by a deductive approach. Specifically, the structure of the power spectrum and auto-correlation function of the zonal mean geopotential is analysed, for the case of a quasi-geostrophic spherical atmosphere subject to a white noise mechanical forcing applied in a single Hough mode and concentrated at a particular level in the vertical, with vertically uniform Newtonian cooling and Rayleigh drag concentrated at a rigid lower boundary. Analytic expressions for the power spectrum are presented together with expressions for an approximate red noise, i.e. a Lorentzian shaped power spectrum. It is found that for an infinitely deep atmosphere the power spectrum can be well approximated by a red noise process for the first few Hough modes (associated with large Rossby heights), provided the distance from the forcing is not larger than about one Rossby height. When a frictional rigid lower boundary is included, however, the approximation is generally bad. The high-frequency part of the power spectrum exhibits near exponential behaviour and the auto-correlation function shows a transition from a rapid decay at short lags to a much slower decay at longer lags. Since observed Northern Annular Mode variability exhibits the same characteristics, the above results lead to the hypothesis that these characteristics may be intrinsic to the linear zonal mean response problem—and may neither need to be explained by slow external forcings, nor by more advanced concepts like deterministic low-order chaos, as suggested in the literature.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
    Publication Date: 2019-06-06
    Description: Time series derived from paleoclimate archives are often irregularly sampled in time and thus not analysable using standard statistical methods such as correlation analyses. Although measures for the similarity between time series have been proposed for irregular time series, they do not account for the time scale dependency of the relationship. Stochastically distributed temporal sampling irregularities act qualitatively as a low-pass filter reducing the influence of fast variations from frequencies higher than about 0.5 (Δtmax) − 1, where Δtmax is the maximum time interval between observations. This may lead to overestimated correlations if the true correlation increases with time scale. Typically, correlations are underestimated due to a non-simultaneous sampling of time series. Here, we investigated different techniques to estimate time scale dependent correlations of weakly irregularly sampled time series, with a particular focus on different resampling methods and filters of varying complexity. The methods were tested on ensembles of synthetic time series that mimic the characteristics of Holocene marine sediment temperature proxy records. We found that a linear interpolation of the irregular time series onto a regular grid, followed by a simple Gaussian filter was the best approach to deal with the irregularity and account for the time scale dependence. This approach had both, minimal filter artefacts, particularly on short time scales, and a minimal loss of information due to filter length.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
    Format: application/pdf
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  • 7
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    Royal Meteorological Society
    In:  Quarterly Journal of the Royal Meteorological Society, 138 . pp. 1970-1982.
    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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  • 8
    Publication Date: 2009-10-01
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 9
    Publication Date: 2003-12-01
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 10
    Publication Date: 2014-06-01
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
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