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  • 1
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    Tropical origins of the severe European winter of 1962/1963 (2015)
    JOHN WILEY & SONS LTD
    In:  EPIC3Quarterly Journal of the Royal Meteorological Society, JOHN WILEY & SONS LTD, 141(686), pp. 153-165, ISSN: 0035-9009
    Details
    Publication Date: 2017-05-30
    Description: A set of relaxation experiments using the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model is used to analyze the severe European winter of 1962/1963. We argue that the severe winter weather was associated with a wave train that originated in the tropical Pacific sector (where weak La Ni˜na 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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
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    Diagnosing the origin and predictability of extratropical circulation anomalies (2011)
    In:  EPIC3EGU General Assembly, 4.4.2011, Vienna, Austria.
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    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 3
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    An analysis of trends in the boreal winter mean tropospheric circulation during the second half of the 20th century (2012)
    AMER GEOPHYSICAL UNION
    In:  EPIC3Geophysical Research Letters, AMER GEOPHYSICAL UNION, 39(L13809), ISSN: 0094-8276
    Details
    Publication Date: 2019-07-17
    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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 4
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    Tropical impact on the East-Asian winter monsoon (2012)
    In:  EPIC3Geophys. Res. Lett., 39(L17801)
    Details
    Publication Date: 2019-07-17
    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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 5
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    Factors influencing Northern Hemisphere winter mean atmospheric circulation anomalies during the period 1960/61 to 2001/02 (2012)
    JOHN WILEY & SONS LTD
    In:  EPIC3Quarterly Journal of the Royal Meteorological Society, JOHN WILEY & SONS LTD, 138(669), pp. 1970-1982, ISSN: 0035-9009
    Details
    Publication Date: 2019-07-17
    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.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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