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  • 1
    Publication Date: 2019-02-01
    Description: The Summer East Atlantic (SEA) mode is the second dominant mode of summer low-frequency variability in the Euro-Atlantic region. Using reanalysis data, we show that SEA-related circulation anomalies significantly influence temperatures and precipitation over Europe. We present evidence that part of the interannual SEA variability is forced by diabatic heating anomalies of opposing signs in the tropical Pacific and Caribbean that induce an extratropical Rossby wave train. This precipitation dipole is related to SST anomalies characteristic of the developing ENSO phases. Seasonal hindcast experiments forced with observed sea surface temperatures (SST) exhibit skill at capturing the interannual SEA variability corroborating the proposed mechanism and highlighting the possibility for improved prediction of boreal summer variability. Our results indicate that tropical forcing of the SEA likely played a role in the dynamics of the 2015 European heat wave.
    Type: Article , PeerReviewed
    Format: text
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  • 2
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    In:  [Talk] In: EGU General Assembly 2018, 08.-13.04.2018, Vienna, Austria .
    Publication Date: 2018-06-28
    Description: A large fraction of interannual variability of the East Asian Summer Monsoon (EASM) can be described by the first two modes of a Multivariate Empirical Orthogonal Functions (MV-EOF) analysis of horizontal wind vectors in the lower and upper troposphere over the East Asian region. The first mode resembles the Pacific-Japan pattern and represents about 20% of the EASM interannual variance. The positive phase of the PJ-pattern is associated with anomalous anticyclonic flow over the tropical western North Pacific in the lower troposphere, leading to enhanced rainfall over the climatological East Asian rain band. Focusing on June/July/August and on the first MV-EOF mode (PJ-mode here) we investigate the relation between tropical intraseasonal variability, namely the Madden-Julian Oscillation (MJO) and the EASM. The second MV-EOF mode had previously been found to be influenced mainly by the Indian Summer Monsoon and is not discussed in this presentation. First, it is found that the MJO modulates the intraseasonal variability of the PJ-pattern, in that early MJO phases, related to enhanced convective precipitation over the Indian Ocean, favour the positive phase of the PJ-mode and late MJO phases, related to enhanced convective precipitation over the Maritime Continent and the western tropical Pacific, favour the negative phase of the PJ-mode. Second, using a decomposition method introduced by Yoo, Feldstein and Lee (2011), we show that interannual variability of the MJO contributes about 11% to the interannual variability of the EASM. Thereby, interannual changes in the frequency of occurrence of the eight standard MJO phases are more important to the variability of the EASM than changes in the circulation patterns associated with the different MJO phases. Some discussion on the involved mechanisms will be given.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    In:  [Poster] In: MiKlip Status Seminar 2018, 28.- 30.05.2018, Berlin, Germany .
    Publication Date: 2018-10-02
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
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    In:  [Poster] In: MiKlip Status Seminar 2017, 28.-30.05.2018, Berlin, Germany .
    Publication Date: 2018-10-02
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2018-11-19
    Description: Experiments using atmosphere-only, as well as coupled forecast models, in which parts of the model atmosphere are constrained towards reanalysis products by relaxation are described. Such experiments have proved useful for determining remote influences, e.g. from the tropics or from the stratosphere, potentially useful for seasonal forecasting boreal winter over Europe. Such techniques can also be used for diagnosing remote influences important in the dynamics of a particular season, a good example being the extreme winter of 1962/63. An example is also given for the boreal summer East Atlantic pattern in which relaxation experiments fail to capture the appropriate influence from the tropics. Possible reasons for this will be given.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
    Publication Date: 2017-05-30
    Description: The phase and amplitude of the North Atlantic Oscillation (NAO) are influenced by numerous factors, including sea-surface temperature (SST) anomalies in both the Tropics and extratropics and stratospheric extreme events like stratospheric sudden warmings (SSWs). Analyzing seasonal forecast experiments, which cover the winters from 1979/1980–2013/2014, with the European Centre for Medium-Range Weather Forecast model, we investigate how these factors affect NAO variability and predictability. Building on the idea that tropical influence might happen via the stratosphere, special emphasis is placed on the role of major SSWs. Relaxation experiments are performed, where different regions of the atmosphere are relaxed towards ERA-Interim to obtain perfect forecasts in those regions. By comparing experiments with relaxation in the tropical atmosphere, performed with an atmosphere-only model on the one hand and a coupled atmosphere–ocean model version on the other, the importance of extratropical atmosphere–ocean interaction is addressed. Interannual variability of the NAO is best reproduced when perfect knowledge of the Northern Hemisphere (NH) stratosphere is available, together with perfect knowledge of SSTs and sea ice, in which case 64% of the variance of winter mean NAO is projected to be accounted for with a forecast ensemble of infinite size. The coupled experiment shows a strong bias in the stratospheric polar-night jet (PNJ), which might be associated with a drift in the modelled SSTs resembling the North Atlantic cold bias and an underestimation of blockings in the North Atlantic/Europe sector. Consistent with the stronger PNJ, the lowest frequency of major SSWs is found in this experiment. However, after removing the bias statistically, a perfect forecast of the tropical atmosphere and allowing two-way atmosphere–ocean coupling in the extratropics seem to be key ingredients for successful SSW predictions. In combination with SSW occurrence, a clear shift of the predicted NAO towards lower values occurs.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2016-04-15
    Description: We examine the interannual variability of the seasonal mean atmospheric circulation in the Southern Hemisphere during austral winter. The three major modes are identified by rotated empirical orthogonal function (REOF) analysis. As expected, REOF1 is associated with the Southern Annular Mode which is dominated by internal atmospheric dynamics. REOF2 displays a wave train, linked to the western North Pacific monsoon and the Pacific-Japan pattern in East Asia in the same season; REOF3 resembles the Pacific-South American pattern. Externally forced variability strongly projects on both REOF2 and REOF3 so that in the ensemble mean, an atmospheric model with prescribed observed sea surface temperature captures considerable parts of the time evolution of REOF2 (50%) and REOF3 (25%), suggesting a potential predictability for the two modes.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 8
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    In:  [Talk] In: EGU General Assembly 2013, 07.-12.04.2013, Vienna, Austria .
    Publication Date: 2014-06-16
    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 by the model experiments with relaxation in the ensemble mean, 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.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 9
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    In:  [Poster] In: Atmosphere and Ocean Dynamics: A Scientific Workshop to Celebrate Professor Dr Richard Greatbatch's 60th Birthday, 10.-11. 04.2014, Liverpool .
    Publication Date: 2014-10-09
    Description: Upper tropospheric zonal winds in the equatorial regions are to some extend associated with variability of the Madden-Julian Oscillation. Regression analysis for December to February reveals a Northern hemispheric extratropical influence via Rossby wave propagation. It is suggested by a simple linear regression model that an improved forecast of the zonal winds in the tropics, in troposphere and stratosphere, could lead to enhanced seasonal forecast skill of the extratropical tropospheric circulation.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
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    In:  (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 85 pp
    Publication Date: 2014-06-11
    Description: In this diploma thesis the interannual variability and trends of the winter mean tropospheric circulation in the northern hemisphere (NH) extratropics in winters from 1960/61 to 2001/02 are investigated. Output is analysed from a recent version of the atmospheric ECMWF model that has been used to perform various hindcast experiments, including experiments with selected regions of the atmosphere relaxed toward reanalysis data (ERA-40), i.e., the tropics and the stratosphere. The results are compared with the reanalysis data to examine the forecasting skill of the single experiments in the NH extratropics. It is found that the stratosphere is influential on the interannual variability of the North Atlantic Oscillation (NAO) related part of the winter tropospheric circulation variability in the North Atlantic sector (NAS), but less important for other modes of variability in the NAS or over the North Pacific sector (NPS). The influence of the stratosphere on the NAO is thought to be caused by the downward propagation of circulation anomalies, for example, caused by sudden stratospheric warmings. Relaxing the tropical atmosphere is influential for the general circulation variability in the NAS, although the influence on the NAO is somewhat smaller than relaxing the global stratosphere. Both regions of the atmosphere therefore are important for a seasonal forecast in the NAS in winter. In the case of tropical relaxation, adding prescribed observed sea surface temperatures and sea ice (SSTSI) from reanalysis data improves the representation of the NAO in the model and is even more influential in reproducing the observed 42 year trend to a more positive NAO index. The stratosphere has a significant impact on the observed positive NAO trend only between 1964/65 and 1994/95, and in this period is comparable with the other forcings, the tropics and observed SSTSI. However, our model experiments are not able to account for more than 25% of the interannual variance of the NAO and not for more than 40% of the observed trend of the NAO in the ensemble means. In the NPS the tropical atmosphere clearly has a strong impact on the interannual variability, and hence the seasonal predictability, which is measured here by means of the Pacific North America (PNA) pattern index. Single realisations of model experiments with tropical relaxation represent, on average, between 40% and 50% of the variance of the observed PNA pattern. The ensemble mean with SSTSI from reanalysis data and no relaxation captures about 25% of the interannual variance of the observed PNA, but leads to a wrong trend in the PNA between 1960/61 and 2001/02 compared with the observations. The observed trend of the PNA is well captured in terms of the trend pattern if the tropics are relaxed to reanalysis data, except that the magnitude of the trend is reduced in the ensemble mean. It is remarkable that the observed PNA trend between 1960/61 and 2001/02 is also found to be within the range of trends of a control experiment, that sees the climatological mean cycles of SSTSI only. The strong impact of the tropical atmosphere on the extratropical atmosphere in the NPS is associated to the strong link between El Nino related variability in the tropical Pacific and the PNA that was confirmed by a number of previous studies.
    Type: Thesis , NonPeerReviewed
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