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  • Other Sources  (105)
  • 2015-2019  (105)
  • 1
    Publication Date: 2018-11-09
    Description: The sea surface temperature (SST) in the eastern tropical Atlantic exhibits pronounced variability on interannual time scales being associated with wind and rainfall anomalies within the tropical Atlantic region. It has been proposed that the interannual variability of SST is partly driven by the variability of the deep equatorial zonal circulation, the so-called equatorial deep jets (EDJs). The EDJs may be described as a superposition of quasi-resonant equatorial basin modes and the direction of vertical phase propagation implies that their energy is propagating towards the surface. Furthermore, recent findings revealed that the EDJs in turn are maintained by intra-seasonal waves that are generated by the barotropic and baroclinic instability of the near-surface circulation. This talk will present the relevant mechanisms that are involved in the conversion of energy from one type of variability to another, i.e. from chaotic intra-seasonal surface variability via deep interannual zonal variability to interannual surface climate variability, with a special focus on the maintenance of the EDJs by intra-seasonal waves. Since EDJs, a key component of the mechanism discussed above, are not well represented in state-of-the-art Ocean General Circulation Models, preliminary findings on the sensitivity of the EDJs to model parameters and configuration are presented.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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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
    Format: text
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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
    Format: text
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  • 5
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    In:  [Invited talk] In: ETH Zürich Switzerland, 05.11.2018, Zürich, Switzerland .
    Publication Date: 2018-11-16
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    In:  [Invited talk] In: Niels Bohr Institute, 18.09.2018, Kopenhagen, Denmark .
    Publication Date: 2018-11-16
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 7
    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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  • 8
  • 9
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    Copernicus Publications (EGU)
    In:  Geoscientific Model Development, 8 (1). pp. 51-68.
    Publication Date: 2017-12-19
    Description: Large-scale fully coupled Earth system models (ESMs) are usually applied in climate projections like the IPCC (Intergovernmental Panel on Climate Change) reports. In these models internal variability is often within the correct order of magnitude compared with the observed climate, but due to internal variability and arbitrary initial conditions they are not able to reproduce the observed timing of climate events or shifts as for instance observed in the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), or the Atlantic Meridional Overturning Circulation (AMOC). Additional information about the real climate history is necessary to constrain ESMs; not only to emulate the past climate, but also to introduce a potential forecast skill into these models through a proper initialisation. We attempt to do this by extending the fully coupled climate model Max Planck Institute Earth System Model (MPI-ESM) using a partial coupling technique (Modini-MPI-ESM). This method is implemented by adding reanalysis wind-field anomalies to the MPI-ESM's inherent climatological wind field when computing the surface wind stress that is used to drive the ocean and sea ice model. Using anomalies instead of the full wind field reduces potential model drifts, because of different mean climate states of the unconstrained MPI-ESM and the partially coupled Modini-MPI-ESM, that could arise if total observed wind stress was used. We apply two different reanalysis wind products (National Centers for Environmental Prediction, Climate Forecast System Reanalysis (NCEPcsfr) and ERA-Interim reanalysis (ERAI)) and analyse the skill of Modini-MPI-ESM with respect to several observed oceanic, atmospheric, and sea ice indices. We demonstrate that Modini-MPI-ESM has a significant skill over the time period 1980–2013 in reproducing historical climate fluctuations, indicating the potential of the method for initialising seasonal to decadal forecasts. Additionally, our comparison of the results achieved with the two reanalysis wind products NCEPcsfr and ERAI indicates that in general applying NCEPcsfr results in a better reconstruction of climate variability since 1980.
    Type: Article , PeerReviewed
    Format: text
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  • 10
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    In:  [Poster] In: EGU General Assembly 2015, 12.–17.04.2015 , Vienna, Austria .
    Publication Date: 2015-04-23
    Description: Variations in the global tropospheric zonal mean zonal wind ([U]) during boreal winter are investigated using Rotated Empirical Orthogonal Functions applied to monthly means. The first two modes correspond to the Northern and Southern Annular Mode and modes 3 and 4 represent variability in the tropics. One is related to El Nino Southern Oscillation and the other has variability that is highly correlated with the time series of [U] at 150 hPa between 5 ◦ N and 5 ◦ S ([U150]e) and is related to activity of the Madden-Julian Oscillation (MJO). High amplitude of the MJO with strong precipitation anomalies over the western tropical Pacific (late MJO phases) are associated with the westerly phase of [U150]e (and vice versa). The extratropical response to [U150]e is investigated using linear regressions of 500 hPa geopotential height onto the [U150]e time series. Use is made of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts (ECMWF) model in which the tropical atmosphere is relaxed towards reanalysis data. Both the 45- year ECMWF reanalysis (ERA-40) and the ERA-Interim reanalysis data sets are used for the relaxation experiment as well as for the regression analysis. Therefore the analysis is covering 52 boreal winters from 1960/61 to 2012/13. The regression analysis reveals a robust shift of the Aleutian low and a wave train across the North Atlantic associated with [U150]e. It is found that the subtropical Rossby waveguides and the link between the North Pacific and North Atlantic are stronger during the easterly phase of [U150]e. The wave train over the North Atlantic is associated with Rossby wave sources over the subtropical North Pacific and North America.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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