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  • 1
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    In:  [Talk] In: IAPSO-IAMAS-IAGA Joint Assembly 2017, 27.08.-01.09.2017, Cape Town, South Africa .
    Publication Date: 2017-11-22
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
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    In:  [Talk] In: DRAKKAR 2018 Annual Workshop, 22.-24.01.2018, Grenoble, France .
    Publication Date: 2018-02-26
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: slideshow
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  • 3
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    Springer
    In:  In: YOUMARES 8 – Oceans Across Boundaries: Learning from each other. Springer, Cham, Switzerland, pp. 7-23. ISBN 978-3-319-93284-2
    Publication Date: 2018-09-11
    Description: Variability in the tropical Atlantic Ocean is dominated by the seasonal cycle. A defining feature is the migration of the inter-tropical convergence zone into the northern hemisphere and the formation of a so-called cold tongue in sea surface temperatures (SSTs) in late boreal spring. Between April and August, cooling leads to a drop in SSTs of approximately 5°. The pronounced seasonal cycle in the equatorial Atlantic affects surrounding continents, and even minor deviations from it can have striking consequences for local agricultures. Here, we report how state-of-the-art coupled global climate models (CGCMs) still struggle to simulate the observed seasonal cycle in the equatorial Atlantic, focusing on the formation of the cold tongue. We review the basic processes that establish the observed seasonal cycle in the tropical Atlantic, highlight common biases and their potential origins, and discuss how they relate to the dynamics of the real world. We also briefly discuss the implications of the equatorial Atlantic warm bias for CGCM-based reliable, socio-economically relevant seasonal predictions in the region.
    Type: Book chapter , PeerReviewed
    Format: text
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  • 4
    Publication Date: 2018-11-22
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
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    In:  [Poster] In: EGU General Assembly 2018, 08.-13.04.2018, Vienna, Austria .
    Publication Date: 2019-01-07
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
    Publication Date: 2019-01-10
    Description: ENSO atmospheric feedbacks are strongly underestimated in state-of-the-art climate models (Bellenger et al. 2014)⁠. Therefore we investigate in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and in CMIP5 models how ENSO atmospheric feedbacks depend on the mean state of the tropical Pacific. Additionally, uncoupled simulations are conducted with the atmospheric component of the KCM to obtain further insight into the mean state dependence. It is found that the strengths of the positive zonal wind feedback µ and the negative heat flux feedback α are both strongly linearly related equatorial sea surface temperature (SST) bias, while at least in the KCM differences in model physics seem to be less important (Bayr et al. 2017)⁠. In observations, strong zonal wind and heat flux feedbacks are caused by a convective response in the western central equatorial Pacific (Niño4 region), resulting from an eastward (westward) shift of the rising branch of the Walker Circulation (WC) during El Niño (La Niña). Climate models with a La Niña-like mean state, i.e. an equatorial SST cold bias in the Niño4 region (a common problem in many state-of-the-art climate models), simulate a too westward located rising branch of the WC (by up to 30°) and only a weak convective response. Thus, the position of the WC determines the strength of both the wind and heat flux feedback, which also explains why biases in these two feedbacks partly compensate in many climate models. Furthermore, a too eastward position of the WC leads to a fundamental change in ENSO dynamics, as ocean-atmosphere coupling shifts from a predominantly wind-driven to a more solar radiation-driven mode. On the other hand, enhanced atmospheric feedbacks lead to a substantial improvement of the non-linearity of ENSO. Differences in the mean state SST are suggested to be a major source of ENSO diversity in current climate models. References: Bayr, T., M. Latif, D. Dommenget, C. Wengel, J. Harlaß, and W. Park, 2017: Mean-State Dependence of ENSO Atmospheric Feedbacks in Climate Models. Clim. Dyn., doi:10.1007/s00382-017-3799-2. Bellenger, H., E. Guilyardi, J. Leloup, M. Lengaigne, and J. Vialard, 2014: ENSO representation in climate models: From CMIP3 to CMIP5. Clim. Dyn., 42, 1999–2018, doi:10.1007/s00382-013-1783-z.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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  • 7
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    In:  [Poster] In: 21st Century Challenges in Regional-scale Climate Modelling, 2nd International Lund RCm Workshop, 04.-08.05.2009, Lund, Sweden .
    Publication Date: 2012-02-23
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 101 pp
    Publication Date: 2012-07-06
    Description: This thesis investigates the influence of arctic sea ice on the winter climate in the 20th century in a high resolution general circulation model (ECHAM5.3, T213). For all seasons in the perturbed simulation sea ice cover (SIC) is replaced by open water at the freezing point, which is then compared with the control run. The effect of completely reduced SIC is then assessed and compared to the effect of an anthropogenic global temperature rise in the projected future summer climate at the end of 21th century (A1B-Szenario scenario), when SIC is reduced to 12% to 20% (August to October)of the SIC in the 20th century. It is shown, that the melting of all sea ice has impacts on the climate in the winter season (January to March) reaching as far south as the subtropics and leads to responses in the dynamic of the atmosphere. The atmospheric warming, caused by an increase of sensible and latent upward heat fluxes from the Arctic Ocean, is conned to the lower troposphere and the high latitudes, whereas temperatures decrease significantly over Siberia. Other than an expected decrease in sea level pressure (SLP), due to the large-scale warming, SLP reduces only in the western Arctic, northeast North America and the North Atlantic, while it increases over northern Asia and Europe as well as over Greenland, which means an intensication and expansion of the Siberian High into the North Atlantic. Due to a smaller meridional temperature gradient, the large-scale atmospheric circulation weakens and moves a little southward, so that the subtropical jet is focused over 30°N. The 10m wind speed decreases in large areas outside the Arctic. Furthermore, the variability of SLP and 2m temperature decrease in the polar region. The leading modes of SLP variability (EOF analysis) are shifted to lower amounts of explained variance, whereby the Arctic Oscillation pattern remains as the dominant mode. Contrary to the 20th century, the planetary circulation in the summer climate of the 21th century strengthens and moves to the North. Zonal wind speeds increase over the mid latitudes in the 21th century, while there is a documented decrease over this area in the 20th century. Atmospheric temperatures rise particularly in the polar regions (as in the 20th century) and in the upper tropics, but the stratosphere cools. There are widespread declines in SLP over the Arctic and subtropics and maximum SLP variability is shifted from the central Arctic to high latitudes. The process which accounts for most of the SLP variability still describes an oscillation between the Arctic and the mid latitudes. The responses of a complete reduction of SIC, analysed in the 20th century, are detectable in a future summer climate projection. The global warming induces processes which partly or totally superimpose, counteract or mitigate the impact of reduced SIC. It could not be claried for good whether these processes have a linear or non-linear relationship.
    Type: Thesis , NonPeerReviewed
    Format: text
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  • 9
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    In:  [Talk] In: EGU General Assembly 2015, 12.–17.04.2015 , Vienna, Austria .
    Publication Date: 2015-04-20
    Type: Conference or Workshop Item , NonPeerReviewed , info:eu-repo/semantics/conferenceObject
    Format: text
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  • 10
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    In:  (Doctoral thesis/PhD), Christian-Albrechts-Universität Kiel, Kiel, Germany, 111 pp
    Publication Date: 2019-02-01
    Type: Thesis , NonPeerReviewed
    Format: text
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