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  • 1
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    Unknown
    AGU / Wiley
    In:  Geophysical Research Letters, 45 (4). pp. 1989-1996.
    Publication Date: 2019-02-01
    Description: Climate models depict large diversity in the strength of the El Niño/Southern Oscillation (ENSO) (ENSO amplitude). Here we investigate ENSO-amplitude diversity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by means of the linear recharge oscillator model, which reduces ENSO dynamics to a two-dimensional problem in terms of eastern equatorial Pacific sea surface temperature anomalies (T) and equatorial Pacific upper ocean heat content anomalies (h). We find that a large contribution to ENSO-amplitude diversity originates from stochastic forcing. Further, significant interactions exist between the stochastic forcing and the growth rates of T and h with competing effects on ENSO amplitude. The joint consideration of stochastic forcing and growth rates explains more than 80% of the ENSO-amplitude variance within CMIP5. Our results can readily explain the lack of correlation between the Bjerknes Stability index, a measure of the growth rate of T, and ENSO amplitude in a multimodel ensemble.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: archive
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  • 2
    Publication Date: 2019-01-10
    Description: El Niño/Southern Oscillation has global teleconnections. Precipitation on the US East Coast, and in particular Southern California, is strongly dependent on ENSO variability in the tropical Pacific: More rainfall is expected during El Niño episodes, and reduced rainfall during La Niña. While this teleconnection is highly dependent on the location, timing, and strength of the sea surface temperature (SST) signal in the tropical Pacific, the associated nonlinearities are often not well represented in current climate models. Moreover, the location and strength of convection over the equatorial Pacific has been shown to be linked to the strength of atmospheric feedbacks in the tropical Pacific, i.e. the wind-SST feedback and the heat flux-SST feedback. The strength of the local atmospheric feedbacks is here shown to not only affecting tropical Pacific ENSO dynamics, but also the teleconnection to California: A strengthening of the atmospheric feedback tends to initiate a stronger wave train to California, bringing significantly higher rainfall. In addition to feedback strength, this study compares coupled and atmosphere-only models with observations in terms of the ENSO teleconnection to California.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
    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
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  • 4
    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
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  • 5
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    In:  (Bachelor thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, n.n. pp
    Publication Date: 2013-01-10
    Type: Thesis , NonPeerReviewed
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  • 6
    Publication Date: 2017-12-20
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 7
    Publication Date: 2018-01-10
    Description: We investigate the dependence of ENSO non-linearity on the mean state in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and in CMIP5 models. 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 spatial ENSO non-linearity, i.e. that El Niño is located further east than La Niña, is underestimated in many state-of-the-art climate models1⁠. This can be explained with the underestimated strengths of the positive zonal wind feedback µ and the negative heat flux feedback α, which are strongly linearly related through sea surface temperature (SST), while at least in the KCM differences in model physics seem to be less important2⁠. 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, which is 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 (Fig. 1a). On the other hand enhanced atmospheric feedbacks lead to a substantial improvement of the non-linearity of ENSO (Fig. 1b). Differences in the mean state SST are suggested to be a major source of ENSO diversity in current climate models.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Publication Date: 2016-05-03
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
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    In:  (Doctoral thesis/PhD), GEOMAR, Kiel, Germany, 161 pp
    Publication Date: 2019-02-01
    Type: Thesis , NonPeerReviewed
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  • 10
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    In:  [Talk] In: 4. International Conference on El Nino Southern Oscillation: ENSO in a warmer Climate, 16.-18.10.2018, Guayaquil, Ecuador .
    Publication Date: 2019-01-10
    Description: Despite improvements in simulating El Niño/Southern Oscillation (ENSO) in the last decades, current climate model still suffering in simulating important ENSO properties of ENSO, like the amplitude, the frequency, the phase locking to the annual cycle or the asymmetry between El Niño and La Nina (e.g. Bellenger et al. 2014). In a recent study of Bayr et al. (2018) the equatorial cold sea surface temperature (SST) bias in the tropical Pacific could be highlighted as one large contributor to biased ENSO dynamics in climate models. The cold equatorial SST bias, a common problem in climate models, causes a La Niña-like mean state with a too westward position of the rising branch of the Walker Circulation (by up to 30°), resulting in an erroneous convective response during ENSO events. This in turn biases the two most important atmospheric feedbacks, the positive (amplifying) wind-SST and negative (damping) heat flux-SST feedback, with error compensation between these two. Bayr et al. (2018) also give some indications how ENSO is tunable, as the study shows that it is possible to reproduce the same spread in cold SST bias and atmospheric feedback strength as seen in a CMIP5 multi-model ensemble with a perturbed physics ensemble of the Kiel Climate Model (KCM). This was achieved by changes in the cloud and convection parameters which are usually used to tune the climate models. Here we present a more detailed and systematic analyses of the influence of the tuning parameters on ENSO simulation in the KCM which employs ECHAM5 as the atmospheric component. We show the impact of the tuning parameters on the cold SST bias, the ENSO atmospheric feedbacks and important ENSO properties and demonstrate the possibility to tune ENSO and important global climate properties at the same time.
    Type: Conference or Workshop Item , NonPeerReviewed
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