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  • 1
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    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
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  • 2
    Publication Date: 2019-03-20
    Description: A long-standing difficulty of climate models is to capture the annual cycle (AC) of eastern equatorial Pacific (EEP) sea surface temperature (SST). In this study, we first examine the EEP SST AC in a set of integrations of the coupled Kiel Climate Model, in which only atmosphere model resolution differs. When employing coarse horizontal and vertical atmospheric resolution, significant biases in the EEP SST AC are observed. These are reflected in an erroneous timing of the cold tongue’s onset and termination as well as in an underestimation of the boreal spring warming amplitude. A large portion of these biases are linked to a wrong simulation of zonal surface winds, which can be traced back to precipitation biases on both sides of the equator and an erroneous low-level atmospheric circulation over land. Part of the SST biases also is related to shortwave radiation biases related to cloud cover biases. Both wind and cloud cover biases are inherent to the atmospheric component, as shown by companion uncoupled atmosphere model integrations forced by observed SSTs. Enhancing atmosphere model resolution, horizontal and vertical, markedly reduces zonal wind and cloud cover biases in coupled as well as uncoupled mode and generally improves simulation of the EEP SST AC. Enhanced atmospheric resolution reduces convection biases and improves simulation of surface winds over land. Analysis of a subset of models from the Coupled Model Intercomparison Project phase 5 (CMIP5) reveals that in these models, very similar mechanisms are at work in driving EEP SST AC biases.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 3
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    In:  [Talk] In: FB1 Seminar, 3.12.2018, GEOMAR Kiel .
    Publication Date: 2019-01-10
    Description: The weather of recent years shows us clearly: Climate change is nothing more that will happen sometime in the distant future, as we already notice its impacts today. But what happens at the political level? For over 25 years, climate change conferences have been looking for a solution to the problem, but at present international climate protection is far from enough if we want to limit global warming to 1.5°C or 2°C by the end of this century! The climate action tracker (https://climateactiontracker.org) projects that global warming will exceed most likely 3°C in 2100, if we follow current policy or even if all countries implement their actual Intended Nationally Determined Contributions (INDCs). In view of this situation, one or the other may ask: Is there still hope that we can mitigate climate change? And is there something more we can do than just to hope that all governments will cooperate one day and will implement a more ambitious climate protection? I argue that each of us can be/is already more effective than we often think! If we change something in our lives towards more sustainability, we often underestimate the long-term positive effect of our actions by thinking too linearly. It can give encouragement when we focus more on where sustainability has already been implemented in recent decades. Each of the positive changes may be very small on its own, but when we look at the sum, a lot has already changed. In my talk, I will discuss why it is so difficult to solve the climate change problem at the political level, but also why there is more hope than we often think. And I will try to link the local action in the here and now into the picture of global climate change and at the same time I want to encourage to commit ourselves to more climate protection and sustainability.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
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    In:  [Talk] In: EGU General Assembly 2018, 08.-13.04.2018, Vienna, Austria .
    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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  • 5
    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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  • 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.
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  • 7
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    Springer
    In:  Climate Dynamics, 42 (5-6). pp. 1631-1648.
    Publication Date: 2016-09-13
    Description: In analysis of climate variability or change it is often of interest how the spatial structure in modes of variability in two datasets differ from each other, e.g. between past and future climate or between models and observations. Often such analysis is based on Empirical Orthogonal Function (EOF) analysis or other simple indices of large-scale spatial structures. The present analysis lays out a concept on how two datasets of multivariate climate variability can be compared against each other on basis of EOF analysis and how the differences in the multivariate spatial structure between the two datasets can be quantified in terms of explained variance in the leading spatial patterns. It is also illustrated how the patterns of largest differences between the two datasets can be defined and interpreted. We illustrate this method on the basis of several well-defined artificial examples and by comparing our approach with examples of climate change studies from the literature. These literature examples include analysis of changes in the modes of variability under climate change for the sea level pressure (SLP) of the North Atlantic and Europe, the SLP of the Southern Hemisphere, the surface temperature of the Northern Hemisphere, the sea surface temperature of the North Pacific and for precipitation in the tropical Indo-Pacific.
    Type: Article , PeerReviewed
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  • 8
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    In:  [Poster] In: AGU Fall Meeting 2013, 09.-13.12.2013, San Francisco, USA .
    Publication Date: 2013-12-16
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  • 9
    Publication Date: 2018-11-20
    Description: In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6-7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean-atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.
    Type: Article , PeerReviewed
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  • 10
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    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Publication Date: 2016-12-19
    Description: Dommenget (2010) found that El Niño-like variability, termed Slab Ocean El Niño, can exist in the absence of ocean dynamics and is driven by the interaction of the atmospheric surface heat fluxes and the heat content of the upper ocean. Further, Dommenget et al. (2014) report the Slab Ocean El Niño is not an artefact of the ECHAM5-AGCM coupled to a slab ocean model. In fact, atmospheric feedbacks crucial to the Slab Ocean El Niño can also be found in many state-of-the-art coupled climate models participating in CMIP3 and CMIP5, so that ENSO in many CMIP models can be understood as a mixed recharge oscillator/Slab Ocean El Niño mode. Here we show further analysis of the Slab Ocean El Niño atmospheric feedbacks in coupled models. The BCCR_CM2.0 climate model from the CMIP3 data base, which has a very large equatorial cold bias, has an El Niño that is purely driven by Slab Ocean El Niño atmospheric feedbacks and is used as an example to describe Slab Ocean El Niño atmospheric feedbacks in a coupled model. In the BCCR_CM2.0, the ENSO-related variability in the 20°C isotherm (Z20), a measure of upper ocean heat content, is decoupled from the first mode of the seasonal cycle-related variability, while the two are coupled in observations, with ENSO being phase-locked to the seasonal cycle. Further analysis of the seasonal cycle in Z20 using SODA Ocean Reanalysis reveals two different regimes in the seasonal cycle along the equator: The first regime, to which ENSO is phase-locked, extends over the west and central equatorial Pacific and is driven by subsurface ocean dynamics. The second regime, extending in observations only over the cold tongue region, is driven by the seasonal cycle at the sea surface and is shifted by roughly six months relative to the first regime. In a series of experiments with the Kiel Climate Model (KCM) with different mean states due to tuning in the convection parameters, we can show that the strength of the equatorial cold bias and the coupling strength between the seasonal cycle of Z20 and ENSO are anti-correlated, i.e. a strong equatorial cold bias suppresses recharge oscillator dynamics and enhances Slab Ocean El Niño atmospheric feedbacks, supporting the results from the BCCR_CM2.0. This can be explained as with a stronger cold bias the second regime of the seasonal cycle in Z20, which extends in observations only over the small cold tongue region, expands westward and becomes more important, so that it decouples ENSO from the seasonal cycle in Z20. This has implications for some major characteristics of the ENSO like the propagation of SST anomalies, the phase locking of SST to the seasonal cycle, or the nonlinearity of ENSO. Dommenget, D., 2010: The slab ocean El Niño. Geophys. Res. Lett., 37, L20701, doi:10.1029/2010GL044888. ——, S. Haase, T. Bayr, and C. Frauen, 2014: Analysis of the Slab Ocean El Niño atmospheric feedbacks in observed and simulated ENSO dynamics. Clim. Dyn., doi:10.1007/s00382-014-2057-0.
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