@article{TotzEliseevPetrietal.2018,
  author    = {Totz, Sonja Juliana and Eliseev, Alexey V. and Petri, Stefan and Flechsig, Michael and Caesar, Levke and Petoukhov, Vladimir and Coumou, Dim},
  title     = {The dynamical core of the Aeolus 1.0 statistical-dynamical atmosphere model},
  series = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  volume    = {11},
  journal   = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  number    = {2},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1991-959X},
  doi       = {10.5194/gmd-11-665-2018},
  pages     = {665 -- 679},
  year      = {2018},
  abstract  = {Here, we present novel equations for the large-scale zonal-mean wind as well as those for planetary waves. Together with synoptic parameterization (as presented by Coumou et al., 2011), these form the mathematical description of the dynamical core of Aeolus 1.0. The regions of high azonal wind velocities (planetary waves) are accurately captured for all validation experiments. The zonal-mean zonal wind and the integrated lower troposphere mass flux show good results in particular in the Northern Hemisphere. In the Southern Hemisphere, the model tends to produce too-weak zonal-mean zonal winds and a too-narrow Hadley circulation. We discuss possible reasons for these model biases as well as planned future model improvements and applications.},
  language  = {en}
}
@article{KretschmerCohenMatthiasetal.2018,
  author    = {Kretschmer, Marlene and Cohen, Judah and Matthias, Vivien and Runge, Jakob and Coumou, Dim},
  title     = {The different stratospheric influence on cold-extremes in Eurasia and North America},
  series = {npj Climate and Atmospheric Science},
  volume    = {1},
  journal   = {npj Climate and Atmospheric Science},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2397-3722},
  doi       = {10.1038/s41612-018-0054-4},
  pages     = {10},
  year      = {2018},
  abstract  = {The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes.},
  language  = {en}
}
@article{TotzPetriLehmannetal.2018,
  author    = {Totz, Sonja Juliana and Petri, Stefan and Lehmann, Jascha and Coumou, Dim},
  title     = {Regional Changes in the Mean Position and Variability of the Tropical Edge},
  series = {Geophysical research letters},
  volume    = {45},
  journal   = {Geophysical research letters},
  number    = {21},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {0094-8276},
  doi       = {10.1029/2018GL079911},
  pages     = {12076 -- 12084},
  year      = {2018},
  abstract  = {Recent studies indicate that the tropical belt has been expanding during recent decades, which can significantly influence precipitation in subtropical climates. Often the location of the tropical border is identified using the Hadley cell edge (HCE) or the subtropical jet stream (STJ), but most studies concentrated on the zonal-mean state, thereby missing regional impacts. Here we detect longitudinal-resolved trends in STJ cores and HCEs over 1979-2016 in both hemispheres at a higher spatial and temporal resolution than previous studies. Besides pronounced regional trend differences in both sign and magnitude, we show that winter HCE and STJ variability increased in the Mediterranean region and decreased over the American and Asian continents. Rainfall variability in these regions changed likewise, and most of those changes can be explained by changes in HCE/STJ variability. This highlights the importance of understanding future tropical belt changes both regionally and in terms of variability. Plain Language Summary We applied a new network-based method to detect motion of the tropical climate border with longitudinal resolution. Depending on the longitudinal position, there are differences in both direction and magnitude of the border motion. In addition, we demonstrate that the rainfall variability is increasing in the Mediterranean region and decreasing over the American and Asian continents, which can be explained by the variability of the tropical belt location.},
  language  = {en}
}
@article{MannRahmstorfKornhuberetal.2018,
  author    = {Mann, Michael E. and Rahmstorf, Stefan and Kornhuber, Kai and Steinman, Byron A. and Miller, Sonya K. and Petri, Stefan and Coumou, Dim},
  title     = {Projected changes in persistent extreme summer weather events},
  series = {Science Advances},
  volume    = {4},
  journal   = {Science Advances},
  number    = {10},
  publisher = {American Assoc. for the Advancement of Science},
  address   = {Washington},
  issn      = {2375-2548},
  doi       = {10.1126/sciadv.aat3272},
  pages     = {9},
  year      = {2018},
  abstract  = {Persistent episodes of extreme weather in the Northern Hemisphere summer have been associated with high-amplitude quasi-stationary atmospheric Rossby waves, with zonal wave numbers 6 to 8 resulting from the phenomenon of quasi-resonant amplification (QRA). A fingerprint for the occurrence of QRA can be defined in terms of the zonally averaged surface temperature field. Examining state-of-the-art [Coupled Model Intercomparison Project Phase 5 (CMIP5)] climate model projections, we find that QRA events are likely to increase by similar to 50\% this century under business-as-usual carbon emissions, but there is considerable variation among climate models. Some predict a near tripling of QRA events by the end of the century, while others predict a potential decrease. Models with amplified Arctic warming yield the most pronounced increase in QRA events. The projections are strongly dependent on assumptions regarding the nature of changes in radiative forcing associated with anthropogenic aerosols over the next century. One implication of our findings is that a reduction in midlatitude aerosol loading could actually lead to Arctic de-amplification this century, ameliorating potential increases in persistent extreme weather events.},
  language  = {en}
}
@article{KretschmerCoumouAgeletal.2018,
  author    = {Kretschmer, Marlene and Coumou, Dim and Agel, Laurie and Barlow, Mathew and Tziperman, Eli and Cohen, Judah},
  title     = {More-Persistent weak stratospheric polar vortex states linked to cold extremes},
  series = {Bulletin of the American Meteorological Society},
  volume    = {99},
  journal   = {Bulletin of the American Meteorological Society},
  number    = {1},
  publisher = {American Meteorological Soc.},
  address   = {Boston},
  issn      = {0003-0007},
  doi       = {10.1175/BAMS-D-16-0259.1},
  pages     = {49 -- 60},
  year      = {2018},
  abstract  = {The extratropical stratosphere in boreal winter is characterized by a strong circumpolar westerly jet, confining the coldest temperatures at high latitudes. The jet, referred to as the stratospheric polar vortex, is predominantly zonal and centered around the pole; however, it does exhibit large variability in wind speed and location. Previous studies showed that a weak stratospheric polar vortex can lead to cold-air outbreaks in the midlatitudes, but the exact relationships and mechanisms are unclear. Particularly, it is unclear whether stratospheric variability has contributed to the observed anomalous cooling trends in midlatitude Eurasia. Using hierarchical clustering, we show that over the last 37 years, the frequency of weak vortex states in mid- to late winter (January and February) has increased, which was accompanied by subsequent cold extremes in midlatitude Eurasia. For this region, 60\% of the observed cooling in the era of Arctic amplification, that is, since 1990, can be explained by the increased frequency of weak stratospheric polar vortex states, a number that increases to almost 80\% when El Nino-Southern Oscillation (ENSO) variability is included as well.},
  language  = {en}
}