@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{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}
}