TY  - JOUR
A1  - Totz, Sonja Juliana
A1  - Eliseev, Alexey V.
A1  - Petri, Stefan
A1  - Flechsig, Michael
A1  - Caesar, Levke
A1  - Petoukhov, Vladimir
A1  - Coumou, Dim
T1  - The dynamical core of the Aeolus 1.0 statistical-dynamical atmosphere model
BT  - validation and parameter optimization
JF  - Geoscientific model development : an interactive open access journal of the European Geosciences Union
N2  - 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.
Y1  - 2018
U6  - https://doi.org/10.5194/gmd-11-665-2018
SN  - 1991-959X
SN  - 1991-9603
VL  - 11
IS  - 2
SP  - 665
EP  - 679
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Kretschmer, Marlene
A1  - Coumou, Dim
A1  - Agel, Laurie
A1  - Barlow, Mathew
A1  - Tziperman, Eli
A1  - Cohen, Judah
T1  - More-Persistent weak stratospheric polar vortex states linked to cold extremes
JF  - Bulletin of the American Meteorological Society
N2  - 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.
Y1  - 2018
U6  - https://doi.org/10.1175/BAMS-D-16-0259.1
SN  - 0003-0007
SN  - 1520-0477
VL  - 99
IS  - 1
SP  - 49
EP  - 60
PB  - American Meteorological Soc.
CY  - Boston
ER  - 
TY  - JOUR
A1  - Kretschmer, Marlene
A1  - Cohen, Judah
A1  - Matthias, Vivien
A1  - Runge, Jakob
A1  - Coumou, Dim
T1  - The different stratospheric influence on cold-extremes in Eurasia and North America
JF  - npj Climate and Atmospheric Science
N2  - 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.
Y1  - 2018
U6  - https://doi.org/10.1038/s41612-018-0054-4
SN  - 2397-3722
VL  - 1
PB  - Nature Publ. Group
CY  - London
ER  -