TY  - JOUR
A1  - Totz, Sonja Juliana
A1  - Tziperman, Eli
A1  - Coumou, Dim
A1  - Pfeiffer, Karl
A1  - Cohen, Judah
T1  - Winter precipitation forecast in the European and mediterranean regions using cluster analysis
JF  - Geophysical research letters
N2  - The European climate is changing under global warming, and especially the Mediterranean region has been identified as a hot spot for climate change with climate models projecting a reduction in winter rainfall and a very pronounced increase in summertime heat waves. These trends are already detectable over the historic period. Hence, it is beneficial to forecast seasonal droughts well in advance so that water managers and stakeholders can prepare to mitigate deleterious impacts. We developed a new cluster-based empirical forecast method to predict precipitation anomalies in winter. This algorithm considers not only the strength but also the pattern of the precursors. We compare our algorithm with dynamic forecast models and a canonical correlation analysis-based prediction method demonstrating that our prediction method performs better in terms of time and pattern correlation in the Mediterranean and European regions.
KW  - precipitation anomaly
KW  - seasonal forecast
KW  - cluster analysis
Y1  - 2018
U6  - https://doi.org/10.1002/2017GL075674
SN  - 0094-8276
SN  - 1944-8007
VL  - 44
SP  - 12418
EP  - 12426
PB  - American Geophysical Union
CY  - Washington
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  -