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The influence of stratosphere-troposphere coupling on cold extremes in mid-latitudes
Jun. 25, 2019, 15:00 - 16:00
KIT, Campus Nord, IMK-ASF, Geb. 435, Seminarraum 2.05

Cold extremes in the densely populated mid-latitudes can cause significant economic and societal damages with severe impacts for the energy, health and transportation sectors. Some of those cold extremes are caused by the variability of the stratospheric polar vortex due to stratosphere-troposphere coupling. The main driver of stratosphere-troposphere coupling are planetary waves (PW). They are generated in the troposphere by orography and land–ocean heating asymmetries and propagate upward into the stratosphere where they either dissipate and initiate a downward-propagating zonal-mean response consistent with the downward migration of the northern annular mode (NAM) or they are reflected downward.
Using cluster analysis, we found two different dominant pattern of increased polar cap height, i.e. weak polar vortex. The first one is zonally symmetric and is associated with PW absorption in the stratosphere leading to a negative phase of the North Atlantic Oscillation in the troposphere and cold spells over Eurasia. The second pattern is zonally asymmetric and is associated with wave reflection over Canada leading to a negative phase of the West Pacific Oscillation and cold spells over Central Canada and the Great Lakes region.
Starting the other way around we investigated three cold spells in winter 2017/18 associated with high latitude blocking. One of these cold spells was caused by PW absorption in the stratosphere with subsequent downward migration of the NAM after a sudden stratospheric warming (SSW). The other two cold spells were caused by wave reflection in the stratosphere but would have been missed with commonly used zonal mean diagnostics to detect wave reflection. Thus further and especially regional studies on wave reflection are needed to improve the forecast skills on cold spells in mid latitudes.

This event is part of the eventgroup Meteorology Colloquium Karlsruhe
Dr. Vivian Matthias, Dr. Marlene Kretschmer

Potsdam Institute for Climate Impact (PIK)
Earth System Analysis, Potsdam
IMK-ASF Administration
Tel: +49 721 608-28271
Mail:sandra leistHpf9∂kit edu
Scientific Staff
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