Stratospheric Influence on Wintertime Tropospheric Climate Change in the Northern Hemisphere

The role of the stratosphere in tropospheric climate response to increased concentrations of the greenhouse gases during Northern Hemisphere winter is addressed by performing and analyzing a set of simulations with the atmosphere general circulation model ECHAM5. Attention is paid to the difference in the response to doubled CO2 concentration and associated sea surface temperature and sea ice concentration anomaly between a low-top and a stratosphere-resolving model version. We find a larger decrease of the Arctic sea level pressure in late winter in the low-top model when compared to the stratosphere-resolving one. The different tropospheric response is shown to originate from different response in the polar stratosphere which is attributable to a stronger Brewer-Dobson circulation response in the stratosphere-resolving model. Additional sensitivity experiments reveal that the magnitude of the Arctic sea level pressure response, but not the difference between the stratosphere-resolving and low-top model responses, depends on the magnitude of SST anomaly in the tropical Pacific. We further test the results found with the ECHAM5 model using a set of CMIP5 models. Similarly to the ECHAM5 study, we diagnose the stratospheric influence by dividing the models into low top and high-top groups and contrasting the climate change simulations between these groups. The results of this study will be presented.