The influence of sea surface temperatures on the Northern winter stratosphere: Ensemble simulations with the MAECHAM5 model

The role of inter-annual variations in sea surface temperatures (SST) on the Northern winter polar stratospheric circulation is addressed by means of an ensemble of 9 simulations performed with the MAECHAM5 model forced with observed SSTs of the 20 year period from 1980 to 1999. Results are compared to ERA40 reanalysis. Three aspects have been considered: The influence of the inter-annual SST variations on the climatological mean state, the response to El Niño Southern Oscillation (ENSO) events, and the influence on systematic temperature changes (trends). The strongest influence of SST variations has been found for the warm ENSO events considered. Namely, it has been found that the large scale pattern associated with the extra-tropical tropospheric response to the ENSO phenomenon during Northern winter enhances the forcing and the vertical propagation into the stratosphere of the quasi-stationary planetary waves emerging from the troposphere. This enhanced planetary wave disturbance thereafter results in a polar warming of a few degrees in the lower stratosphere in late winter and early spring. Consequently, the polar vortex is weakened and the warm ENSO influence clearly emerges also in the zonal mean flow. In contrast, the cold ENSO events considered do not appear to have an influence distinguishable from that of internal variability. It is also not straightforward to deduce the influence of the SSTs on the climatological mean state from the simulations performed, because the simulated internal variability of the stratosphere is large, a realistic feature. Moreover, the results of the ensemble of simulations provide weak to negligible evidence for the possibility that SST variations during the two decades considered are substantially contributing to trends in the polar temperature in the winter lower stratosphere.