Stratosphere-troposphere coupling during two ENSO flavors in CMIP5 models

The seasonal evolution of the Northern Hemisphere (NH) stratospheric signals in response to two different flavors of El Nino, the Eastern Pacific (EP) and the Central Pacific (CP) El Nino, have been explored in an ensemble of simulations from models with a well resolved stratosphere in comparison with observations. The large number of events in the CMIP5 simulations shed light on some of the previously reported concerns regarding the small number of events considered in the observational record. CMIP5 results reveal a significantly warmer and weaker Northern Hemisphere polar vortex during EP El Nino events, while no significant response is found during CP El Nino. This is related to differences in the timing of the intensification of wavenumber 1 El Nino anomaly and its constructive interference with the climatological wavenumber1, which occurs earlier during EP than CP events. These differences are also consistent with differences in the timing of the tropospheric teleconnections in the Pacific-North American region. The role of the stratosphere in driving tropospheric El Nino teleconnections in the North-Atlantic European region in late winter and early spring is also investigated. The CMIP5 models show this coupling only for EP El Nino events, facilitated by the SSWs, whose frequency of occurrence is significantly higher in this case. For CP El Nino events, CMIP5 models do not support a stratospheric pathway for NH teleconnections. Thus, the type of El Nino event is key to potentially improved climate predictability over Europe.