Variability of the North Atlantic response to sudden stratospheric warming events in a simplified atmospheric model

Sudden stratospheric warming (SSW) events are often followed by a surface impact, most commonly by a negative phase of the North Atlantic Oscillation (NAO). Recent work has emphasized the large variability among the tropospheric response after these events, showing that only about two thirds of the SSWs are dominated by this canonical negative NAO response. In this study, we use an idealized atmospheric model forced with seasonally varying sea surface temperatures to examine the influence of the pre-existing tropospheric conditions on the North Atlantic response to stratospheric forcing. In the model, the negative phase of the NAO is found to be the most common response to SSWs, occurring after ~85% of the SSWs (under climatological SST forcing).  For the remaining ~15% of the SSW events, the response is associated with a positive phase of the NAO. In the search for the origin of the different tropospheric response in the North Atlantic, the role of synoptic wave propagation from the eastern Pacific on the downward response to SSWs is investigated. By systematically varying the strength of the North Pacific circulation, we are able to assess the sensitivity of the downward response to tropospheric variability in the Pacific, and shed light on its contribution to the persistence of the downward impact of SSWs in the idealized model.