Sensitivity of EQBO’s boreal winter teleconnections and surface impacts to SSWs

The Quasi-Biennial Oscillation (QBO) is known to influence the boreal winter surface circulation. The QBO affects extratropical surface circulation and temperature over Asia as well as the North Atlantic and Europe. While these surface responses are undoubtedly associated with the QBO, it is less clear whether they arise solely due to modulations in the frequency of extreme stratospheric events such as sudden stratospheric warmings (SSWs) or due to other pathways of the QBO. SSWs rapidly evolve in the stratosphere but can initiate persistent hemispheric scale surface temperature and circulation responses and hence, may account for some of the surface response to the QBO. While not reproduced in climate models, reanalysis tends to show that the frequency of SSW is higher during easterly QBO (EQBO), which could suggest the SSW do in fact account for some of the EQBO surface response. However, the QBO has multiple pathways to influence boreal winter surface conditions and hence may affect surface conditions independently of SSWs.
Here, we study the teleconnections and extratropical surface responses to EQBO by prescribing EQBO in an ensemble of branched simulations derived from a control simulation run without a QBO. The EQBO branched simulations, which run with repeating annual cycles of sea surface temperatures and sea ice, reproduce many of the observed EQBO teleconnections and surface impacts. After subsampling the branched runs for members
with and without SSWs, it becomes clear that SSWs are necessary to fully realize both the polar and subtropical routes of EQBO influence. However, there are regions such as east Asia where the surface circulation is able to be modulated by the EQBO even in the absence of SSWs.