Relative roles of the MJO and stratospheric variability in North Atlantic and European winter climate

摘要

European and eastern United States wintertime weather is strongly influenced by large-scale modes of variability in the Northern Hemisphere such as the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). The negative phase of the NAO has been linked to both the Madden-Julian Oscillation (MJO) phase with convection in the West Pacific (phases 6 and 7) and to stratospheric sudden warmings (SSW), but the relative role of each phenomenon is not clear, and the two phenomena are themselves linked, as more than half of SSW events were preceded by phases 6 and 7 of the MJO. Here we disentangle the relative roles of MJO phase 6/7 and stratospheric variability for Northern Hemisphere surface weather during boreal winter. We show that stratospheric variability leads to significantly different North Atlantic anomalies if it is preceded by MJO phase 6/7. Furthermore, MJO phase 6/7 leads to a long-lived negative AO pattern only if it modulates the stratosphere first. Hence, proper attribution of their respective influence on surface weather needs to take into consideration the linkages between these two phenomena. Finally, MJO phase 6/7 events that lead to SSW can be differentiated from those which do not by their characteristics within the tropics: only MJO phase 6/7 events in which enhanced convection propagates into the South China Sea, which rarely occurs in winter, lead to SSWs.