Time-varying structure of the wintertime Eurasian pattern: role of the North Atlantic sea surface temperature and atmospheric mean flow

The Eurasian (EU) pattern was shown to be closely linked to the North Atlantic sea surface temperature (SST) in boreal winter, and this study suggest that such linkage varies with time and that the structure of the EU pattern also changes accordingly. During the period with strong EU-SST linkage (1961–1977 and 1993–2013, denoted as HIGH epoch), the EU pattern is a well-defined barotropic wave train stretching from the North Atlantic towards East Asia-western North Pacific, and it facilitates cooling and slight reduction of precipitation over East Asia and enhanced precipitation over Europe in its positive phase. During the period with weak EU-SST linkage (1948–1960 and 1978–1992, denoted as LOW epoch), in contrast, its centre over Northern Europe weakens sharply, and the wave-like structure of the EU pattern is not well established. In this case, its influences on precipitation over Europe almost vanish, but its influences on both the surface air temperature and precipitation over East Asia amplify significantly. The Atlantic centre of the EU pattern forms from different mechanism during the two epochs. It can be excited by the underlying North Atlantic SST via eddy-mediated processes during the HIGH epoch, while it is likely generated by an incoming Rossby wave train from upstream region during the LOW epoch. Nevertheless, its location and intensity remain similar during the two epochs. Numerical experiments with a barotropic model suggest that the contrasting structure of the EU pattern is mainly attributed to the changes of the atmospheric mean flow.