To shed light on where the DICE-related atmospheric anomalies are forced, we estimated wave activity flux of stationary Rossby waves on the zonally-varying climatological mean flow, applying a formula of W developed by Takaya and Nakamura (1997) to Z500 anomalies in the linear regression maps for the individual SST modes (Fig. 3). It is shown that W is independent of wave phase, even if unaveraged, and parallel to the local group velocity. Since W is a flux of pseudomomentum that is conserved for a stationary Rossby wave packet under the non-acceleration conditions, W should be divergent where anomalies are forced. Note that reversing signs of the anomalies yields no change in W. For the first SST mode, W is strongly divergent over SAFZ where a vigorous stormtrack resides, and W is equatorward in the subtropics. Hence, the PNA pattern must be forced in midlatitudes. For the second SST mode, W is,divergent above STFZ and predominantly poleward to the north, suggestive of SST-related forcing in the subtropics. Around 20｡?, there appears to be a hint of reinforcement of wave activity coming in from the lower latitudes. For the third SST mode, southeastward W is converging over Mongolia. The flux is found to be originated in the North Atlantic.
Time evolution of the 3 leading modes is presented in Fig. 4 with their PCs, which have been extended into the period before 1967 by projecting the smoothed SST anomalies for individual years onto the EOFs.
