Relative role of tropical SST forcing in the 1990s periodicity change of the Pacific‐Japan pattern interannual variability

The interannual variability and periodicity change of the Pacific‐Japan (PJ) pattern during 1979–2008 are investigated using observation/reanalysis data sets and Atmospheric Model Intercomparison Project (AMIP)‐runs of 20 atmospheric general circulation models (AGCMs). The PJ pattern is represented by the first leading empirical orthogonal function mode (EOF1) of 850 hPa vorticity over the East Asian and western North Pacific regions in boreal summer. The leading principal component (PC1) is characterized by a dominant periodicity of 4–5 (2–3) years before (after) 1993. Analyses reveal that the 4–5 year periodicity may be forced by sea surface temperature anomalies (SSTAs) in the tropical western Indian Ocean (IO). In contrast, the 2–3 year periodicity is the combined effect of the SSTAs surrounding the maritime continent (MC) and in the central Pacific (CP), in which the latter may play a primary role. AGCMs which reasonably capture above responses are inclined to have higher PC1 skills and vice versa. To confirm the source of forcing diagnosed based on AMIP simulations, a series of sensitivity experiments is done using the CAM4 model. Without SST variation in the MC or CP after 1993, the 2–3 year variability of the PJ pattern weakens. The periodicity change of PJ variability around 1993 is shown to be correlated with the change of El Niño–Southern Oscillation (ENSO) variability. Analysis of observations indicates that the anomalous summertime SST and circulation patterns over the Indo‐Pacific region correspond to decaying ENSO phase before 1993 and to developing phase after 1993.