the continent. The significant temperature variability of 45-70 year period is seen almost in midlatitude western North America in spring with a further penetration toward the inside of the land than for the 10-25 year band. It is noteworthy, in contrast to the existence of the pentadecadal SLP signal in both winter and spring, the temperature signal on pentadecadal timescale is evident only in spring and absent in winter. The absence of the winter pentadecadal signal over North America might be resulted from the fact that the SLP signal in winter is more confined to the west than that in spring (Fig. 2c, d).
The seasonality of the pentadecadal signal reflects in the difference of means between two periods. The sign reversals of the pentadecadal variability corresponds to the regime shifts in 1920's, 1940's and 1970's (Minobe, 1997; Mantua et al., 1997). Figure 4 shows the winter and spring SLP difference between successive two periods, which are separated at 1924/25, 1947/48, 1976/77. As shown in Fig. 4, the regime shifts are detectable in both in winter and spring. Between two successive periods of a regime, the spatial distributions of SLP differences exhibit approximately the same patterns, with strongest anomalies over the central northern North Pacific and a weaker anomaly with the opposite sign over western North America. These patterns are related to the Pacific/North American teleconnection pattern in the atmospheric
