circulation aloft (Wallace and Gutzler, 1981).

On the other hand, in consistent with the MEM assessment of the air-temperature, the air-temperature difference between two periods in midlatitude North America is significant mainly in spring, but not in winter (Fig. 5). The relation between the SLP and air-temperature is a consequence of the strengthened (weakened) Aleutian low enhancing (reducing) the advection of warmer air onto the west cost of North America (van Loon and Williams, 1976). The warming after the mid 1970s, which occurred over a wider area than the earlier three changes, might be due in part to the anomalous atmospheric circulation associated with the North Atlantic Oscillation (Hurrel, 1995).

The linkage between the air-temperature and Aleutian-low strength is likely to be modulated on century time scale (Fig. 6) An out-of-phase relation holds for the air-temperature in Alaska and NPI through the present century, with the increasing period of the oscillation. For the air-temperature in the western North America, however, corresponding relation holds after 1930.

4. CONCLUSIONS

We have shown that the two dominant interdecadal timescales, about 20 years (bidecadal) and 50-70 years (pentadecadal), exhibit significant regional and seasonal differences in its distributions. The bidecadal variability is found in the wintertime SLP in the central North Pacific and also in the winter air-temperature over North America. The air-temperature in western (eastern) North America is high (low) when the Aleutian low is strong. The pentadecadal signal is evident in SLP both in the winter and spring, but the corresponding air-temperature variabili1y is found only in the springtime air-temperature in western North America. The out-of-phase relationship between the air-temperature and Aleutian low strength holds throughout the present century in Alaska, but after 1930 in midlatitude western North America.

The difference between the bidecadal and pentadecadal oscillations must be taken into account for the assessments of the socio-economic influences of these interdecadal variations, and for a planning of paleoclimate reconstructions. For example, Minobe (1997) showed that the spring air-temperature reconstructed from tree-rings over western North America exhibits the bidecadal variability at least last three centuries. The results of the present study indicates that this region is a most suitable region for capturing the pentadecada1 variability in North American continent linked with the Aleutian low strength. However, the bidecadal variability in the Aleutian low is most likely to reflect more strongly to the air-temperature in Alaska than in the western North America. Therefore, for a specific timescale of the variability, one should choose proper paleoclimate proxies in terms of their regional and seasonal dependencies.

　

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