where Δθ is the confidence interval, ωj is the frequency, and κxy is the squared coherency (see Storch and Zwiers 1999; Hanan 1970). When the phase spectral is significantly different from zero and from 180°at a frequency of a significant coherency peak, we accept the phase-lag relation as meaningful.
3. Results
3.1. SLPs
Figure 1 shows the real part of wintertime relative MWF SLPs with respect to 50｡? 165｡? from 1910 to 1980 at 10 year sampling intervals. In 1910, strong amplitudes confined over Alaska in the Pacific sector. As year progressed, the pacific action center moved into the central North Pacific from 1920 to 1950. The BO amplitude takes its maximum around 1950 with an oval pattern, and then the large amplitude region has been elongated southwest-northeastward direction accompanied by slight decrease in its amplitudes. In the Atlantic sector, the BO signatures appear as a pattern similar to the North Atlantic Oscillation. However, the contribution from the mid-latitudes is strong in the early 20th century, whereas in the late 20th century, the amplitudes are larger in the Arctic region than in the mid-latitudes. The enhanced arctic amplitudes make the overall pattern resemble the Arctic Oscillation (Thompson and Wallace, 1998) near the end of the 20th century. Consequently, the BO has migrated from the high latitude to mid-latitude in the North Pacific, and in contrast the region of the larger contributions changed from mid-latitudes to high-latitudes in the North Atlantic.
In order to know the reliability of the pattern changes of the BO captured by MWF, we examine EOFs for the SLP data of three segments. First, we separate 102-year data of the wintertime SLPs into three 34-year segments (1899-1932, 1933-1966, 1967-2000). Then, a 10-30-year band-pass filter is applied to each data segment, independently. Thus, end effects for filtering are expected to influence three data segments equally. Then, EOFs are calculated for the each of the three filtered SLP data-segments over the North Pacific region (30-70｡?, 120｡?-80｡?), and then regression coefficients between the PC-1 and SLPs over the northern hemisphere (north of 20｡?) are obtained (Fig. 2). The regression coefficients exhibit the century scale changes of the BO consistent with the MWF results. In the first epoch (1899-1932), the variability occurred over Alaska, but in the second epoch (1933-1966) the signal is found in the central North Pacific. In the last epoch (1967-2000), the maximal amplitude slightly reduced and the pattern is elongated in the north-eastern and south-western direction. In the Atlantic sector, the meridional dipole pattern is common in the three regression maps, but amplitudes in high-latitudes (mid-latitudes) are stronger in the last (first) epoch, with roughly similar amplitudes between the high and mid latitudes in the second epoch. These features obtained by the EOFs are consistent with the results of the MWF described above.
