Consistent with previous studies of the SST changes of the BO (e.g., Mann and Park 1996; White and Cayan 1998), EOF-1 exhibits oval-shape anomalies in the central North Pacific accompanied by horseshoe-shape anomalies with the opposing polarity to the east.
If the pattern of the bidecadal SST variation changed associated with the BO migration in SLPs, most prominent changes may have occurred around the boundary between the oval and horseshoe regions in the central northern North Pacific (roughly 46-50｡? 160｡?-140｡?). Within this physically interesting region, the data from 170-140｡? have better availability. Thus, we have chosen to focus the SST data over 46-50｡? and 170-140｡? (referred to as border SST). Also, we examine time series averaged over 26-34｡?, 170｡?-160｡? (oval SST) and 26-50｡?, 130-110｡? (horseshoe SST).
Figure 8 shows the 10-30-year band-pass filtered time series of the border SST along with the oval and horseshoe SSTs. The oval and horseshoe SSTs are out of phase though the record, indicating that the overall combination of the overall and horseshoe unchanged in the 20th century. The border SST, however, was used to be in-phase (out-of-phase) with the oval (horseshoe) SST in the early 20th century, but a reversed phase relationship was evident from 1980 to the present. Consequently, the oval (horseshoe) SST dominated the border region in the first (last) few decades, and hence the boundary between the oval and horseshoe SSTs in the first few decades was inferred to be located to the north of the present location. This result of the SST analysis supports the southward migration of the BO observed in the SLP field.
Now we examine the structure changes of the BO in land-surface air-temperatures. Figure 9 shows a latitude-time section of air temperature averaging zonally from 180-115｡? filtered with the 10-30-year band-pass filter. The BO signature in high-latitudes is evident around 60｡? through the century, but penetrations of the temperature anomalies from the high-latitudes to mid-latitudes are limited from the 1930s to the early 1970s. This is consistent with the previous wavelet analysis of air-temperatures, which showed that the BO signature in mid-latitude western North America was observed only from 1930-1970 (Minobe 2000). The timing of the beginning of the penetration coincides with the timing of the BO signature in SLPs reached to 50｡? supporting the southward migration of the BO in the latter field. The absence of the mid-latitude penetration of the BO signature from the late 1970s to the present cannot be explained by the southward migration of the BO, but it might be related with the weaker recent BO amplitudes over North America shown in Fig. 1; The resultant weakened zonal gradient of the SLP may be related with the weakened meridional winds and hence weakened meridional advection of temperatures. Consequently, the BO signature in LAT field partially supports the BO migration in the SLP field.
4. Conclusions and Discussion
The newly developed data-analysis method, Multi-variate Wavelet Filter (MWF), have succussfully captured century scale changes of the BO in the SLP field over the North Pacific and North Atlantic. The MWF have indicated the southward migration of the BO in the 20th century over the North Pacific; The BO started from Alaska in the early 20th century, and penetrated into the North Pacific from 1920 to 1950. At the same time, the DO contribution in the Atlantic shifted from the mid-latitudes to high-latitudes. These features are confirmed by an EOF analysis for the three 34-year segments of the SLP records with the 10-30-year band-pass filter.
