Century-scale changes of bidecadal oscillation over the North Pacific

Based on wavelet transform, a new method referred to as Multivariate Wavelet Filter (MWF) is proposed to extract a climate variation, which changes its pattern in association with its change of period. MWF is applied to the bidecadal oscillation over the North Pacific. The bidecadal oscillation has been known to change its timescale through the 20 th century; the period increased from 15 years to 20 years from 1930 to 1950 (e.g., Minobe 1999, 2000). MWF successfully revealed the changes of the spatial structure corresponding the changes of the dominant frequency. The bidecadal oscillation concentrate to the North in the beginning of the present century and then migrate southward probably corresponding to the period increase. In contrast, in the Atlantic sector, the bidecadal oscillation coherent with the Pacific one is likely to migrate from the south to the North. The amplitude of the bidecadal oscillation seems to have its maximum around 1950 and decreased slightly to the present. The relating analysis of the land-air temperature and SSTs will be presented.

The structure changes pose significant implications for the mechanisms of the bidecadal oscillation. Also, the lack of the simultaneous phase reversal between the bidecadal and pentadecadal oscillation may be related with the century scale structure change of the bidecadal oscillation. These points will be discussed.