Minobe 2001
01/02/28
3.6 Century-Scale changes of the bidecadal oscillation over the North Pacific.
Shoshiro Minobe (minobe@ep.sci.hokudal.ac.jp)
Division of Earth and Planetary sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan /Frontier Research System for Global Change, Tokyo, Japan
Teruko Manabe and Akiko Shouji
Japan Meteorological Agency, Tokyo, Japan
Abstract
Based on wavelet transform, a new method referred to as Multivariate Wavelet Filter (MWF) is proposed to extract a climate variation, that changes its pattern corresponding to the change of period. MWF is applied to northern hemisphere SLPs to extract SLP changes associated with the Bidecadal Oscillation (BO). The bidecadal oscillation has been known to change its timescale through the 20th century over the North Pacific, with the period increasing 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 to the changes of the dominant frequency. In the Pacific sector, the bidecadal oscillation is centered over Alaska in the first few decades of the 20th century and then migrated southward to the central North Pacific from 1920-1950, with maximal BO amplitudes in the middle of the century. On the other hand, the BO in the Atlantic sector exhibited that the contribution from mid-latitudes (high-latitudes) is larger in the early (late) of the 20th century. Due to the larger contribution from the high-latitudes in the Atlantic sector, combined with the amplitude reduction from 1950 to the present in the Pacific sector, the recent pattern of the BO somewhat resembles to the Arctic Oscillation pattern.
The analysis of the SST data gridded from COADS and newly digitized Kobe collections exhibits that the southward migration in the SLP field is consistent with the changes of the BO captured by the SSTs. Also, analysis of the land-surface air-temperature over North America partially supports the century scale BO changes in the SLP field.
The implications for Pacific and Atlantic roles for the BO are discussed, based on the spatial structure changes of the BO. Also, the possibility that the absence of a prominent regime shift around 1 900 was resulted from the poleward-shifted BO is presented.
1. Introduction
Recent studies have revealed that one of the most prominent earth's climate variability on decadal-centennial timescales is the bidecadal oscillation (BO) (Mann and Park, 1994; 1996; White et al., 1997; White and Cayan, 1998). The BO distributes globally, but has the largest amplitude in the central North Pacific in association with the variability of the Aleutian low (e.g., Mann and Park 1996; White and Cayan, 1998). The BO over the North Pacific has been analyzed from various aspects; Royer (1989) first reported the evidence of the BO from the analysis of air and water-temperatures in Alaska. The upper water temperature or mixed layer depth were reported to exhibit the bidecadal signal (Lagerloef 1995, Polovina et al. 1995, Tourre et al. 1999; Deser et at, 2000), and bidecadal variations were captured in the analyses of basin-scale or global surface tempearture and/or SLP (Ghil and Vautard 1991; Kawamura 1994; Polovina et al. 1995; Mann and Park 1994, 1996; White et al., 1997; White and Cayan 1998; Zhang et al. 1998; Tourre et al 1999; Chao et al., 2000).
