Spatio-temporal structure of the pentadecadal variability over the North Pacific
Introduction
A growing body of literature has revealed that there were climatic regime shifts in the 1920s, 1940s and 1970s which had significant influences on the physical and biological environments over and around the North Pacific. Nitta and Yamada (1989) and Trenberth (1990) reported that in the 1970s the Aleutian low abruptly strengthened. This abrupt change has been referred to as a climatic regime shift. A climatic regime shift is defined as a transition from one climatic state to another within a period substantially shorter than the lengths of the individual epochs of each climate states (e.g., Minobe, 1997). A number of analyses have shown the significant changes occurred in the physical environments in association with the 1970s regime shift (e.g., Graham, 1994; Tanimoto, Iwasaka, Hanawa & Toba, 1993; Trenberth & Hurrell, 1994; Miller, Cayan, Barnett, Graham & Oberhuber, 1994; Miller, Cayan, & White, 1998; Polovina, Mitchum & Evans, 1995; Lagerloef, 1995; Nakamura, Lin & Yamagata, 1997; Zhang & Levitus, 1997; Schneider, Miller, Alexander & Deser, 1999; Tourre, Kushnir & White, 1999; Zhang & Liu, 1999; Suga, Kato & Hanawa (2000)).
Several analyses indicate that similar regime shifts also occurred in the 1920s and 1940s with alternating polarities (Kondo, 1988, Hare & Francis, 1995, Dettinger & Cayan, 1995; Zhang, Wallace & Battisti, 1997; Minobe, 1997; Mantua, Hare, Zhang, Wallace & Francis, 1997). Mantua et al. (1997) called the low-frequency variability associated with the regime shifts the Pacific (Inter) Decadal Oscillation (PDO). From the point of view of the representative time scale, Minobe (1999) described the low-frequency variability as a pentadecadal oscillation. All three climatic regime shifts observed in the 20th century have had a significant influence on the marine ecosystems, notably in dramatic changes of several stocks of commercial fish (i.e., Mantua et al., 1997; Kodama, Nagashima & Izumi, 1995; McGowan, Cayan & Dorman, 1998; Yasuda, Sugusaki, Watanabe, Minobe & Oo-zeki, 1999). The pentadecadal variability has also been detected in tree-ring records over the United States and Canada during the 18th and 19th centuries, but with a somewhat longer timescale (from 50–70 years) than the 50 year timescale observed in instrumental data in the present century (Ware, 1995, Minobe, 1997, Shabalova & Weber, 1999, Ware & Thomson, 2000).
The three climatic regime shifts are not only important scientific issues intrinsically, but are also important because of their influence on interannual climate variations. Minobe and Mantua (1999) showed that the interannual variability in the wintertime Aleutian low is strong in a regime with a stronger regime-mean Aleutian low, with coherent signatures in the SLP, 500 hPa geopotential height, SST, and wind fields. Gershunov and Barnett (1998) showed that the influence of El Niño and Southern Oscillation (ENSO) on North America is modulated by the regime shifts. Power, Casey, Folland, Colman and Mehta (1999) also indicated that the ENSO influence on Australian rainfall is dependent on the PDO.
An interesting feature of these regime shifts is the rapidity with which the transition from one regime to another occurs. Analyzing a representative SLP time series, the North Pacific Index (NPI) defined by Trenberth and Hurrell (1994), Minobe (1999) explained that all three climatic regime shifts in the 20th century have involved simultaneous phase reversals between the pentadecadal (a period of about 50 years) and bidecadal (a period of about 17 years) oscillations. An epoch of a regime is equivalent to a half period of the pentadecadal variation, and corresponds to one and half periods of the bidecadal variation. In other words, the pentadecadal and bidecadal oscillations are synchronized with a relative period of three. A wavelet analysis of the NPI revealed that the pentadecadal oscillation prevailed both in the winter and spring, but the bidecadal oscillation occurred only in the winter. From these seasonal differences between the bidecadal and pentadecadal variations, it has been inferred that these two climate oscillations are generated by different mechanisms. The bidecadal variability over the North Pacific has been analyzed from various aspects; Royer (1989) first reported the bidecadal variability from an analysis of air and water-temperatures in Alaska. The upper water temperature, or mixed layer depth, was reported to exhibit the bidecadal signal (Lagerloef, 1995, Polovina et al., 1995, Tourre et al., 1999), and bidecadal variations were also detected in analyses of basin-scale or global surface temperature and/or SLP (Ghil & Vautard, 1991, Kawamura, 1994, Polovina et al., 1995, Mann & Park, 1994, Mann & Park, 1996, White & Cayan, 1998; Zhang, Sheng & Shabbar, 1998; Tourre et al., 1999).
In these previous studies, however, the seasonality and regionality of the pentadecadal variability or regime shifts have not been fully clarified. The pentadecadal signature was evident in the SLP in both the winter and spring seasons (Minobe, 1999), whereas the signature in the air-temperature in mid-latitude western North America is only found in spring (Minobe, 1997). Why there is this different seasonality between the SLP and air-temperature has not yet been explained. In this paper, in order to examine the seasonal and regional dependency more closely, we analyze the SLP and air-temperature by using a MTM–SVD. It is also interesting to examine whether the resonance between the bidecadal and pentadecadal variations shown for the NPI by Minobe (1999) is common to other large scale climate indices, such as the PDO index (PDOI) proposed by Zhang et al. (1997). The PDOI also captures the three climatic regime shifts in the present century (Mantua et al., 1997). So, we have performed a series of the wavelet analyses for the PDOI and area averages for several air-temperatures time series.
Section snippets
Data
We analyze the following monthly 5°×5° gridded SLP and land air-temperature. The SLP data from January 1899 to June 1999 was provided from NCAR, as the updated version of the SLP data of Trenberth and Paolino (1980). The air-temperature data from January 1880 to June 1999 are the gridded version of the Global Historical Climate Network (Vose et al., 1992), the gridding method described by Baker, Eischeid, Karl and Diaz (1995).
As representative climate proxies, the North Pacific Index (NPI) and
Winter–spring MTM–SVD
Fig. 1 shows the LFV spectra of the winter–spring combined MTM–SVD for the SLP over the North Pacific and western North America. A significant peak on interdecadal timescale occurs at 0.030 cycle yr−1 (33-year period). As noted in the Appendix, the frequency at the LFV peak does not necessarily give a representative timescale, which can be obtained from the reconstruction of a representative time series. The reconstructed time series has about two cycles in a one-hundred year record suggesting
Discussion
From the point of view of the superposition of the bidecadal and pentadecadal variations, only the three previously noted climatic regime shifts were accompanied by the simultaneous phase reversals of bidecadal and pentadecadal variations in the present century. Several papers documented that a series of atmospheric and oceanographic changes occurred over the North Pacific in 1988/89, and a certain portion of the changes was opposite to the changes which occurred in 1976/77 (Tachibana, Honda &
Conclusions
This paper examines aspects of the spatio-temporal structure of the atmospheric and oceanic variability over the North Pacific. The structure of the pentadecadal variability was analyzed by using a winter–spring combined MTM–SVD. The pentadecadal SLP variability in winter is distributed over the entire zonal extent of the Pacific basin, but the springtime SLP pattern is shifted toward the east with the strong zonal SLP gradient only within 10° longitude from the west coast of North America.
Acknowledgements
I thank Y. Kushnir, M. Scheledinger, and M. Shabalova for invaluable discussions, R.-H. Zhang, N. Schneider, L. D. Talley and D.M. Ware for preprints and reprints, N. Mantua for Pacific Decadal Oscillation Index, M.E. Mann for source code of the MTM–SVD. Insightful advises by N. Mantua and an anonymous reviewer are helpful to improve the present paper. Some figures are produced with the GrADS developed by B. Doty. This study is supported by grants from the Japanese Ministry of Education,
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