BIDECADAL AND PENTADECADAL CLIMATIC OSCILLATIONS OVER THE NORTH PACIFIC AND NORTH AMERICA
Shoshiro Minobe*
Hokkaido University, Sapporo, Japan
1. INTRODUCTION
Recent investigations have revealed that the variability in the North Pacific is strongly related to the strengthening/weakening of the Aleutian low, which dominantly influences the climate over the North Pacific and western coast of North America from interannual to interdecadal timescales. On the latter timescale, two oscillatory variations have been identified from the analyses of instrumentally observed data; one is bidecadal (〜20 years) (e.g., Royer 1989: White et al., 1997; Mann and Park 1996) and the other is a 50-70 year variability (Minobe, 1997; Mantua et al. 1997). The pentadecadal variability is closely associated with three climatic regime shifts in mid 1920's, late 1940's and mid 1970's (Kondo, 1988; Mantua et al., 1997; Minobe, 1997). The last regime shift has attracted large attentions of climate researchers (e.g., Nitta and Yamada, 1989; Trenberth, 1990). Although the existence of the two major timescales has been reported in the North Pacific/North American sector, the features of these timescales have not been fully investigated. The purpose of this paper is, therefore, to clarify the characteristic of the variations of these two timescales. In particular, we focus our attention on similarities and differences of these two interdecadal timescales concerning with their regional and seasonal distributions in the present century.
2. DATA
We examine two gridded datasets: sea-level pressure (SLP) and air temperatures. The monthly SLP data are the updated version from January 1899 to June 1996 of Trenberth and Paolino (1980). The seasonal air-temperature data were complied by Baker et al. (1994) based on monthly temperature data collected as the Global Historic Climate Network (Vose et. al., 1992), which is the climate dataset created from 15 data sources including, World Weather Records at National Climatic Data Center, Climate Anomaly Monitoring System at Climate Analysis Center, World Monthly Surface Station Climatology at National Center of Atmospheric Research, and Jones' Temperature database for the world.
3. RESULTS
In order to examine the seasonality of the interdecadal variations of the Aleutian low, we examine the existence of significant spectrum peaks in the North Pacific Index (NPI) for each month using Maximum Entropy Method (MEM). The significance MEM spectrum is tested by using a Monte-Carlo Method, in which an AR (1) or Red-Noise model is employed. The NPI represents the strength of the Aleutian low, and is defined as the SLP averaged over 160°E-140°W, 30°-65°N (Trenberth and Hurrel 1994). The MEM spectra exhibit significant peaks around the period of 20 years from December to January, with most of the spectrum power concentrating in January (Fig. 1). The other cluster of significant spectrum peaks found from 50 to 70 years from December to May. In short, bidecadal variability was observed only in wintertime, whereas pentadecadal variability was found both in winter and spring.
The spatial distribution of the bidecadal and pentadecadal signals are identified by examining MEM spectra for years at each grid point in winter (Dec.-Feb.) and spring (Mar.-May) separately. The results are summarized into spectrum amplitudes and their significance in two bands of 15-25 and 45-70 year periods (Fig. 2). These band ranges are chosen so that the bidecadal and pentadecadal variability is separated. The spectrum amplitudes and their significance are shown for the most significant peak in each band. The distributions of the MEM spectra indicate that the significant spectrum peak occurs in Aleutian region in winter for the 15-25 year band, and both in winter and spring for the 45-70 year band, as expected from the analysis of the NPI in Fig. 1. On both timescales, these two peaks are well organized in the central North Pacific.
In parallel to the above MEM assessment for the SLP, MEM spectra were calculated for the surface air-temperature at each grid and in each season over North America (Fig, 3). Again, significant signals on two interdecadal timescales are found with a distinction between winter and spring in North America. For the 15-25year band, the significant peak is found in the both sides of
* Corresponding author address, Shoshiro Minobe, Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan; e-mail: minobe@geophys.hokudai.ac.jp
