Spatio-temporal structure of the pentadecadal variability over the North Pacific

Abstract Using a Multi-Taper frequency domain-Singular Value Decomposition (MTM–SVD), a pentadecadal oscillation was detected in the winter–spring sea-level pressure (SLP) field over the North Pacific and surface air-temperature in North America which was significant at the 95% confidence level. The MTM–SVD captured the different SLP and air-temperature distributions between the winter and spring seasons in a consistent manner. The pentadecadal SLP signature in the spring season is centered nearer the west coast of North America than in the winter season. This zonal displacement is consistent with the prominent springtime pentadecadal air-temperature variability in mid-latitude western North America. A wavelet analysis of the Pacific Decadal Oscillation Index (PDOI) showed that the regime shifts in the 1920s, 1940s and 1970s involved simultaneous phase reversals of the bidecadal and pentadecadal variations. The two interdecadal variations are synchronized with one another such that a half period of the pentadecadal oscillation (one epoch of an individual regime) corresponds to one and half periods of the bidecadal oscillation. These results are consistent with the wavelet analysis of the North Pacific Index (NPI). Similar resonance between the bidecadal and pentadecadal variations is evident in air-temperatures over Alaska. The bidecadal and pentadecadal signals have different seasonality in these time series, suggesting that although the two interdecadal variations arise from two different physical mechanisms, they interact with each other. The most distinct seasonal difference was observed in mid-latitude western North America, where the bidecadal variation prevails only in the winter season and the pentadecadal variation only in the spring season. Alaska air-temperatures in the winter and winter–spring of 1999 were the coldest since 1977, as were springtime air-temperatures in mid-latitude western North America, in contrast to the warm anomalies that prevailed in this region during 1977–98. The NPI and PDOI also exhibited an opposite polarity in 1999 to the respective regime mean polarities. These anomalous conditions in winter and spring seasons of 1999 may signify a major regime shift in 1998–1999. In order to verify whether or not a regime shift did occur in 1998–1999, a careful examination of additional data in coming ten or so years will be necessary.