A 2290-year reconstruction of warm-season Tasmanian temperatures from tree rings has been analyzed for decadal-scale fluctuations. Spectral analyses indicate the existence of four oscillatory modes with mean periods of 31, 56, 79, and 204 years. The waveforms of these oscillations, estimated by singular spectrum analysis, appear to be reasonably stable through time, although each exhibits varying degrees of amplitude and phase modulation. The temperature oscillations, especially at periods of 31 and 56 years, may be related to the expansion and contraction of the circumpolar vortex around Antarctica and the compensating north-south movement of the subtropical high-pressure belt off eastern Australia. However, the cause of such sustained oscillatory behavior remains a mystery. Self-sustained internal forcing related to ocean circulation dynamics and deep-water formation is one possibility. Another possibility, related principally to the 79- and 204-year terms, is external forcing caused by long-term solar variation. In particular, the 79-year oscillation appears to have been phase-locked since 1700 with the envelope of solar cycle length associated with the sunspot numbers. Regardless of their cause(s), these decadal- and century-scale temperature oscillations seem to be important features of the climate system in the Tasman Sea Region that may need to be considered when searching for evidence of greenhouse warming.
Compared to the Northern Hemisphere (e.g., Lamb, 1977), the Southern Hemisphere (SH) has relatively few high-resolution, instrumental climate records that extend back more than 100 years (Barry, 1978). To properly characterize and evaluate recent climatic trends and decadal-scale fluctuations apparent in SH instrumental records (e.g., Fletcher et al., 1982; Jones et al., 1986c; Hansen and Lebedeff, 1987), however, much longer proxy climate records are needed. In this paper, we describe and analyze one such series: a 2290-year temperature reconstruction for Tasmania developed from a precisely dated annual tree-ring chronology. In so doing, we document the probable existence of interdecadal oscillations that appear to be important features of the SH climate system in the Tasmania-New Zealand sector.