Deterministic signals in tree‐rings from North America

Maximum entropy spectrum analysis of 305 tree‐ring chronologies from North America yields evidence for two terms with periods 18.6 ± 1.3 years (in 286 out of 305 series) and 10.5 ± 0.5 years in 244 instances. Statistical tests show the terms to be significant at confidence levels of 99.9 and 98 per cent, respectively. These signals are identified as the luni‐solar 18.6‐year M n and the 10–11 year solar cycle S c terms. Amplitude and phase of the M n signal are non‐stationary with respect to both time and geography. In particular, abrupt 180° phase changes in wave polarity are often observed. During the seventeenth, and most of the eighteenth century, most of western North America experienced wet ‘W’ conditions at epochs of maximum in tidal forcing. At epochs 1787.2 and 1805.8 radical changes in phasing occurred, and from 1805.8 to present most of the region has been dry ‘D’ at epochs. A physical mechanism that can explain these phenomena is described briefly, and the implications for economic science briefly discussed.