Deterministic signals in USA precipitation records: Part II

Power spectra of 1203 American rainfall records show evidence for two peaks with mean periods of 18.5 ± 1.8 and 10.6 ± 0.6 years, and statistical tests show that both are significant at confidence levels of 99.9 per cent. These are identified as induced by the luni‐solar 18.6‐year constituent tide and a 10–11‐year solar cycle variation in the sun's luminosity of the order of 0.1 per cent. Amplitude and phase of the 18.6 year wavetrains are highly non‐stationary, and abrupt 180° phase changes in wave polarity are often observed. Wave maxima/minima are generally in phase or out of phase with epochs (epochs are dates of maximum in tidal forcing which occurred at 1898.9, 1917.5, etc.). Polarity maps show that for epochs 1898.9, 1917.5, 1936.1 and 1954.7 the major part of the nation's landmass experienced droughty conditions, which reached a maximum in both amplitude and areal extent at 1936.1, the Dust Bowl era. At mid‐epoch 1964.0 and epoch 1973.3 so many waves changed phase by 180° that the major part of the landmass was wet at the epoch. Analogous phenomena have been found in tree‐ring data from several continents, and implications for economic science are briefly discussed.