Deterministic signals in precipitation in the northwestern United States

Abstract

Evidence for the 18.6‐year lunisolar M N signal is found in 91 out of 102 rain gauge records (89% detection ratio) in the states of Montana, Wyoming, and Idaho, while the 10‐ to 11‐year solar cycle S C term is detected in 67 of these time series (66% detection ratio). Lunar nodal 19‐year M N wave trains for these three states show complex patterns, with many experiencing a bistable 180° phase switch at epoch 1973.3 (epochs are dates of maximum in tidal forcing). At epoch 1954.7 ± 2.5 years, Wyoming, Montana, and eastern Idaho experienced drought conditions. By epoch 1973.3 ± 2.5 years, a wet cell of opposite polarity had invaded the eastern half of Montana and parts of eastern Wyoming; concomitantly, drought had expanded westward to include virtually all of Idaho. Bistable 180° phase switches in climate parameters were first reported in Patagonian tree ring data (Currie, 1983) and were later found for a flood index in India (Currie, 1984 a ), a drought index for Beijing (Currie and Fairbridge, 1985), in flooding of the Nile and St. Lawrence rivers (Currie, 1987 a ; Currie and O'Brien, 1989), in air temperature and air pressure records worldwide (Currie, 1987 b ), and in rainfall for the northeastern United States and the corn belt (Currie and O'Brien, 1989, 1990).