PERIODICITY IN MIDWEST US CLIMATIC DATA - THE HALE CYCLE SIGNAL OR THE LUNI-SOLAR SIGNAL

Cohen and Sweeter (1975) found the 20 to 22-year Hale (double sunspot) cycle signal in the maximum entropy spectra of sunspot and Atlantic t ropical cyclone data. Currie (1996) corroborated and extended their an alysis, but he argued that this signal is the 18.6-year luni-solar tid al constituent. Currie maintains that Cohen and Sweeter mistakenly con jectured that this long term periodicity was induced by the 20 to 22-y ear Hale cycle signal. However, no further investigation of periodicit y in the extracted wave forms corresponding to the 18.6-year luni-sola r signal was conducted. In this study, we follow Currie's signal proce ssing procedures to extract the wave forms corresponding to the 18.6-y ear luni-solar signal. In order to investigate the periodicity in the extracted wave forms, multi-taper method (MTM) is used for harmonic an alysis. Band pass filters are then designed to extract the wave forms corresponding to the individual components identified in the MTM harmo nic analysis. The investigation results of the monthly precipitation a nd Palmer's drought severity index (PDSI) data in three of the midwest ern states - Illinois, Indiana and Ohio - show that two periodic compo nents, the 20 to 22-year Hale cycle signal and the other component wit h periods between 16.9 and 13.5 years, are identified. The bistability phenomenon, which Currie found in these wave forms, is more likely to result from the superposition of these two periodic components, rathe r than from a nonlinear mechanism. Besides, a periodic component with an approximate period of 33 years is detected.