The quasi-periodic fluctuations in the drought indices over the North American Great Plains

The maximum entropy spectral technique is used to search for periodicities in drought indices over the Great Plains of North America, covering a period of at least 70 years. The results show that in general, quasi-biennial, quasi-triennial and quasi-five-year cycles dominate the spectra of drought indices of many stations in the region. These significant periodicities in the 2–6 year waveband are, however, sporadic in their spatial distribution; they can be evident at one station while being absent at another a few hundred kilometres away. From the results of the spectral analysis, it is concluded that significant peaks obtained in the drought indices are only short-lived time variabilities in drought as recorded by individual stations and do not represent any persistent drought characteristics over a broad geographical region. In general, the drought series display only what appears to be a random variation, with the possible exception of a systematic quasi-biennial oscillation.