Power spectra and coherence of drought in the interior plains

Mean growing season (April‐September) precipitation series at 407 stations in the Interior Plains of North America, dating back to 1985 in some stations, are used to derive a Bhalme and Mooley‐type drought index for the region. Linear trend, representing first order non‐stationarity, is first removed from the drought series. Then the periodogram via the Fast Fourier Transform (FFT), Maximum Entropy (ME) and Non‐Integer (NI) spectral techniques are applied to search for significant periodicities in the temporal characteristics of the drought series. Cross‐spectral technique is also used to characterize the extent of spectral coherence in the drought series. It is found that no significant power in drought is present at periods greater than about 6 years. Spectra of drought in the Interior Plains are dominated by three peaks in the quasi‐biennial, quasi‐triennial and quasi‐5‐year oscillations. The periodic 186‐year lunar nodal cycle obtained by others for the Drought Area Index (DAI) for the region is completely absent in the spectra of the analysed drought index series. Also the well‐known 11‐year sunspot cycle is only evident in the drought index series for a few of the stations. In general there is an ill‐defined drought recurrence interval. Moreover the areal extent of peaks contained within these oscillations do not indicate any significant spatial coherence. These periodicities can be evident at one station while being absent at another a few hundred kilometres away. On the basis of the spectral results, it is concluded that obtained significant periodicities 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. A cross‐spectral analysis of the drought series with sunspot numbers shows no evidence to indicate a significant relation between drought and the single sunspot cycle. In general, the growing season drought series in the Interior Plains displays only what appears to be a random variation, with the possible superimposition of the quasi‐biennial oscillation.