Assessing drought cycles in SPI time series using a Fourier analysis.

Moreira E.E., Martins D.S., Pereira, L.S.
Natural Hazards and Earth System Sciences Discussions, 2, 2731-2759.

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In this study, drought in Portugal was assessed using 74 time series of Standardized Precipitation Index (SPI) with a 12 month time scale and 66 years length. A clustering analysis on the SPI Principal Components loadings was performed in order to find regions where SPI drought characteristics are similar. A Fourier analysis was then applied to the SPI time series included in each cluster to investigate the existence of cycles that could represent the return periods of droughts. The most frequent and significant cycles that compound the time series in each of the three clusters were therefore identified and analysed. Results show that droughts periodicities vary among the three clusters identified. The results point to a cycle with 6 years return period of droughts occurrence across the country and a cycle with 9.4 years period of droughts with very high presence in central and southern Portugal. Both these cycles are likely showing influences of the North Atlantic Oscillation on the occurrence and severity of droughts in Portugal.