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| Titel |
Assessing drought cycles in SPI time series using a Fourier analysis |
| VerfasserIn |
E. E. Moreira, D. S. Martins, L. S. Pereira |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Sciences ; 15, no. 3 ; Nr. 15, no. 3 (2015-03-13), S.571-585 |
| Datensatznummer |
250119372
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| Publikation (Nr.) |
 copernicus.org/nhess-15-571-2015.pdf |
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| Zusammenfassung |
| In this study, drought in Portugal was assessed using 74 time series of
Standardized Precipitation Index (SPI) with a 12-month timescale 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
considering one SPI value per year relative to every month.
The analysis focused on the December SPI time series grouped in each of the three
identified clusters to investigate the existence of cycles that could be related
to the return periods of droughts. The most frequent significant cycles in each
of the three clusters were identified and analysed for December and the other
months. Results for December show that drought periodicities vary among the three
clusters, pointing to a 6-year cycle across the country and a 9.4-year cycle in central
and southern Portugal. Both these cycles likely show the influence of the North
Atlantic Oscillation (NAO) on the occurrence and severity of droughts in Portugal.
Relative to other months it was observed that cycles varied according to the common
occurrence of precipitation: for the rainy months – November, December and January – cycles
are similar to those for December; for the dry months – May to September – where the lack of precipitation masks the occurrence of drought, the
dominant cycles are of short duration and cannot be related to the NAO or other large
circulation indices to explain drought variability; for the transition
months – February, March, April and October – 6-year and 3-year cycles were
identified, the latter being more strongly apparent in central and southern Portugal.
NAO influence is again identified relative to the 6-year cycles. The short cycles are
apparently associated with positive SPI, thus with wetness, not drought. Overall,
results confirm the importance of the NAO as a driving force for dry and wet periods. |
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