Wavelet Empirical Orthogonal Functions of Space-Time-Frequency Regimes and Predictability of Southern Africa Summer Rainfall
Publication: Journal of Hydrologic Engineering
Volume 12, Issue 5
Abstract
Wavelet-based empirical orthogonal function (WEOF) analysis was used to analyze the nonstationary spatial, temporal, and frequency regimes of the regional variability in southern African summer (October–March) rainfall. The leading modes of rainfall variability were then used to establish associations with gridded scale-averaged wavelet power of the sea surface temperature (SST) for the Indian and Atlantic Oceans. The WEOF revealed that southern African rainfall is out of phase between areas north and south of and that areas north of and northern South Africa experienced decreased rainfall between 1970 and 1997. The decrease in rainfall was modulated by periods of between 2 and 8 years. Using judiciously selected windows of April-May-June SST data for the Atlantic and Indian Oceans as predictors in the artificial neural network-genetic algorithm (ANN-GA), high prediction skill of standardized summer rainfall of southern Africa was achieved. For the validation period 1988–97, Pearson correlation between 0.83 and 0.98 (i.e., 69–96% of observed rainfall variability), Hanssen Kuipers skill scores of between 0.2 and 1.0, and root-mean-square errors of between 0.25 and of standardized rainfall were found between observed and predicted summer rainfall at a 3-month lead time.
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Acknowledgments
This study is partly funded by Natural Science and Engineering Research Council (NSERC) of Canada, and the first author is supported by the commonwealth scholarship of CIDA, Canada. The U.K. meteorological office provided both the SST grid data for the Indian and Atlantic Oceans (part of MOHSST6) and the rainfall data for eastern Africa. The wavelet analysis was done using the software of Torrence and Compo (1998) downloaded from http://www.paos/colorado.edu/research/wavelets.
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Published In
Journal of Hydrologic Engineering
Volume 12 • Issue 5 • September 2007
Pages: 513 - 523
Copyright
© 2007 ASCE.
History
Received: Feb 1, 2005
Accepted: Aug 31, 2006
Published online: Sep 1, 2007
Published in print: Sep 2007
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