Is multidecadal prediction of flood patterns possible for infrastructure planning purposes using the 10000 year cosmogenic isotope (10Be and 14C) record?

We compare spectral decomposition of flood data from three sites in Australia (south-east coast, east-inland, east-central) with the Southern Oscillation Index (SOI), and with those from the Brahmaputra River (Bangladesh) and Nile River (Egypt). All show clear evidence of spectral maxima at medium periods approximating 50, 85, 130 and 200 years, which correspond closely to the maxima in the power spectra of the cosmogenic 10Be and 14C observations obtained from ice cores with ages covering the past 10000 years. We find that the Gleissberg cycle (85 yr period) for Australian sites is out of phase with that for the Brahmaputra River. All sites also show spectral maxima at short periods 6-20 yr as expected from the ENSO cycle; flood associations varying over these short periods are generally accepted. We explore the possibility that the medium periods can be used to assist in the prediction of flood and drought activity several decades into the future. We consider the hypothesis of a correlation or causal relationship existing between solar activity (including its effects on the intensity of galactic cosmic rays on the Earth) and flood cycles for the medium periods. The Australian SE coast and east-inland sites, and the Brahmaputra River, show strong medium-period maxima. The phase of the medium periods is obtained by optimized fitting of multiple sine curves with periods obtained from the spectra; the Australian SE coast and east-inland sites show summed sine curves with high correlation. The Brahmaputra River shows similar correlation at medium periods (in particular the Gleissberg 85-year period) but in opposite phase. The Australian east-central site (Murray-Darling Basin) and the Nile River (Egypt) show only weak evidence for the medium-period maxima which suggests ocean proximity is a factor for these influences. The short duration SOI record shows weaker evidence for medium-period spectral maxima, and the Southern Annular mode (SAM) and Indian Ocean Dipole (IOD) show no obvious correlation with observed medium-period flood patterns at the selected sites. We speculate that the strong medium-period flood patterns are associated with the solar and/or cosmic ray cycles, observed in the cosmogenic record, where the causative mechanisms are yet to be established. We conclude that the association of floods in medium-period cycles in addition to the association with the better-known short period variations associated with the ENSO cycle, provides opportunity for empirical predictions of flood patterns over ~80 years, and for the further investigation of possible causative mechanisms linking solar phenomena to oceanic indices and multi-decadal flood patterns.

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