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Time-series analysis of extreme rainfall and flood events in two water catchments of Eastern New South Wales shows an indicative link to Gleissberg 87 yr cycles
  • Michael Asten,
  • Ken McCracken
Michael Asten
Earth Insight, Hawthorn Vic Australia

Corresponding Author:michael.asten.monash@gmail.com

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Ken McCracken
JelloreTechnologies, Mittagong, NSW, Australia
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Two sites in adjacent catchments located in eastern NSW provide hydrological data over 200 years since European settlement: (a) height of the Hawkesbury River at Windsor, within the Sydney Basin (HR); (b) level of the ephemeral Lake George, sited 100 km inland (LG). HR has experienced 43 moderate to major floods since 1799 with the timing of floods grouping into approximate 40-year segments of greater or lesser flood frequency. LG has a reconstructed history of annual levels (Short et al, 2020) which shows obvious spacings with range 50 to 80 years. Three features are clear. The close correlation in time between HR floods, and the deep LG records, in separate hydrological catchments, suggests that these were not random occurrences. The sunspot record shows clear correlation in timing of occurrence (but not of amplitudes) of anomalously weak sunspot maxima with high rainfall/flood-prone segments. High sunspot maxima are associated with dryer 40 yr segments. Both datasets yield meaningful spectra via Lomb-Scargle spectral analysis (the data lengths being too short for reliable Fourier spectra). These power spectra show maxima at periods 82-88 yr (HR) and 80 yr (LG). Subsidiary peaks at periods 50, 30, 20and 11 yr appear on both. These peaks align with the first three of the six named periodicities in solar activity; the Schwabe(11yr); Hale (22yr) and the Gleissberg (87 yr) periodicities. These three periodicities are present in the sunspot record from1609 CE, and in the cosmogenic (Be and C ) record of the past 50+ kyr where high sunspot activity correlates with low cosmic radiation and high total solar insolation (TSI). In particular HR for 1810-20 CE and LG for 1819-20 CE coincide with the last portion of the sunspot Dalton Minimum, the last of the “Little Ice Ages” experienced worldwide. The timing of HR floods correlates closely with La Nina events, and a subset correlates with solar cycle terminators described by Leamon et al (2021). We believe it is significant that a subset of terminators associated with dryer segments also approximate a pattern consistent with the 87 yr Gleissberg cycle. We conclude that just as solar cycle terminators appear to have predictive value for La Nina events, recognition of the Gleissberg cycle may have predictive value for ~80 yr cycles of flood-prone and drought-prone times.