Biannual Cycles in Intertidal Beach Volume Variations in New Zealand

Interannual beach variations in intertidal beach volumes have been shown to correlate with climatic drivers. In New Zealand, the main climatic drivers on the north east coast are the Pacific Decadal Oscillation (PDO) and the Southern Oscillation Index (SOI), which drive temperature and wind variations in the south Pacific. Lomb-scargle spectra of in situ intertidal beach volume measurements collected at 19 north-east beach sites and indices representing these two climate drivers show significant energy at biannual frequencies, yet no variation in wave climate at the same biannual timescale. In addition, the resolution of volume data precludes a detailed cross-correlation analysis. To explore the source of this biannual variation, a wave hindcast and an existing shoreline change model calibrated with 6 years of video-derived shoreline variations on one of the beaches was used to hindcast shoreline variations over (1) the time range of the in situ measurements (~17 years) and (2) over the time range of the hindcast (~33 years). Results show that the size of annual variations in the shoreline depend on the mean summer wave energy relative to the winter, which is inversely correlated with the energy difference the previous year (p=0.95). Moreover, the summer mean southern oscillation index had a similar inverse behaviour (p=0.90), and the two patterns are cross-correlated (p=0.95). Cross-correlation of the Pacific decadal oscillation index does not have the same pattern, likely due to the confounding effect of longer-term cycles in this index. Finally, results are not significant for the longer time range indicating that the biannual cycle is transient. Results show that very subtle variation in between the winter and summer wave climate can drive significant shoreline variations.