Climate-Driven Cross-Shore Change and Shoreline Rotation Revealed by Satellite Observations

Satellite derived shoreline position data from “CoastSat” has provided novel insights on the underlying climatic patterns driving cross-shore shoreline movement and shoreline rotation at many sites around the globe. Here we use CoastSat shoreline and wavelet coherence analysis to identify common scales between wave components and cross-shore shoreline change/alongshore shoreline rotation along the New Zealand coast.  Wavelet-based decomposition was performed on two contrasting wave-climate regimes, comprising 250 km of west coast shoreline exposed to the Southern Ocean and 260 km of east coast shoreline influenced by the Southeast Pacific. A key challenge inherent to wavelet analysis of highly noisy satellite-derived data is determining statistical significance. Our approach estimates null hypothesis empirically using Monte Carlo red-noise simulations using the same effective number of degrees of freedom as presented in the real data. Thresholds change when scales or results from different sections (transects) of the coast are combined. Transect-averaged wavelet results indicate that, along the west coast, changes in wave height are consistently accompanied by changes in wave direction over the analysed period (1999-2024), implying that higher waves are associated with a single dominant direction. In contrast, the east coast exhibits multiple coherent signals, indicating that similar wave heights can occur under different directional regimes. Global coherence for the east coast (0.66 to 0.76) shows a high coherence in the seasonal band and the dominance of the alongshore component of the wave radiation stress in explaining beach rotation.