Bimodal climate control of shoreline change influenced by Interdecadal Pacific Oscillation variability along the Cooloola Sand Mass, Queensland, Australia

Abstract

Wave climate in southeast Queensland, Australia has been correlated to changes in the Southern Oscillation Index (SOI), but observations of shoreline change associated with the variable wave climate have been limited due to a paucity of aerial photography, LiDAR, and other beach profiling datasets in this region. A multi-decadal, sub-annual temporal resolution shoreline dataset spanning 1996 to 2017 was produced using satellite imagery collected by Landsat 5, 7, and 8. A total of 147 shoreline positions were delineated using the Modified Normalized Difference Water Index on cloud-free imagery and corrected for horizontal offsets forced by variable tide stages at the satellite flyover time. The relative influence of SOI, the Southern Annular Mode (SAM), Pacific Decadal Oscillation (PDO), and Subtropical Ridge Latitude (STR-L) and Pressure (STR-P) on shoreline dynamics along the Cooloola Sand Mass in Queensland are assessed by performing cross correlations between their respective index values and shoreline change distances calculated by the Digital Shoreline Analysis System. A bimodal climate control of shoreline change is observed dependent upon the phase of the Interdecadal Pacific Oscillation (IPO). IPO modulates the impacts of ENSO on eastern Australia through its control over the position of the South Pacific Convergence Zone and elevating/lowering of tropical Pacific sea surface temperatures. During negative IPO, SOI is negatively correlated to the Cooloola Sand Mass shoreline indicating that the shoreline retreats during negative SOI phases. During positive IPO, the impacts of SOI are weakened and the STR becomes the primary driver of shoreline change with shoreline response being contingent upon its orientation. The SW-NE aligned Noosa North shoreline erodes in response to poleward movement of the STR, likely due to the enhanced cross-shore wave attack as a result of anti-clockwise wave rotation. The opposite response is shown at the SE-NW aligned Rainbow Beach shoreline, which is oriented almost parallel to the incident wave direction and is sheltered by the Double Island Point headland. These results suggest that climate control on shoreline change at Cooloola Sand Mass, and likely other sand islands in the region, is two-tiered, whereby interdecadal variability of the IPO governs the relative influence of SOI and STR under the different IPO phases. The linked climate and shoreline variability correlation shown in this study provides significant insight into how the Cooloola Sand Mass shoreline will respond to future climate changes under a global warming scenario.