Southern Annular Mode influences on shoreline variability at an embayed beach in southern Australia

In coastal areas where Southern Ocean swells are observed, the Southern Annular Mode (SAM) climate index has been shown to be a useful predictor for wave conditions. However, the relationship between SAM and beach morphology change in these areas is not well known. In this study, empirical orthogonal function statistical analysis was applied to satellite-derived shoreline data from 1987 to 2021 at Grassy Beach, King Island, Australia. The dominant modes of shoreline variability were calculated and the interconnection between SAM and shoreline position investigated. Cross-shore beach movement and seasonal beach rotation were the dominant sources of shoreline variability, accounting for ~64 and ~18% of total shoreline variability respectively. Shoreline retreat (advance) was observed at the eastern (south-western) end of the embayment during winter and, conversely, shoreline retreat (advance) at the south-western (eastern) end during summer. A clear connection between the SAM and shoreline variability was found, the SAM modulating beach rotation depending on the SAM phase. When SAM was positive, beach rotation increased with a greater difference in beach orientation between summer and winter owing to an increased shoreline retreat at the eastern end of the embayment by 4.1 m on average. This increased retreat was attributed to more powerful south-westerly waves in winter for positive SAM. This research demonstrated a strong relationship between SAM and shoreline change at a sandy embayment in southern Australia and may have implications for the current and future morphodynamics at other beaches influenced by the Southern Ocean wave climate.