Investigating Southern Annular Mode (SAM) as a Driver for Longshore Transport on Adelaide's Managed Beaches in South Australia

Perry, B.; Huisman, B.A.; Roelvink, D.; Hesp, P.A., and da Silva, GM., 2024. Investigation Southern Annular Mode (SAM) as a driver for longshore transport on Adelaide's managed beaches in South Australia. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 246-250. Charlotte (North Carolina), ISSN 0749-0208. A one-line shoreline hindcast model has been utilised to investigate the temporal variability and climatic drivers of shoreline change on Adelaide's Managed Beaches in South Australia. This coastline is a 28km long, highly developed, sandy beach shoreline with a number of coastal management interventions including rock revetments, training walls, offshore breakwaters and beach nourishment aimed at maintaining beach widths. There is a seasonally varying longshore transport gradient with a net northerly direction, which results in the erosion of southern beaches and shorelines downdrift of hard structures. The bimodal wave climate, which drives the longshore transport, is significantly correlated with the climate drivers in the region (the Southern Annular Mode, Indian Ocean Dipole and Southern Oscillation Index). In this study a 32 year hindcast time-series of longshore transport rates have been deduced for the system as well as spatially along the extent of Adelaide's Managed Beaches and correlated with the Southern Annular Mode (SAM), which is the most relevant anomaly for the longshore transport. The correlation between SAM and (northerly) transports is typically weak to moderate, but varies spatially with correlation coefficients as strong as 0.6 for the beach section north of Semaphore. Visualizations are made for the temporal and spatial relationship between the Southern Annular mode and the net, gross, southerly and northerly longshore transport, showing that gross and northward transport decrease with a strengthening of SAM, while southern transport increases with strengthening of SAM. This implies that less erosion of the recreational beaches takes place if SAM strengthens. Wave climate indices can thus have a substantial impact on coastal management requirements for the coastline. How SAM will change in the future as a result of climate change is, however, not understood well, and will be key for understanding future scenarios for Adelaide.