Impacts of El Niño-Southern oscillation on multi-scale morphodynamics of an embayed beach in southern China

Increased coastal erosion and extreme oceanographic forcing driven by El Niño-Southern Oscillation (ENSO) along the Pacific coast are increasingly receiving concerns. Despite considerable attention, the specific impact of these phenomena on Chinese shorelines, particularly along the South China beaches, remains inadequately understood. To address this gap, this study conducted high-frequency surveys on the Qing'an Bay beach in South China and employ a profile fitting model to refined waterline extraction from monthly satellite images. The goal was to investigate the relationship between high-frequency shoreline changes and interannual climate variability (such as ENSO). The findings indicate that the coastal evolution is intricately linked to a combination of strong wave events, summer storms, winter cold waves, and the amplifying effects of ENSO. During La Niña phase, the sea level in the western Pacific experiences a rise, with associated storms proving particularly destructive to the beach environment, leading to violent oscillations in the coastline. Specifically, in the strong La Niña phases, significant wave heights markedly surpass neutral conditions, precipitating intensified sediment movement. However, it is imperative to recognize that shoreline changes alone do not fully characterize coastal erosion. Assessment must also encompass alterations in beach volume. By calculating beach volume per unit width that accounts for the influence of ENSO variability on the beach, the seasonal characteristics of the beach showed sediment losses in summer and autumn, and gains in winter and spring, which was more obvious during strong La Niña. Additionally, conceptual model analysis reveals that during the La Niña phase, the bay is fully exposed, whereas during the El Niño phase, significant headland sheltering is observed.