Interannual Response of Reef Islands to Climate-Driven Variations in Water Level and Wave Climate

Abstract

Coral reef islands are among the most vulnerable landforms to climate change. However, our understanding of their morphodynamics at intermediate (seasonal to interannual) timescales remains poor, limiting our ability to forecast how they will evolve in the future. Here, we applied a semi-automated shoreline detection technique (CoastSat.islands) to 20 years of publicly available satellite imagery to investigate the evolution of a group of reef islands located in the eastern Indian Ocean. At interannual timescales, island changes were characterized by the cyclical re-organization of island shorelines in response to the variability in water levels and wave conditions. Interannual variability in forcing parameters was driven by El Niño Southern Oscillation (ENSO) cycles, causing prolonged changes to water levels and wave conditions that established new equilibrium island morphologies. Our results present a new opportunity to measure intermediate temporal scale changes in island morphology that can complement existing short-term (weekly to seasonal) and long-term (decadal) understanding of reef island evolution.

Identifying Metocean Drivers of Turbidity Using 18 Years of MODIS Satellite Data: Implications for Marine Ecosystems under Climate Change

“…It is likely that these turbidity drivers (higher water levels, wave power and rainfall) along with cyclones (due to the immense, but short-lived, wave power and winds) are the mechanism behind increased turbidity in the Gulf during La Niña phases of ENSO. We found many of these drivers (waves, rainfall, higher water levels) were intensified in the Exmouth Gulf region during the La Niña phase of ENSO (Figure 5) and this has been shown by others [45,77,93]. Furthermore, cyclonic activity is increased during La Niña phases in north Western Australia [94] and strong turbidity peaks are known to be associated with these events [47].…”

“…Close to the mouth of the North Gulf region is Eva Island and Exmouth Reef, which are heavily exposed to NNW swell wave and southerly wind-wave energy. Island shoreline change on Eva Island is known to be sensitive to the metocean conditions that occur with ENSO, displaying an oscillating orientation of erosion-accretion that aligns with ENSO events [45]. Seasonal turbidity patterns in this region are like those in the NE Coast region, with higher turbidity occurring during winter months, coinciding with the winter intensification of easterly wind forcing and swell waves.…”

“…The northern Gulf is influenced by wind-driven waves to 1.5 m, with a seasonally variable wave climate where longer period energy (peak period~7-10 s) ocean swell waves propagate into the Gulf during austral winter from the NNW and shorter period, locally generated sea waves (peak period~5 s) during the austral summer from the SSW [45]. Regional wind patterns are due to low pressure troughs extending over the inland Pilbara during summer resulting in a mostly south to south-easterly regime.…”

“…Wave data was extracted from a 20-year regional-scale, fully coupled wave-circulation hindcast (Delft3D-SWAN [45]) that has been extensively validated along the Pilbara coast [57]. Point data was extracted for the 13 locations from 2003 to 2019.…”

Primary drivers of multidecadal spatial and temporal patterns of shoreline change derived from optical satellite imagery

“…On microtidal beaches, satellite-derived shoreline (SDS) errors are typically under 10 m (e.g. Vos et al, 2019a;Bishop-Taylor et al, 2019;Cuttler et al, 2020). Therefore in such environments SDS can be used to improve the understanding of, for instance, embayed beach rotation (Di Luccio et al, 2019) or the dominant timescales of shoreline variability (Vos et al, 2019a).…”