Drivers and predictability of extreme still water level trends and interannual variability along the coast of Australia across different time scales

Still water levels, representing the height above a datum of a smooth water surface without surface waves, are typically measured by tide gauges in ports. In this study, we present an improved model for predicting non-stationary extreme still water levels (ESWL), which accounts for both deterministic ESWL driver components and a probabilistic component to account for stochastic influence of storm events using 30-year records (1990–2019) from 76 tide gauges around the Australian coast. The deterministic formulation of the model represents the non-stationarity of the trends in the overall ESWL driven by climate change, along with the individual contribution to interannual variability made by astronomical tidal cycles and ENSO variability. The probabilistic formulation of model represents the nature of remaining stochastic storm surge occurrence.

Our findings discuss significant ESWL trends, with and without background mean sea level rise included. A 4.4-year lunar tidal cycle significantly affects ESWL variability in northern Australia. While La Niña raises ESWLs at most sites, many in southeastern Australia are unaffected.

We use the model to examine high impact future climate contributions, which project an additional six-mm/year sea level rise and a 16% increase in ENSO by the end of the century. Our results suggest that sea level rise will dominate the other drivers and the projected increase in ENSO variability across Australia.

This model enables more precise predictions, identifying emerging thresholds for ESWL impacts in the coming decades. Here, improved coastal modelling efforts can inform planning and adaptation measures in response to rising sea levels.