Global Environmental Forcing of Coastal Waterline Variability Across Seasonal to Interannual Timescales

Abstract The position of the coastal waterline—the interface between land and sea—varies over time due to sedimentary and hydrodynamic processes influenced by climate variability. While previous studies have mainly focused on local morphodynamic or storm‐driven shoreline changes, the large‐scale climatic controls on global waterline variability remain poorly understood. This study addresses this gap by analyzing how climate‐driven environmental factors govern coastal waterline dynamics across seasonal to interannual timescales. Using a globally consistent, satellite‐derived data set of coastal waterlines from 1999 to 2019, combined with reanalysis‐based estimates of sea‐level anomalies, wave setup and freshwater runoff, we use an empirical mode decomposition framework to disentangle the contributions of these drivers. This approach isolates the dominant temporal components of waterline variability and identifies regional relationships with environmental forcing. Our results reveal two main regimes of climate‐driven waterline variability: (a) a seasonal pattern that intensifies with latitude and is primarily linked to storm‐driven wave energy flux and sea‐level oscillations; and (b) an interannual pattern that peaks in the tropics and is strongly modulated by large‐scale climate oscillations, such as ENSO, and by variability in fluvial discharge. We demonstrate that regional sea‐level and freshwater anomalies can alter the waterline by amounts comparable to those caused by waves. This challenges the conventional view that wave‐driven morphodynamics dominate waterline change. By framing the waterline as a climate‐sensitive indicator of coastal change, this study establishes the first global‐scale, data‐driven framework for diagnosing and predicting waterline variability. The results provide a basis for developing climate‐informed waterline forecasts and improving assessments of coastal vulnerability in the context of ongoing sea‐level rise.