Quantifying the Seasonal Cycle of Sea-Level Using Satellite Altimetry

Abstract

Sea level change is a major consequences of anthropogenic climate change. However, sea-level does not change uniformly: it exhibits pronounced spatial and temporal variability driven by processes including atmospheric forcing (wind and pressure), ocean mass redistribution due to land-ice loss, steric effects linked to temperature gradients, and changes in ocean circulation. Understanding this variability, particularly on seasonal timescales, is essential for improving sea-level projections and informing coastal adaptation and protection strategies. Despite its importance, the seasonal dependence of regional sea-level variability remains insufficiently understood. Therefore, this study investigates how and why the spatial and temporal characteristics of the seasonal cycle of regional sea-level vary during the satellite altimetry era (1993 – present). Satellite altimetry observations of sea-level anomalies (SLA) are used to characterise seasonal patterns. The influence of large-scale climate variability is assessed through correlations between SLA and major climate indices, including ENSO, PDO, NAO and AMO. Ocean bathymetry data is used to quantify the difference between continental shelf regions (> -200 m) and the open ocean (< -200 m), enabling evaluation of the role of ocean bottom pressure (OBP) anomalies and associated barotropic processes. Results show enhanced seasonal sea-level anomalies in equatorial regions, semi-enclosed and shallow basins, along coastlines, and along the eastern margins of large Northern Hemisphere landmasses. Sea-level maxima typically occur during the autumn of the respective hemisphere. Large-scale climate modes exhibit clear regional and seasonal dependencies: ENSO and PDO show strong correlations with SLA in the tropical Pacific and Indian Ocean, predominantly during DJF and SON, while NAO-related variability dominates the North Atlantic during DJF. The relative importance of baroclinic and barotropic processes also varies regionally. Baroclinic effects play a larger role in the open ocean, whereas barotropic processes are more influential at high latitudes and over shallow continental shelves. Seasonal amplitudes are frequently amplified on continental shelves, where stronger correlations with OBP indicate a dominant barotropic contribution. Together, this demonstrates that the seasonal sea-level cycle is highly regional and dynamically complex and highlights the importance of incorporating seasonal sea-level dynamics into coastal risk assessments and adaptation planning.