Observed Spatiotemporal Variability in the Annual Sea Level Cycle Along the Global Coast

Changes in the seasonal sea level cycle can modulate the flooding risk along coastlines. Here, we use harmonic analysis to quantify changes in the amplitude and phase of the annual component of the sea level cycle at 798 tide gauge locations along the global coastline where long records are available. We identify coastal hotspots by applying clustering methods revealing coherent regions with similar patterns of variability in the annual sea level cycle. Results show that for most tide gauges the annual amplitude reached its maximum after 1970 and its peak typically occurs during the fall season of the respective hemisphere. Many tide gauges exhibit non-stationarity in the annual cycle in terms of amplitude and/or phase. For example, at 226 tide gauges we find significant trends in the amplitude (either increasing or decreasing) for the time period after 1970; while several sites (50 in total), mostly in the Mediterranean and around Pacific islands, experienced phase changes leading to shifts in the timing of the peak of the annual cycle by more than a month over their entire record. Our results highlight the importance of accounting for potential non-stationarity in seasonal mean sea level cycles along coastlines.