Seasonal Phase Alignment Governs How Long Coastal High‐Water Thresholds Are Exceeded Worldwide

Coastal extreme sea level is conventionally characterized by exceedance frequency or height, with the seasonal cycle removed as a nuisance. Using hourly records from 730 tide gauges worldwide, we quantify how the annual sea‐level cycle (ASLC) governs the time coastal thresholds spend exceeded. At each station we separate the astronomical tide, excluding the annual and semiannual constituents, reconstruct the ASLC, identify its annual‐harmonic phase and the storm‐surge season, and compare observations with a seasonally flattened counterfactual. The amplification of annual exceedance time increases monotonically with the ratio r of ASLC range to tidal range (Spearman ρ = 0.70) and weakens as r increases beyond about 1, ranging from negligible where the tide dominates to about +150% where the seasonal cycle does. After controlling for r , amplification decreases as the annual‐harmonic and storm‐season phases separate (partial ρ = −0.43, p < 0.001); in the best‐sampled r stratum, median amplification is 29% larger for in‐phase than anti‐phase stations (ratio 1.29, 95% CI 1.18–1.39). The association strengthens as co‐located gauges are merged. At matched exceedance quantile the mean episode lengthens by only 1.8%: the seasonal cycle multiplies exposure chiefly by adding episodes, not extending them. The ASLC moves 43 stations (5.9%) across a threshold marking an annual‐cycle‐dominated intertidal regime. Two negative results bound the scope: alignment shows no coherent global trend, and it does not affect near‐mean‐sea‐level emergence duration, which depends on amplitude alone. Seasonal phase alignment thus governs annual exceedance time in a way frequency‐based metrics do not capture.