Nonlinear Interactions of Sea‐Level Rise and Storm Tide Alter Extreme Coastal Water Levels: How and Why?

Sea‐level rise (SLR) increasingly threatens coastal communities around the world. However, not all coastal communities are equally threatened, and realistic estimation of hazard is difficult. Understanding SLR impacts on extreme sea level is challenging due to interactions between multiple tidal and non‐tidal flood drivers. We here use global hourly tidal data to show how and why tides and surges interact with mean sea level (MSL) fluctuations. At most locations around the world, the amplitude of at least one tidal constituent and/or amplitude of non‐tidal residual have changed in response to MSL variation over the past few decades. In 37% of studied locations, “Potential Maximum Storm Tide” (PMST), a proxy for extreme sea level dynamics, co‐varies with MSL variations. Over all stations, the median PMST will be 20% larger by the mid‐century, and conventional approaches that simply shift the current storm tide regime up at the rate of projected SLR may underestimate the flooding hazard at these locations by up to a factor of four. Micro‐ and meso‐tidal systems and those with diurnal tidal regime are generally more susceptible to altered MSL than other categories. The nonlinear interactions of MSL and storm tide captured in PMST statistics contribute, along with projected SLR, to the estimated increase in flood hazard at three‐fourth of studied locations by mid‐21st century. PMST is a threshold that captures nonlinear interactions between extreme sea level components and their co‐evolution over time. Thus, use of this statistic can help direct assessment and design of critical coastal infrastructure. Plain Language Summary Sea‐level rise poses a significant threat to coastal communities worldwide, but the level of risk varies across different areas, making accurate estimation challenging. This study analyzed the tidal data from around the Globe to understand the complex interactions between tides, surges, and mean sea level (MSL) fluctuations. The research found that in most locations, tides and non‐tidal residuals have changed due to variations in MSL over recent decades. This research proposes a conservative proxy for extreme sea level dynamics, called “Potential Maximum Surge Tide” (PMST). By mid‐century, the median PMST is projected to be 20% higher over all monitoring stations. Simply shifting storm tide predictions up based on projected SLR may underestimate the flooding risk up to fourfold. The interactions between MSL and storm tides, captured throug