Extended Water Level Trends at Long-Record Tide Gauges Via Moving Window Averaging and Implications for Future Coastal Flooding

Quantifying the nature and patterns of sea level rise (SLR) is a critical challenge for the sustainability and resilience of coastal zones. Studies of SLR typically consider time-averaged mean sea level (MSL) trends, however, coastal flooding is more sensitive to extended water level (EWL), which includes time-variable components such as tides and seasonal variability. Here, we describe a new way to determine long-term trends of peak extended water levels (PEWL) considering combined monthly averaged MSL and tidal range and their changes in linear rates and variability, which are compared to trends in mean extended water levels (MEWL) over moving 20-year time windows at long-period tide gauges over two epochs: century-scale, and half-century-scale. Results show that a majority of locations exhibit different PEWL trends than MEWL trends in both epochs, and this difference has intensified in the past 50 years. There are strong regional coherencies observed: positive PEWL trends along western boundaries of basins (US Atlantic Coast, Japan, and Australia), and negative PEWL trends in most of Europe and the US Pacific Coast. Strong correlations to the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation are also observed at many locations. Significant contributions from changes in tidal range and/or variability highlights the importance of considering multiple components of the water level spectrum beyond MSL. Our results are further analyzed in the context of IPCC projections to demonstrate that increased rates of PEWL may have additional implications for future flood levels at locations which have large projected increases in SLR.

Automated summary

This study investigates the impact of sea level rise on coastal zones, focusing on the sensitivity of extended water levels to coastal flooding. Results show differences in PEWL trends compared to MEWL trends at most locations, with this difference intensifying in the past 50 years.