High water above the annual mean had no network trend at 173 UHSLC tide gauges in 1980-2021; local trends exceeded chance but did not extrapolate to 2010-2021

Question: does the height of high water above the annual mean sea level change, and is the cause the astronomical tide or the non-tidal part? Approach: we used UHSLC/JASL Research Quality hourly sea level at 173 stations (6,528 station-years, 1980-2021). For each station-year we fitted 31 tidal constituents by least squares. The primary metric was the 99th percentile of hourly sea level above the annual mean (P99). Trend models included 18.61-year and 4.43-year terms. A joint-year block-permutation null kept the correlation between stations. Results: the network median P99 trend was +0.17 mm per decade (95% interval -0.62 to +1.16). The median absolute P99 trend was 0.11 times the median absolute relative mean sea-level trend. But 35 stations had |t| > 1.96, against a null 95th percentile of 22 (p = 0.004). Models fitted on 1980-2009 with a linear trend had larger 2010-2021 errors than models without a trend (median MAE ratio 1.062, interval 1.033-1.089; trend better at 29% of stations). Tide trends explained more of the cross-station variance than non-tidal trends (R2 0.59 versus 0.32), but five large-tide stations caused this difference. Exploratory: on the US Atlantic and Gulf coasts (28 stations) P99 rose by a median 5.9 mm per decade (permutation p = 0.007), mainly in the 30-day-mean non-tidal part. This trend also did not extrapolate. Elsewhere the fast non-tidal part and skew surge decreased (p = 0.003 and 0.016), and instrument changes can explain this. Limitations: uneven network with few European stations, data end in 2021, one-year tide fits, post hoc regions.

extreme sea level · tide gauge · UHSLC · storm surge · skew surge · tidal change · mean sea level · nodal cycle · out-of-sample test · permutation test · coastal flooding