The Zone of Influence: Matching sea level variability from coastal
altimetry and tide gauges for vertical land motion estimation
Abstract. Vertical land motion (VLM) at the coast is a substantial contributor to relative sea level change. In this work, we present a refined method for its determination, which is based on the combination of absolute satellite alimetry (SAT) sea level measurements and relative sea level changes recorded by tide gauges (TG). These measurements complement VLM estimates based on GNSS (Global Navigation Satellite System) by increasing their spatial coverage. Trend estimates from SAT and TG combination are particularly sensitive to the quality and resolution of applied altimetry data as well as to the coupling procedure of altimetry and tide gauges. Hence, a multi-mission, dedicated coastal along-track altimetry dataset is coupled with highfrequent tide gauge measurements at 58 stations. To improve the coupling-procedure, a so-called `Zone of Influence’ is defined to identify coherent zones of sea level variability on the basis of relative levels of comparability between tide gauge and altimetry observations. Selecting 20 % of the most representative absolute sea level observations in a 300 km radius around the tide gauges results in the best VLM-estimates in terms of accuracies and uncertainties. At this threshold, VLM_SAT-TG estimates have median formal uncertainties of 0.59 mm/year. Validation against GNSS VLM estimates yields a root-mean-square (RMS_VLM) of VLM_SAT-TG and VLM_GNSS differences of 1.28 mm/year, demonstrating the level of accuracy of our approach. Compared to a reference 250 km radius selection of sea level anomalies, the 300 km Zone of Influence improves trend accuracies by 12 % and uncertainties by 28 %. With progressing record lengths, the spatial scales of coastal sea level trend coherency increase. Therefore the relevance of the ZOI for improving VLM_SAT-TG accuracies decreases. Further individual Zone of Influence adaptations offer the prospect of bringing the accuracy of the estimates below 1 mm/year.