Spatio-temporal changes in interannual sea level along the world coastlines

To investigate how coastal sea level evolves at interannual time scales from one region to another, we perform an Empirical Orthogonal Function (EOF) decomposition of coastal sea level time series derived from reprocessed coastal altimetry data over a 20-year long time span (2002−2021), at 1132 virtual coastal stations homogeneously distributed along the world coastlines. This analysis is first performed globally and then over selected coastal regions. Consistent with previous studies, the results show the dominant influence of internal climate modes, in particular ENSO (El Niño Southern Oscillation), on the interannual coastal sea level variability. But our study also reports novel findings in coastal sea level: (1) a regime shift between 2008 and 2012, with increased sea level rate after that date in many coastal regions, and (2) a 6-year cycle, notably along the northeast and northwest coasts of America (north of 40°N) and along the Indian coast of Indonesia. Additional EOF analyses are done using both gridded altimetry sea level data from the Copernicus Climate Change Service and an ensemble mean of four ocean reanalyses, in three successive coastal bands: 0-50 km, 50-100 km and 100-500 km from land. They confirm the strong influence of internal climate modes at the coast but also show that results in the cross-shore direction towards the open ocean are similar to those at the coast. This study based on three different datasets shows that the strong influence of internal climate modes on sea level interannual variability in the world coastal zones is not limited to the open ocean but also dominates sea level changes very close to the coast. It also shows evidence of a regime shift in the rate of change of coastal sea level between 2008 and 2012 and the presence of a 6-year oscillation in coastal sea level, possibly linked to the recent discovery of a 6-year cycle in the whole climate system.