Sea level at the coast : what are the major contributors to its variability and trend and how do they differ from the open ocean ?

Since 1993, satelite altimetry indicates that sea level has been rising fast in response to global warming, but with large regional variations. However, standard altimetric data do not allow to study sea level variability and rise close to the coast, where the socio-economic impacts of sea level rise are the greatest.

Here, we first study how sea level variability and trend spatially evolve from the open to the coastal ocean (from 200 to ~10 km off the coast) over the period 1993-2012 and for two regions: off the coast of western Africa and in the western Tropical Pacific. To do so, we analyse along-track altimetric data that have recently been reprocessed at LEGOS/CTOH using algorithms adapted to coastal regions to recover information in coastal zones (XTRACK dataset). We show that over the two regions of study, sea level variability is enhanced coastward.

When getting even closer to the coast (in the last 10 km), processes not included in altimetric data also contribute to sea level variability. In a second step, we therefore evaluate the relative importance of processes causing coastal sea level variability. A case study is developed in the Gulf of Guinea. Contributions from the altimetry-derived sea-level, wind set-up, atmospheric loading, wave-induced run-up and set-up, and astronomical tides are estimated from observational datasets and reanalyses. As these processes impact the coast differently, evaluating their importance is essential for assessment of the local coastline vulnerability. The leading contributors to sea level variability differ depending on the time-scales considered. The trend is dominated by processes included in altimetric data and to a lesser extent by swells run-up. The latter dominates interannual variations, while subannual variability is mainly due to tides. Extreme events are due to high tides and large swells and waves run-up, exhibiting a clear seasonal cycle with more events in summer.