Present-Day Sea Level Variations in Coastal Areas from Daily to Multidecadal Time Scales: Observations and Causes

Climate-related sea level changes near the coast may deviate from what we observe in the open ocean. In effect, coastal changes result from the superposition of the global mean sea level rise, regional sea level trends, plus local contributions related to processes specific to nearshore areas. The latter operate on a broad range of timescales, from subdaily to multidecadal. They include ocean tides, atmospheric forcing, wind and waves, trapped coastal waves, coastal currents, river discharge in deltas and estuaries, natural climate modes and remote steric and mass effects. Changes in coastal ocean circulation driven by bathymetry or in coastal morphology and changing forcing factors can also impact coastal sea level. Such coastal processes not only produce coastal sea level changes, but also mediate the coastal response to open-ocean forcing. Vertical crustal motions are an additional factor causing sea level to vary with respect to the ground, in a proportion often larger than current climate-related sea level changes. Due to all involved processes, coastal sea level can significantly vary from one region to another. Here, we first discuss how coastal sea level is measured by in situ and space-based observing systems. Next, we review all processes causing sea level variations at the coast, from high-frequency extreme events to long-term multidecadal changes. We finally discuss different types of models developed for process understanding.