Sea-level rise is one of the most damaging consequences of climate change and is projected
to displace millions of people and cause billions of dollars in economic losses by 2100. Robust
estimates of the rate of sea-level rise are crucial to risk assessments, and thus, to the design
of risk-reducing strategies for sea-level rise. However, obtaining such estimates remains a
major scientific challenge, particularly in coastal areas where they are needed most. Despite
remarkable progress in understanding the causes of global and regional sea-level rise, we still
do not fully understand how coastal sea-level changes relate to open-ocean forcing. It is widely
recognized that coastal sea level can fluctuate coherently over large distances along the coast
and be decoupled from changes in the nearby open ocean. Numerical simulations suggest
that this phenomenon is closely tied to boundary waves. If wave decay is weak, this would
both enhance alongshore coherence and restrict the transmission of oceanic signals towards
the coast. Yet differences in observed sea-level trends between the coast and the open ocean
are inconclusive in this respect and often inconsistent across studies. Despite the expectation
that the continental slope can act to insulate the coastal zone from open-ocean changes, we
do not understand exactly how, where, and to what extent, this happens.

The two key gaps in our knowledge are: (1) the extent to which open-ocean changes affect
coastal sea level and (2) how this relates to alongshore coherence. Tide gauges provide sea-level
measurements right at the coast but not on the continental shelf and they are spatially
sparse. Satellite altimetry provides observations at ungauged locations and also on the shelf
and in the open ocean, but such observations tend to be unreliable within about 20 km from
the coast due to land contamination of the altimeter signal.

This ISSI project will address these knowledge gaps through new coastal altimetry observations
that leverage recent advances in processing and technology to improve the quality of the
coastal observations. The analysis of the new altimetry data together with model simulations
and data from tide gauges and satellite gravimetry will answer fundamental questions about
the cross-shelf structure of coastal sea-level changes, their alongshore scale, and the role of
local and remote forcing. We will also aim to provide specific recommendations for future
improvement of the space observations (altimetry and gravimetry) and coastal oceanographic
observations for future projects.