A Mechanistic Link Between Coastal Sea Level Rise and Offshore Subsurface Warming

In this work, using a newly developed 1/12th degree regional ocean model, we establish a link between U.S. east coast sea level variability and offshore upper-ocean density change. This link is realized as a mass redistribution response to thermosteric and halosteric changes offshore of and deeper than the shelf-break. A significant fraction of coastal sea level change, from seasonal to inter-annual timescales, is explained as a result of this mass redistribution and shown to depend on both regional ocean hypsometry and the depth dependence of density changes over time. The ability to establish this link and explain sea level changes at the coast importantly depends on model resolution of continental shelf and slope bathymetry. We specifically focus on a break in along-coast sea level covariance north and south of Cape Hatteras apparent in separate multi-year periods throughout the 1993-2019 model run. This break can be largely explained as a result of differing patterns of density change north and south of the Gulf Stream extension and below the seasonal thermocline. These results reveal a mechanism that connects any point on the coast to a broader region and identifies the influence of regional ocean circulation and heat content changes on coastal sea level. They likewise present opportunity to establish and improve seasonal to inter-annual sea level predictions with an increased understanding of how regional climate changes are communicated to the coast.