CoAuthors

Christopher Piecuch (WHOI, United States); Benjamin Hamlington (NASA JPL, United States ); Sloan Coats (University of Hawaii Manoa , United States ); Phillip Thompson (University of Hawaii Manoa , United States )

Thirty years of satellite-altimeter measurements show that rates of sea level rise are increasing across much of the global ocean. Such sea-level acceleration is particularly pronounced along the U.S. Gulf of Mexico coastline. Here we use model and observational data to identify the underlying mechanisms responsible for the enhanced rates of coastal sea-level rise in this region. Specifically, we quantify that subsurface warming in the Gulf of Mexico contributes importantly to sea-level rise at the coast. Using the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate, we find changes in ocean bottom pressure are the main contributor to coastal sea-level changes in this region on inter-annual to decadal timescales.These ocean bottom pressure changes reflect both a net mass flux into the Gulf, but more importantly a redistribution of mass within the Gulf, which can be understood as an isostatic ocean response to warming of the Gulf of Mexico below the seasonal mixed layer. The nature of this response is shaped by basin geometry and the depth of warming. This relationship, between ocean bottom pressure at the coast and subsurface warming over the deep Gulf of Mexico, is then tested using observations of ocean bottom pressure from the Gravity Recovery and Climate Experiment (GRACE), coastal relative sea level from tide gauge measurements, sea-surface height from altimetry, and Argo profiles of upper ocean temperature and salinity. Results show that changes in coastal ocean bottom pressure predicted from observed subsurface warming explain a dominant fraction of observed coastal sea-level changes. For example, at St. Petersburg, Florida over the 2008-2017 period, we find subsurface-warming-driven ocean-mass-redistribution effects account for ~50% of the observed coastal sea-level trend — a greater fraction than that due to mass flux into the Gulf. This analysis identifies a physical mechanism by which coastal sea level responds to open-ocean subsurface warming. It motivates future studies of the open-ocean influence on coastal sea level in other regions.