Forcing of subannual-to-decadal sea level variability and the recent rapid rise along the U.S. Gulf Coast

From the mid-late 2000s through at least the mid-2010s, dynamic sea level (DSL) along the U.S. Gulf Coast rose at a rate of ~5-7 mm/yr, almost twice the rate of global mean steric plus barystatic sea level rise. Previous statistical and numerical modeling studies have suggested a number of hypotheses for this enhanced rise. However, the atmospheric and hydrologic forcing contributions to this enhanced rise have not been quantified. This study quantifies forcing contributions to DSL using adjoint sensitivities from the observationally-constrained Estimating the Circulation and Climate of the Ocean (ECCO) state estimation framework. Convolving the adjoint sensitivities with air-sea fluxes from the ECCO state estimate and JRA55-do river discharge shows that Gulf Coast DSL at subannual and interannual timescales is mostly driven by wind stress along the Gulf and U.S. Atlantic coasts, consistent with the propagation of coastal Kelvin and topographic Rossby waves. In contrast, the decadal-timescale enhanced rise in DSL can be attributed to trends in wind stress and heat fluxes across the North Atlantic, from the Caribbean to the subpolar latitudes. A decline in Mississippi river discharge during this period produced a ~1 mm/yr decrease in western Gulf Coast DSL, tempering the rapid rise in the down-waveguide direction from the Mississippi outflow. Reconstructions of the decadal Gulf Coast DSL using ERA5 atmospheric reanalysis were not as successful at reproducing the timing of the rapid rise, suggesting the importance of dynamically-consistent ocean data assimilation in constraining estimates of decadal-timescale air-sea flux variations.