Quantifying the contributions of surface forcing to sea level variability along the US Gulf Coast

Dynamic sea level (DSL) along the US coast of the Gulf of Mexico varies on synoptic-to-multidecadal timescales, with implications for coastal hazard mitigation, infrastructure, and ecosystems. On the decadal-to-multidecadal timescale, the DSL exhibited a substantial rise of ~5–6 mm yr^-1 from the mid‐to-late 2000s through the late 2010s, almost twice the rate of global mean sea level rise. Numerous hypotheses have been proposed to explain this rapid rise, involving mechanisms such as wind-forced coastal waves, subsurface warming, and the Atlantic Meridional Overturning Circulation.

Delman et al. (2026) use an adjoint model from the observationally-constrained Estimating the Circulation and Climate of the Ocean (ECCO) state estimate to quantify the contributions of surface atmospheric and hydrologic (river discharge) forcing to sea level. The adjoint model is integrated to compute the tangent-linear dependence of sea level in a region (e.g., eastern or western Gulf coast) on surface fluxes of momentum, heat, and freshwater for every grid cell globally and at varying lead times.

Reconstructions of DSL using ECCO adjoint sensitivities, convolved with ECCO air-sea fluxes and JRA55-do river discharge, indicate the dominance of wind-driven coastal Kelvin and topographic Rossby waves at subannual timescales and, to a lesser extent, at interannual timescales. However, decadal/multidecadal variations are driven by a wider range of forcings across the Gulf, Caribbean, and North Atlantic. Of the 2006–2017 DSL trend along the US Gulf Coast, only 20–30% is driven by winds along the Gulf and US Atlantic Coasts. Approximately 40% of the trend is driven by winds and heat fluxes in the tropical Caribbean and Atlantic, and ~30% by winds and heat/ freshwater fluxes in the interior subtropical-to-subpolar North Atlantic. The influence of Mississippi river discharge on regionally-averaged Gulf coast sea level is smaller but not negligible: ~20% of the positive 2006–2017 DSL trend along the western portion of the Gulf coast was offset by decreasing Mississippi river discharge during this period.

This study indicates the importance of assessing regional sea level changes using fully dynamical ocean models, constrained by observations where possible to improve accuracy. It also illustrates that an adjoint model can be a powerful tool for identifying the causes of sea level variations in the historical record and in quantifying the impacts of forcing on sea level in future model projections.

This research was supported by the NASEM Gulf Research Program and the NASA Physical Oceanography program.