<p>The relative contributions of local and remote wind stress and air-sea buoyancy forcing to sea-level variations along the East Coast of the United States are not well quantified, hindering the understanding of sea-level predictability there. Here, we use an adjoint sensitivity analysis together with an Estimating the Circulation and Climate of the Ocean (ECCO) ocean state estimate to establish the causality of interannual sea-level variations near the Nantucket island, the approximate geographic center of the northeast US coast where sea-level fluctuations are coherent. Wind forcing explains 68% of the Nantucket interannual sea-level variance, while wind and buoyancy forcing together explain 97% of the variance. Wind stress contribution is near-local, primarily from the New England shelf northeast of Nantucket. We disprove a previous hypothesis about Labrador Sea wind stress being an important driver of Nantucket sea-level variations and another hypothesis suggesting local wind stress being a secondary driver. Buoyancy forcing, as important as wind stress in some years, includes local contributions as well as remote contributions from the subpolar North Atlantic that influence Nantucket sea level a few years later. Our rigorous adjoint-based analysis corroborates previous correlation-based studies that sea-level variations in the subpolar gyre and the northeast US coast can both be influenced by subpolar buoyancy forcing. Forward forcing perturbation experiments further indicate remote buoyancy forcing affects Nantucket sea level mostly through slow advective processes, although waves can cause rapid Nantucket sea level response within a few weeks. Our results quantifying the spatial distribution of forcing contributions to Nantucket sea-level variations are also useful for the development of machine-learning models for predicting sea-level variation off the northeast US coast.</p>

Satellite observations are used to establish the dominant magnitudes, scales, and mechanisms of intraseasonal variability in ocean dynamic sea level (z) in the Persian Gulf over 2002-15. Empirical orthogonal function (EOF) analysis applied to altimetry data reveals a basinwide, single-signed intraseasonal fluctuation that contributes importantly to z variance in the Persian Gulf at monthly to decadal time scales. An EOF analysis of Gravity Recovery and Climate Experiment (GRACE) observations over the same period returns a similar large-scale mode of intraseasonal variability, suggesting that the basinwide intraseasonal z variation has a predominantly barotropic nature. A linear barotropic theory is developed to interpret the data. The theory represents Persian Gulf average z (z) in terms of local freshwater flux, barometric pressure, and wind stress forcing, as well as z at the boundary in the Gulf of Oman. The theory is tested using a multiple linear regression with these freshwater flux, barometric pressure, wind stress, and boundary z quantities as input and z as output. The regression explains 70% 6 9% (95% confidence interval) of the intraseasonal z variance. Numerical values of regression coefficients computed empirically from the data are consistent with theoretical expectations from first principles. Results point to a substantial nonisostatic response to surface loading. The Gulf of Oman z boundary condition shows lagged correlation with z upstream along the Indian subcontinent, Maritime Continent, and equatorial Indian Ocean, suggesting a large-scale Indian Ocean influence on intraseasonal z variation mediated by coastal and equatorial waves and hinting at potential predictability. This study highlights the value of GRACE for understanding sea level in an understudied marginal sea.

Local sea-level changes deviate from the global mean and unforced variability often masks sea-level changes driven by greenhouse forcing. Both cause difficulties when local observations are compared to projections. We present two analyses of local sea level aiming at improving understanding local causes of sea-level rise and variability. First, we analyse local sea-level budgets at 557 tide-gauge locations from 1993-2019. On average, the sum of contributing processes explains 49% of the observed inter-annual variance. Sterodynamic processes explain most of the variability. The average observed trend is 2.6 mm yr-1 with contributors summing up to 2.7 mm yr-1. Secondly, we extrapolate the current trajectory of sea-level rise and estimate how unforced variability can mask or exaggerate future long-term sea-level changes. Unforced variability can cause sea-level changes up to multiple decimeters on 30-year time scales, and the differences between projections and the trajectory are thus generally not significant.

Satellite observations are used to establish the dominant magnitudes, scales, and mechanisms of intraseasonal variability in ocean dynamic sea level (z) in the Persian Gulf over 2002-15. Empirical orthogonal function (EOF) analysis applied to altimetry data reveals a basinwide, single-signed intraseasonal fluctuation that contributes importantly to z variance in the Persian Gulf at monthly to decadal time scales. An EOF analysis of Gravity Recovery and Climate Experiment (GRACE) observations over the same period returns a similar large-scale mode of intraseasonal variability, suggesting that the basinwide intraseasonal z variation has a predominantly barotropic nature. A linear barotropic theory is developed to interpret the data. The theory represents Persian Gulf average z (z) in terms of local freshwater flux, barometric pressure, and wind stress forcing, as well as z at the boundary in the Gulf of Oman. The theory is tested using a multiple linear regression with these freshwater flux, barometric pressure, wind stress, and boundary z quantities as input and z as output. The regression explains 70% 6 9% (95% confidence interval) of the intraseasonal z variance. Numerical values of regression coefficients computed empirically from the data are consistent with theoretical expectations from first principles. Results point to a substantial nonisostatic response to surface loading. The Gulf of Oman z boundary condition shows lagged correlation with z upstream along the Indian subcontinent, Maritime Continent, and equatorial Indian Ocean, suggesting a large-scale Indian Ocean influence on intraseasonal z variation mediated by coastal and equatorial waves and hinting at potential predictability. This study highlights the value of GRACE for understanding sea level in an understudied marginal sea.