Emerging high-resolution global ocean climate models are expected to improve both hindcasts and forecasts of coastal sea level variability by better resolving ocean turbulence and other small-scale phenomena. To examine this hypothesis, we compare annual to multidecadal coastal sea level variability over the 1993-2018 period, as observed by tide gauges and as simulated by two identically-forced ocean models, at $\sim$1$^{\circ}$ (LR) and $\sim$0.1$^{\circ}$ (HR) horizontal resolution. Differences between HR and LR, and misfits with tide gauges, are spatially coherent at regional alongcoast scales. Resolution-related improvements are largest in, and near, marginal seas. Near attached western boundary currents, sea level variance is several times greater in HR than LR, but correlations with observations may be reduced, due to intrinsic ocean variability. Globally, in HR simulations, intrinsic variability comprises from zero to over 80\% of coastal sea level variance. Outside of eddy-rich regions, simulated coastal sea level variability is generally damped relative to observations. We hypothesize that weak coastal variability is related to large-scale, remotely-forced, variability; in both HR and LR, tropical sea level variance is underestimated by $\sim$50\% relative to satellite altimetric observations. Similar coastal dynamical regimes (e.g., attached western boundary currents) exhibit a consistent sensitivity to horizontal resolution, suggesting that these findings are generalizable to regions with limited coastal observations.

Changes in the amplitude of decadal climate variability over the 20th century have been noted, with most evidence derived from tropical Pacific sea surface temperature records. However, the length, spatial coverage, and stability of most instrumental records are insufficient to robustly identify such non-stationarity, or resolve its global spatial structure. Here, I find that the long-term, stable, observing platform provided by tide gauges reveals a dramatic increase in the amplitude and spatial coherence of decadal (11-14 year period) coastal sea level (𝜁) variability between 1960 and 2000. During this epoch, western North American 𝜁 was approximately out of phase with 𝜁 in Sydney, Australia, and led northeastern United States 𝜁 by approximately 1-2 years. The amplitude and timing of changes in decadal 𝜁 variability are consistent with changes in the spatial structure of atmospheric variability. In particular, central equatorial Pacific wind stress (𝜏 𝐶 𝑃) and Labrador sea heat flux (𝑄 𝐿𝑆) are highly coherent with 𝜁 and exhibit contemporaneous, order-of-magnitude increases in decadal power. These statistical relationships have a mechanistic underpinning: along the western North American coastline, 𝜏 𝐶 𝑃 variability is known to drive rapidly propagating 𝜁 signals along equatorial and coastal waveguides; while a 1-2 year lag between 𝑄 𝐿𝑆 and northeastern United States 𝜁 , is consistent with a remotely-forced, buoyancy-driven, mechanism. Tide gauges thus provide strong independent support for an increase in inter-basin coherence on decadal timescales over the second half of the 20th century, with implications for both the interpretation and prediction of climate and sea level variability. SIGNIFICANCE STATEMENT: Decadal climate variability influences the frequency and severity of many natural hazards (e.g., drought), with considerable human and ecological impacts. Understanding observed changes and predicting future impacts relies upon an understanding of the physical processes and any changes in their variability and relationship over time. However, identifying such changes requires very long observational records. This paper leverages a large set of tide gauge records to show that global decadal-timescale coastal sea level variability increased dramatically in the second half of the 20th century in many locations. The increase was driven by a shift in the amplitude, spatial pattern, and inter-basin coherence, of atmospheric pressure, wind, and sea surface temperature variability.

Projections of the sea level contribution from the Greenland and Antarctic ice sheets rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous CMIP5 effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

The difference between North Atlantic subpolar gyre sea surface temperatures (SPG SSTs) and hemispheric- or global-scale surface temperatures has been utilized as an index of centennial-timescale changes in Atlantic Meridional Overturning Circulation (AMOC) strength. Here, using Community Earth System Model ensembles, we show that surface temperature-based indices (STIs) proposed to date largely reflect global-scale temperature trends and thus do not reflect dynamical relationships with AMOC. More broadly, we find that relationships between STIs, SPG SSTs, and AMOC strength differ greatly in significance and magnitude over different time periods because they are dependent upon the nature of external forcing. In the 20th century, characterized by offsetting greenhouse gas and aerosol forcing, the relationship between SSTs and AMOC strength varies widely and changes sign across a 20-member ensemble. We conclude that STIs and SPG SSTs are poor predictors of centennial-timescale AMOC strength variations.

A wealth of new climate model simulations have recently become available through the Coupled Model Intercomparison Project, Phase 6 (CMIP6). Evaluation of the representation of the Antarctic ocean across CMIP6 models is critical: projections of near-ice sheet temperature change will be used as input into sea level projections, and previous CMIP ensembles show substantial biases with a wide inter-model and inter-region spread. However, the ocean over the Antarctic continental shelf remains sparsely sampled, posing challenges for model-data comparison. Here, we assess a new clustering-based, grid-independent, methodology to identify and compare regional water masses, focusing on the Pacific sector of the Antarctic continental shelf. We find that temperature is insufficient to differentiate water masses, given the complexity and diversity of hydrographic profiles on the continental shelf. In contrast, clustering approaches applied to World Ocean Atlas 2018 temperature and salinity profiles identify “source” and “mixed” regimes that have a physically interpretable basis. For example, meltwater-freshened coastal currents in the Amundsen Sea, and High Salinity Shelf Water formation regions in the western Ross Sea, emerge naturally from the algorithm. We compare the location and properties of observed regimes to those found in the modern hydrographic state of the Community Earth System Model, version 2. Although CESM2 biases can be substantial, the locations of distinct regimes, and inter-cluster differences in water mass properties, are relatively consistent with observations. Differences in the locations and properties of hydrographic regimes are consistent with those expected from missing or poorly-represented physical processes (e.g. katabatic winds, ice shelf basal melting). We note other applications of this method, including the assessment of seasonal variability, and model-data comparison with different CMIP6 simulations and higher resolution regional ocean models.