Coats, Sloan, Philip R Thompson, Christopher G Piecuch, John Fasullo, Benjamin D Hamlington, Kristopher B Karnauskas, R Steven Nerem, Angelica R Rodriguez, Jacob M Steinberg, and Julius J M Busecke, October 2025: Understanding the role for internal variability in driving past and future ocean dynamic sea level trends in CMIP6 simulations. Journal of Climate, 38(20), DOI:10.1175/JCLI-D-24-0336.15685-5699. Abstract
Herein, spatial variations of sea level trends from the altimeter record are compared to contemporaneous (1993–2014) and future trends in ocean dynamic sea level from state-of-the-art climate models. A multiclimate model ensemble of CMIP6 historical simulations is analyzed (n = 560), and little agreement is found in the global pattern of ocean dynamic sea level trends across the ensemble. While some simulations have regional ocean dynamic sea level trends that are a close match to the altimeter record, none are a good match globally (maximum pattern correlation globally of 0.47 and 5%–95% range from −0.20 to 0.26), and simultaneously matching the altimeter record in the tropical and North Pacific and tropical and North Atlantic is particularly challenging. Our focus in this study is on differences across the individual historical simulations and the role for internal variability, external forcing, and structural factors in driving these differences. A close relationship is found between patterns of sea surface temperature trends and those in sea level, and both can be related to the trajectories of common modes of atmosphere–ocean variability, with centers of action in the Indian Ocean and the tropical and North Pacific. Using preindustrial control simulations, we determine where external forcing has and will produce local (i.e., gridpoint level) ocean dynamic sea level trends that are significant relative to internal variability. At the present (1992–2023), climate models suggest that ocean dynamic sea level trends over ∼15% of the ocean area are significant relative to internal variability, with this number increasing to 37% by 2050 under a high-emission scenario (33% under a low-emission scenario).
Plain Language Summary: We detail a new climate model hierarchy, CM4X. CM4X has two model configurations, CM4X-p25 and CM4X-p125, that differ only in the ocean/sea ice horizontal grid spacing. CM4X-p125 outperforms CM4X-p25 for certain climate processes, while maintaining skill levels seen in previous generations for other results. CM4X-p125 requires about 10 times less time than CM4X-p25 to reach pre-industrial control thermal equilibration. Also, CM4X-p125 equilibrates to an ocean state with roughly 400 ZJ less heat content than present-day, consistent with estimates of anthropogenic heat uptake since 1870, whereas CM4X-p25 equilibrates to a state with roughly 1100 ZJ more heat than present-day. Consequently, the CM4X-p125 ocean state has not drifted far from observational estimates. We propose the mesoscale dominance hypothesis to interpret the relatively rapid thermal equilibration of CM4X-p125 to a cooler and more realistic pre-industrial state. Such ocean models result from negligible spurious mixing (from numerical truncation errors) along with an active mesoscale transport and realistic parameterization of small scale (diapycnal) mixing. Noting the preliminary nature of our results, and with caveats detailed in this paper, we suggest that the more rapid thermal equilibration possible from mesoscale dominant ocean models greatly reduces the computational energy footprint of models that are not mesoscale dominant.
Plain Language Summary: We examine simulations from a new climate model hierarchy, referred to as Climate Model version 4X (CM4X). The finer grid component of the hierarchy, CM4X-p125, out shines its coarser sibling, CM4X-p25, for certain processes of interest for climate studies, though in others the results are not dramatically distinct. Each case study reveals the advances made by moving from the predecessor CM4.0 climate model to finer grid spacing in either the atmosphere or ocean. Even so, there remain many unresolved problems that help to guide further research and development goals and strategies.
Steinberg, Jacob M., Elizabeth Yankovsky, Sylvia T Cole, and Laure Zanna, November 2025: A landscape of mesoscale eddy vertical structure: The influence of bathymetric slope and roughness on kinetic energy. Journal of Physical Oceanography, 55(11), DOI:10.1175/JPO-D-25-0044.11987-2004. Abstract
Surface and upper-ocean measurements of mesoscale eddies have revealed the central role they play in ocean transport, but their interior and deep ocean characteristics remain undersampled and underexplored. In this study, mooring arrays, sampling with high vertical resolution, and a high-resolution global atmosphere–ocean coupled simulation are used to characterize full-depth mesoscale eddy vertical structure. The vertical structure of eddy kinetic energy, e.g., partitioning of barotropic to baroclinic eddy kinetic energy or vertical modal structure, is shown to depend partly on bathymetric slope and roughness. This influence is contextualized alongside additional factors, such as latitude and vertical density stratification, to present a global landscape of vertical structure. The results generally reveal eddy vertical structure to decay with increasing depth, consistent with theoretical expectations relating to the roles of surface-intensified stratification and buoyancy anomalies. However, at high latitudes and where the seafloor is markedly flat and smooth (approximately 20% of the ocean’s area), mesoscale eddy vertical structures are significantly more barotropic by an approximate factor of 2–5. From a climate modeling perspective, these results can inform the construction, implementation, and improvement of energetic parameterizations that account for the underrepresentation of mesoscale eddies and their effects. They also offer expectation as to a landscape of eddy vertical structure to be used in inferring vertical structure from surface measurements.
Zilberman, Nathalie, William Llovel, Jacob M Steinberg, Benoit Meyssignac, Michaël Ablain, and R Fraudeau, August 2025: Deep ocean steric sea level change in the subtropical northwest Atlantic Ocean. Geophysical Research Letters, 52(16), DOI:10.1029/2024GL114158. Abstract
The non-closure of the global sea level budget, detected since 2017, stimulates the need to better understand limitations of satellite altimetry and gravimetry measurements, and breakdown in situ measurement contributions and gaps. Here, temperature and salinity profiles collected in the subtropical Northwest Atlantic Ocean between 2017 and 2022 by Deep Argo floats are used to partition steric sea level variability into contributions as a function of depth. Interannual steric sea level variability near the surface is of the same order of magnitude over the western boundary and abyssal plain, but fluctuations below 2,000 m over the western boundary are seven times larger and seem affected by local wind forcing. This analysis demonstrates how Deep Argo enables new evaluation of regional sea level budgets and comparison to geodetic products. Differences between float measurements and GLORYS12 highlight the need for more deep-ocean measurements that can be assimilated in the development of ocean reanalysis products.
Using a recently developed 1/12th degree regional ocean model, we establish a link between U.S. East Coast sea level variability and offshore upper ocean heat content change. This link manifests as a cross-shore mass redistribution driven by an offshore thermosteric sea level response to subsurface warming or cooling. Approximately 50% of simulated monthly to interannual coastal sea level variance south of Cape Hatteras can be statistically accounted for by this mechanism, realized as a function of regional ocean hypsometry, gyre scale warming, and the depth dependence of density change. This response to offshore warming explains the nonstationarity of U.S. East Coast sea level covariance, a specifically observed and modeled behavior after ~ 2010. Since approximately 2010, elevated rates of sea level rise south of Cape Hatteras can be partly explained as the result of shoreward mass redistribution due to offshore subsurface warming within the North Atlantic subtropical gyre. These results reveal a mechanism that connects local coastal sea level to a broader region and identifies the influence of regional heat content changes on coastal sea level. This analysis presents a framework for identifying new regions that may be susceptible to enhanced sea level rise due to ocean warming and helps bridge the gap between quantifying large scale change and anticipating local coastal impacts that can make flooding and storm surge more acutely damaging.
We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence.
