Understanding the role for internal variability in driving past and future ocean dynamic sea-level trends in CMIP6 simulations

Herein spatial variations of sea-level trends over the global ocean 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 multi-climate model ensemble of Coupled Model Intercomparison Project phase 6 (CMIP6) historical simulations are analyzed (n = 167) and weak and largely zonally homogenous sea level trends are found in the ensemble mean, implying little agreement in the pattern of sea level trends across the ensemble. While some CMIP6 simulations have regional sea level trends that are a close match to the altimeter record, none are a good match globally (maximum centered pattern correlation statistic globally of 0.45 and 5-95% range of -0.20-0.26), and simultaneously reproducing spatial variations of sea level trends from 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 (SST) 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 extratropical Southern Hemisphere and the tropical and North Pacific. Using pre-industrial control simulations (with no transient external forcing and from the same climate models), we determine where external forcing, principally anthropogenic in origin, has and will produce local (i.e., grid point level) trends in ocean dynamic sea level that are significant relative to internal variability. At present, climate models suggest that ocean dynamic sea level trends over ~15% of ocean area are significant relative to internal variability, with this number increasing to 37% by 2050 under a high emissions scenario (32% under a low emissions scenario).