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

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 multi-climate model ensemble of CMIP6 historical simulations are 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 0.45 and 5–95% range of −0.20−0.26), 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 pre-industrial control simulations, we determine where external forcing has and will produce local (i.e., grid point level) ocean dynamic sea-level trends that are significant relative to internal variability. At present (1992–2023), 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 (33% under a low emissions scenario).