While it is well established that there is a global mean sea level trend due to long-term man-induced atmospheric warming, the magnitude and patterns of sea level trends can differ from place to place and over different time periods. Multiple ocean model simulations (where the global mean sea level trend was removed) show that while much of the regional pattern in the residual sea level changes can be explained through both natural and man-made atmospheric forced variability (e.g. through changes in winds, air-sea exchanges etc.), in a substantial fraction (38-47%) of the global ocean, the influence of this atmospheric forcing on sea level trends can be masked by the spontaneous chaotic variability of the ocean.
Llovel et al.  show that this ocean “noise” effect is particularly evident in the strong flows of the western boundaries and of the Southern Ocean where chaotic eddies are commonly found. Although the intrinsic ocean variability appears stationary in time, the atmospherically forced part of the trend can change from year to year. This suggests that long-term measurements (more than 20 years) are needed in these regions so as to be able to accurately distinguish the atmospherically forced regional sea level trends from the inherent random ocean variability.
Citation: Llovel, W., Penduff, T., Meyssignac, B., Molines, J.‐M., Terray, L., Bessières, L., & Barnier, B. . Contributions of atmospheric forcing and chaotic ocean variability to regional sea level trends over 1993–2015. Geophysical Research Letters, 45. https://doi.org/10.1029/2018GL080838
—Janet Sprintall, Editor, Geophysical Research Letters