Anatomy of Interannual to Decadal Sea Level Changes in the GECCO3 Ocean Synthesis

The GECCO3 synthesis is used to infer a dynamically consistent decomposition of sea level changes (variability and trends) on interannual to decadal periods.Over most parts of the global ocean equatorward of 60 latitude, sea level variability and trends can be explained almost completely by the upper ocean steric component alone.We find a strong but negative relation between the deep steric component and the manometric changes on decadal and longer time scales.Such a relation, albeit weaker, can also be identified on annual to decadal timescales in CMIP6 MPI-ESM climate model results.Sea level variability over all shallow regions and some fraction of the Southern Ocean appears to be dominated by the manometric component.Enhanced but short spatial scale manometric contributions exist also in boundary current regions, such as the North Atlantic or the Antarctic Circumpolar Current region.Our findings suggest that without a full depth steric sea level information available, observed sea level change on decadal and longer time scales might best be explained by just using the steric estimate available from the top 2000m depth as provided by Argo.Adding additionally information from the GRACE bottom pressure fields will likely degrade the decomposition of sea level in many regions of the world ocean.Implications of our findings for estimating ocean heat content changes from altimetry remain to be explored.We encountered large differences between the GECCO3 manometric component and what the GRACE project provides, suggesting remaining uncertainties to exist in the GRACE ocean products and/or GECCO3 fields.