Are ocean reanalyses useful for Earth rotation research?

Oceanic circulation and mass-field variability play important roles in exciting Earth’s wobbles and changes in length-of-day (ΔLOD), on time scales from days to several years. Quantifying these effects require estimates of ocean angular momentum (OAM), which are typically drawn from numerical forward models or coarse-resolution ocean state estimates. A little-tested alternative in this regard is the emerging suite of ocean reanalyses, operating at eddy-permitting horizontal resolution (1/4°) and with sequential data assimilation (DA) schemes. Here, we compute geophysical excitation series from three identically configured global ocean reanalyses that are based on the same hydrodynamic core and input data (e.g., altimetry, Argo, sea surface temperature) but different DA schemes. The resulting OAM time series are compared both with each other and with atmosphere-corrected geodetic excitation from 2007 to 2011. For periods less than 120 days, the reanalyses series typically explain 40–50% of the residual observed polar motion excitation, somewhat less than a widely used ocean state estimate. By contrast, the reanalyses’ skill in accounting for oceanic signals in ΔLOD is more varied, particularly when seasonal and longer time scales are included in the comparison. This result points to a misclosure of the global mass balance constraint among atmosphere, ocean, and terrestrial hydrology, in part depending on whether or not a specific reanalysis considers the discharge of continental freshwater into the ocean. Together, our analyses provide insight into the kinematic consistency of newly available ocean reanalyses and give a quantitative understanding of the influence of different DA schemes on OAM estimates.