Assessing Global Ocean and Continental Mass Change from 17 years of GRACE/-FO: the role of coastal buffer zones

Following the ESA CCI Sea Level Budget Closure (SLBC) project's work of 2017–2019, we present updated time-series of

• Global Ocean Mass Change (OMC) and
• Continental Mass Change (CMC),

both including continuous measurements by GRACE-FO. By least-squares adjusting a multi-parameter fit to monthly resolved OMC, we estimate the linear trend of Global Ocean mass change over all presently available GRACE/-FO months since April 2002 to be 2.41 ± 0.22 mm/a, with an acceleration of 0.10 mm a^-2 over the same period.

A systematic analysis of the computed ocean response ("monthly fingerprints") to on-shore mass changes by means of solving the sea level equation implies that the common method of re-scaling the 'inner' buffered ocean may lead to OMC overestimations up to 10 per cent. We present this effect as a function of coastal buffer width for a global and for a 'truncated' global ocean (Lat ≤ ±65°), since the latter case, as seen e.g. in conjunction with radar altimetry, can lead to more than five per cent overestimated OMC. Furthermore, the use of coastal buffers seems to induce phase bias in the annual oscillation, which may explain the observed phase shift in the monthly budget between GRACE OMC and the sum of contributing continental components (i.e. ice sheets, glaciers, land water storage).

As a supplementary product of the SLBC project, we present GRACE/-FO derived mass change series 2002-04/present for continents (excluding Greenland and Antarctica). Consistent integration of out-leaking signal over coastal buffer zones, and subtraction of GAD-corrected mean OMC therein, leads to agreement with independently-assessed joint land water and glacier mass change data, well within uncertainty bounds.