Numerical Simulations of Short-Term Non-tidal Ocean Mass Anomalies

Short-term non-tidal ocean mass anomalies are currently reduced within the standard GRACE gravity field processing by means of the Ocean Model for Circulation and Tides (OMCT). Numerical sensitivity tests performed while defining the model configuration to be used for the atmosphere-ocean de-aliasing product (AOD) RL04 include the consideration of the European Centre for Medium-Range Weather Forecasts (ECMWF) short-term forecasts including accumulated wind stresses, the consideration of atmospheric and continental freshwater fluxes, and the treatment of time-variations of the total oceanic mass. Analyses of simulated ocean mass anomalies suggest that benefits of accumulated wind stresses do not outweigh errors contained in the forecasts with respect to the corresponding analyses. The impact of freshwater fluxes is generally small on ocean mass anomalies. Continental freshwater fluxes are in particular dominant on monthly time-scales and longer, allowing to neglect them for AOD purposes. Consequently, the total ocean mass is artificially held constant at every time-step in the operational simulations. Reasonable seasonal variations could have been only obtained after a yearly de-trending of the data, which cannot be performed in an operational setup.