Verification and intercomparison of global ocean Eulerian near-surface currents

Abstract

The OceanPredict task team for Intercomparison and Validation (IV-TT) has established the CLASS4 data standard for routine forecast verification against selected reference observing platforms. The set of CLASS4 reference data has been recently extended to include near-surface currents derived from the trajectories of drifting buoys drogued at 15 m. We have applied these data to the Ocean Model, Analysis and Prediction System (OceanMAPS) at the Australian Bureau of Meteorology (ABoM) for verification and inter-comparison with Mercator Océan International ocean forecast system (MOi), the operational models of the Met Office, UK: Forecast Ocean Assimilation Model (FOAM) and Coupled Atmosphere-Land-Ocean-Ice Data Assimilation (CPLDA) systems, and the Global Ice Ocean Prediction System (GDPS-GIOPS) at the Canadian Centre for Meteorological and Environmental Prediction (CCMEP). The aims for this verification analysis are to extend the routine monitoring of the operational system; to assess the OceanMAPS skill against other models; and to inform our stakeholders of the OceanMAPS performance.We have evaluated the impacts of adding Stokes drift and tidal currents from separate global wave and global tide models to the model currents on the verification of currents. Including Stokes drift leads to significant improvements in representation and the metrics for model skill while inclusion of tides has no significant impact on the global statistics. Zonal currents have a stronger signal than the meridional currents as indicated by their high correlation with observations, and lower errors. This indicates zonal currents on average are more predictable and persistent than the meridional currents, resulting in improved representation in the models.Overall, MOi, and the new version of OceanMAPS (4.0i), and FOAM show marginally better performance against the observations relative to other models. Although there are significant differences in the configurations of the eight models under evaluation, all the models are shown to be remarkably statistically equivalent with consistent spatial and temporal patterns indicating the main differences are attributable to unrepresented processes. We therefore conclude that there is significant scope to further improve the representation of the modelled currents with the observations.