Using Drifter Velocity Measurements to Assess and Constrain Coarse-Resolution Ocean Models

Properly fitting ocean models to observations is crucial for improving modelperformance and understanding ocean dynamics. Near-surface velocity measurementsfrom the Global Drifter Program (GDP) contain valuable information about upper oceancirculation and air-sea fluxes on various space and time scales. This study exploreswhether GDP measurements can be used for usefully constraining the surfacecirculation from coarse-resolution ocean models, using global solutions produced bythe consortium for Estimating the Circulation and Climate of the Ocean (ECCO) as anexample. To address this problem, a careful examination of velocity data errors isrequired. Comparisons between an ECCO model simulation, performed without anydata constraints, and GDP and Ocean Surface Current Analyses Real-time (OSCAR)velocity data, over the period 1992--2017, reveal considerable differences inmagnitude and pattern. These comparisons are used to estimate GDP data errors inthe context of the time-mean and time-variable surface circulations. Both instrumentalerrors and errors associated with limitations in model physics and resolution(representation errors) are considered. Given the estimated model-data differences,errors and signal-to-noise ratios, our results indicate that constraining ocean stateestimates to GDP can have a substantial impact on the ECCO large-scale time-meansurface circulation over extensive areas. Impact of GDP data constraints on the ECCOtime-variable circulation would be weaker and mainly limited to low latitudes.Representation errors contribute substantially to degrading the data impacts.