The Demand of Total Ocean Surface Current Measurement

Ocean surface current system contains multi-scales, including large-scale geostrophic current, basin-wide gyres, and ageostrophic currents, such as Ekman currents, Stokes, tides, etc. Understanding the full ocean surface current system requires measuring the total ocean surface current vector at multi-scales. Today's satellite observations cannot directly measure the total ocean surface current vector with global coverage, even though the large-scale geostrophic current can be derived from spaceborne altimetry based on the geostrophic balance assumption. This study uses the most recent 5-year GDP (Global Drifter Program) drifter data to derive near-surface ocean current velocity. After correcting the wind-induced slippage contribution from drifter observations, GDP drifter-derived ocean current velocity is compared with the existing commonly-used ocean current datasets, i.e., OSCAR (Ocean Surface Currents Analyses Real-time) and AVISO (Archiving Validation and Interpolation of Satellite Oceanographic Data) geostrophic current datasets. There are discrepancies among these three datasets, and the significant discrepancies are evident, especially around the equator regions since the geostrophic assumption is not valid at the equator. Our results indicate that the existing ocean current datasets cannot fully resolve the variability of total ocean surface current. A clear solution is to measure the global total ocean surface current vector, which is highly desirable.