Removing spurious low-frequency variability in drifter velocities

Abstract

Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, tominimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergenceor tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from subergence to tether strain sensors. In this study,a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset. © 2013 American Meteorological Society.