Removing Spurious Low-Frequency Variability in Drifter Velocities

Rick Lumpkin * NOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida

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Semyon A. Grodsky Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland

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Luca Centurioni Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Marie-Helene Rio Space Oceanography Division, CLS, Toulouse, France

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James A. Carton Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland

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Dongkyu Lee Department of Oceanography, Pusan National University, Busan, South Korea

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Abstract

Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or 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 submergence 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.

Corresponding author address: Rick Lumpkin, NOAA/AOML, 4301 Rickenbacker Cswy., Miami, FL 33149. E-mail: rick.lumpkin@noaa.gov

Abstract

Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or 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 submergence 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.

Corresponding author address: Rick Lumpkin, NOAA/AOML, 4301 Rickenbacker Cswy., Miami, FL 33149. E-mail: rick.lumpkin@noaa.gov
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