Global characteristics of coherent vortices from surface drifter trajectories

An algorithm is developed that can automatically identify loopers in Lagrangian trajectory data, i. e., looping trajectories that complete at least two orbits, in a significant update to Griffa et al. (2008). This algorithm is applied to the Global Drifter Program data set, and over 15,000 looping trajectory segments are identified worldwide. While two third of these segments are 14-39 days long, some persist for hundreds of days; the longest looper in the record persisted for 287 days. The paths taken by the vortices at the center of these looper trajectory segments can be calculated from these data. The Lagrangian integral time scale can also be estimated for the looper segments, and is generally very close to the orbital perioda value several times larger than the integral time scales characterizing nonloopers. Fundamental timemean quantities such as total kinetic energy and velocity are shown to be significantly different between loopers and nonloopers. These results suggest that a careful approach to the data might require separately calculating means of the nonloopers and loopers, and only afterward combining the weighted results for an overall time-mean picture. While many of the loopers with large radii orbit vortices identified in altimeterderived eddy census data, many with smaller radii do not match vortices resolved in altimetry. The data from this study are available at http://www.aoml.noaa.gov/phod/loopers/.