Statistical Behavior of ALACE Floats at the Surface of the Southern Ocean

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Autonomous Lagrangian Circulation Explorer (ALACE) floats were designed to measure subsurface velocities throughout the global ocean. In order to transmit their data to satellite, they spend 24 h at the ocean surface during each 10–25-day cycle. During this time the floats behave as undrogued drifters. In the Southern Ocean, floats tend to advect downwind and, in accordance with Ekman theory, slightly to the left of the wind during their time at the surface. Mean displacements are likely to carry floats northward and, correspondingly, with each cycle, the Southern Ocean floats will move into warmer water with higher dynamic height. Because of large variability, the northward trend may not be discernible for any single float: in 2 years' worth of 10-day cycles, a typical float will be displaced 100 ± 270 km northward relative to a float that never surfaces. Float surface velocities and wind speed are statistically correlated at the 95% confidence level. Compared with drogued drifters, floats tend to move more rapidly, are advected more strongly downwind, and are more sensitive to changes in wind speed. Regression coefficients estimated from the differences between float and drogued drifter velocities suggest that floats may be used to estimate the mean upper ocean currents in regions where drogued drifter data are not available.