Spurious trends in global surface drifter currents

The Global Drifter Program (GDP) has been measuring near-surface ocean currents with surface drifters since 1979. At least half of the World Ocean now has drifter velocity time series longer than 15 years. The availability of this data opens new opportunities to explore observationally how ocean circulation responds to changing surface forcing. In this paper we report evidence of an apparently spurious acceleration of global surface drifter currents. This rapid acceleration occurs in a pattern reflecting the geographic distribution of mean surface winds. For example, in the westerly wind region of the Southern Ocean this strengthening is at least 0.5 cm/s per year eastward, while in the easterly trade wind region of the tropics this strengthening is on average 0.25 cm/s per year westward. One possible explanation we explore is that the bias is due to the presence of some undrogued drifters whose frequency of occurrence changes in time and whose windage is significantly greater than that of the drogued drifters. 1 This paper is dedicated to the memory of Professor Peter Niiler, who first suggested this explanation