Measuring surface currents with Surface Velocity Program drifters : the instrument , its data , and some recent results

For centuries, our knowledge of the oceans’ surface circulation was inferred from the drift of floating objects. Dramatic examples include wrecked Chinese junks and Japanese glass fishing balls which have washed ashore on the US west coast (Sverdrup et al., 1942). Such observations could only provide crude ideas of gyre-scale currents, as there was no way to tell the exact beginning (in time or space) of the drifter’s journey, or the trajectory it had taken. Currents can be more accurately inferred from ship drift measurements. A ship’s motion relative to the surrounding water is measured by the ship log; its absolute motion is estimated from navigational fixes. In the absence of wind and the “sailing” force of flow around the hull and keel, the difference between absolute and relative motion is the velocity of the water (the current). However, due to relatively large navigational errors in the mostly pre-GPS data set of ship drifts, such current estimates can have errors of O(20 cm/s) (Richardson and McKee, 1984). In addition, a drifting ship is exposed to both currents and wind, making the relative role of the two forces difficult to separate. Comparison of ship drifts with less windage-prone measurements have revealed significant differences in the tropical Pacific (Reverdin et al., 1994) and Atlantic (Richardson and Walsh, 1986; Lumpkin and Garzoli, 2005). To reduce the wind force, investigators in the early 1800s began using drift-bottles to map surface currents. These bottles were typically weighed down so that they were almost entirely submersed, and typically carried a note indicating their launch location and time (Sverdrup et al., 1942). Bottles have been used to map currents in regions such as the North Sea (Fulton, 1897; Tait, 1930) and northwestern Pacific Ocean (Uda, 1935). A major step towards collecting true Lagrangian time series of velocity was made by attaching a sea anchor, or “drogue”, to a buoyant object that would not extend far above the surface but could be tracked by triangulation from a fixed point (such as an anchored ship). Observations of this nature were collected off the US east coast as early as the mid-1700s (Franklin, 1785; Davis, 1991) and were collected worldwide in the famous 1872–1876 Challenger oceanographic survey at most of the 354 hydrographic stations (Thomson, 1877; Niiler, 2001). With the advent of radio, drifter positions could be transmitted from small, low-drag antennae and triangulated from shore (Davis, 1991). Drifters of this type are still manufactured today, often inspired by the cruciform-shaped design used in the Coastal Ocean Dynamics Experiment (CODE). In CODE, 164 drifters were used to map currents and their variability and to calculate Lagrangian integral scales and dispersion off the California coast (Davis, 1985).