Mapping upper ocean horizontal velocity shear from drifting buoys and floats

In this study, upper ocean shear is examined globally using drogued and undrogued drifters and Argo floats at the ocean surface. First, an empirical model using altimetry and winds is tuned to match the drogued observations as closely as possible. This is then interpolated to the undrogued drifters and floats, and differences are examined in a downwind/crosswind coordinate system. It is shown that the windage factor, i.e., the fraction of the wind that must be added to the downwind undrogued drifter to match a drogued drifter, varies with marked spatial structure as shown in previous studies. By examining undrogued drifters of different diameter and weight, the varying buoyancies of the drifters can be eliminated as a factor. Because a similar geographic distribution is found between drogued drifters and Argo floats at the surface, which are less susceptible to direct wind forcing and Stokes drift, it is concluded that the distribution primarily reflects variations in upper ocean shear between 15m depth and the surface. The most striking differences in this distribution is the sharp gradient spanning the tropical Pacific basin between the North Equatorial Countercurrent and the seasonally shoaling Equatorial Undercurrent.