Stokes drift and ExB drift are the same thing.

Measurements of Lagrangian fluid particle drifts, such as Stokes drift, rarely match theoretical predictions. Because the classical prediction of Stokes drift is dependent on the wave model, it has been hard to identify what feature or features of the measured flows (e.g. reflux, vorticity, finite system size) violate the assumptions of the predictions to cause the discrepancy. It would therefore be desirable to understand if there is a general mechanism that produces drift that does not depend as particularly on the details of the model. In this talk, a technique for analyzing Lagrangian fluid particle kinematics is developed that models a particle trajectory as a circular gyration plus a steady drift, a path known as a "trochoid". We show that a particle undergoing such motion is experiencing in its frame a rotating centripetal acceleration proportional to its speed plus a constant cross-axis acceleration. This is exactly what happens in the ExB drift of charged particles, so that Stokes drift and ExB drift are special cases of the same phenomenon. In the case of Stokes drift we find that the cross-axis acceleration is connected with an upward Eulerian stress, suggesting that differences in that parameter may be why theory and experiment disagree.