Initial and mean parcel labels in a surface wave
= Initial and mean parcel labels in a surface wave
A <fluid displacement> measured from an initial position must vanish at the initial time. Integrating a harmonic <velocity> therefore introduces phase-dependent constants. Mean-orbit labels absorb those constants and leave purely oscillatory displacements. The instantaneous displacement correction to velocity can differ under the two reference conventions, but its period mean gives the same leading <Stokes drift>.