  Recently, Banner et al. (2014) highlighted a new fundamental property of open
ocean wave groups, the so-called crest slowdown. For linear narrowband waves,
this is related to the geometric and dynamical phase velocities $U_d$ and $U_g$
associated with the parallel transport through the principal fiber bundle of
the wave motion with $\mathit{U}(1)$ symmetry. The theoretical predictions are
shown to be in fair agreement with ocean field observations, from which the
average crest speed $c=U_d+U_g$ with $c/U_d\approx0.8$ and
$U_{g}/U_d\approx-0.2$.
