Ensemble-averaged ⟨d²⟩ of the quadratic distance travelled by tracers as a function of time (solid dark red line), power law fits (lighter red and cream lines), and (inset) corresponding diffusion coefficients as determined by Eq. (5). The thin coloured lines with markers every 24 h show the history of d² for individual tracers with larger-than-average excursions on long times. The surface trajectories of these tracers are shown with the same markers and colours in Figs. 3 and 4.