We combine two point velocity and position data from surface drifter
observations in the Benguela upwelling region off the coast of Namibia. The
compensated third order longitudinal velocity structure function
⟨Δuℓ3⟩/s shows a positive plateau
for inertial separations s roughly between 9km and 120km
revealing an inverse energy cascade with energy transfer rate
ε≃1.2±0.1⋅10−7m3/s2. Deviations from
Gaussianity of the corresponding probability distribution P(Δuℓ∣s) of two-point velocity increments Δuℓ for given pair
separation s show up in the nth antisymetric structure functions
S−(n)(r)=∫un(P(u)−P(−u)du, which scale in agreement with
Kolmogorov's prediction, S−(n)(r)∼r(n/3), for n=2,4,6. The
combination of ε with Richardson dispersion
⟨s2(t)⟩=gεt3, where ⟨s2(t)⟩ is mean squared pair separation at time t, reveals a
Richardson-Obhukov constant of g≃0.11±0.03.Comment: 6 pages, 5 figure