  Pair-separation statistics of in-situ and synthetic surface drifters deployed
near the \emph{Deepwater Horizon} site in the Gulf of Mexico are investigated.
The synthetic trajectories derive from a 1-km-resolution data-assimilative Navy
Coastal Ocean Model (NCOM) simulation. The in-situ drifters were launched in
the Grand LAgrangian Deployment (GLAD). Diverse measures of the dispersion are
calculated and compared to theoretical predictions. For the NCOM pairs, the
measures indicate nonlocal pair dispersion at the smallest sampled scales. At
separations exceeding 100 km, pair motion is uncorrelated, indicating absolute
rather than relative dispersion. With the GLAD drifters however the statistics
suggest local dispersion (in which pair separations exhibit power law growth),
in line with previous findings. The disagreement stems in part from inertial
oscillations, which affect the energy levels at small scales without greatly
altering the net particle displacements. They were significant in GLAD but much
weaker in the NCOM simulations. In addition the GLAD drifters were launched
close together, producing few independent realizations and hence weaker
statistical significance. Restricting the NCOM set to those launched at the
same locations yields very similar statistics.
