Scale-dependent distribution of kinetic energy from surface drifters in the Gulf of Mexico

The scale-dependent distribution of kinetic energy is probed at the surface in the Gulf of Mexico using surface drifters from the Grand Lagrangian Deployment (GLAD) experiment. The second-order velocity structure function and its decomposition into rotational and divergent components are examined. The results reveal that the divergent component, compared to the rotational component, dominates at scales below 5km, and the pattern is reversed at larger scales. The divergent component has a slope near 2/3 below 5km, similar to an energy cascade range (k(-5/3)). The third-order velocity structure function at scales below 5km is negative and implies a forward cascade of energy to smaller scales. The rotational component has a steeper slope, roughly 1.5, from scales of 5km up to the deformation radius. This is similar to a 2-D enstrophy cascade, although the slope is shallower than the predicted 2. There is a brief 2/3 range from the deformation radius to 200km, suggestive of a 2-D inverse cascade.