Lagrangian Eddy Scales in the Northern Atlantic Ocean

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Abstract
Eddy time and length scales are calculated from surface drifter and subsurface float observations in the northern Atlantic Ocean. Outside the energetic Gulf Stream, subsurface timescales are relatively constant at depths from 700 m to 2000 m. Length scale and the characteristic eddy speed decrease with increasing depth below 700 m, but length scale stays relatively constant in the upper several hundred meters of the Gulf Stream. It is suggested that this behavior is due to the Lagrangian sampling of the mesoscale field, in limits set by the Eulerian eddy scales and the eddy kinetic energy. In high-energy regions of the surface and near-surface North Atlantic, the eddy field is in the “frozen field” Lagrangian sampling regime for which the Lagrangian and Eulerian length scales are proportional. However, throughout much of the deep ocean interior, the eddy field may be in the “fixed float” regime for which the Lagrangian and Eulerian timescales are nearly equal. This does not necessarily imply that the deep interior is nearly linear, as fixed-float sampling is possible in a flow field of O(1) nonlinearity.