On variations in static stability along Lagrangian trajectories

By causing an isopycnal float to rise and sink to neighboring density surfaces, one can measure layer thickness along a float trajectory and thus variations in static stability. We report here on layer thickness measurements that were obtained from the nearly 100 isopycnal floats that were deployed in the 1993-1995 Lagrangian study of the North Atlantic Current. Layer thickness variations depend significantly upon location and process: In the high velocity core of the meandering North Atlantic Current, layer thickness changes correlate with curvature of the flow-stretching in troughs, compression in crests, but this correlation fails in unsteady flows or outside the high velocity center of the current. Floats also register significant changes in stratification when they cross between the warm waters in the Newfoundland Basin and the cold waters from the Labrador Sea. As floats enter and exit what appear to be coherent eddy structures, significant but not necessarily predictable changes in layer thickness can occur. While the technology to measure layer thickness variations performed as well and reliably as we had hoped for, the presence of ubiquitous small-scale features in the density field together with the fact that the thickness measurements sampled only O(100) m of the total water column limited our ability to isolate and identify the cause of thickness variations to only relatively energetic features.