On the horizontal variability of the upper ocean

The last decade has seen a tremendous increase in the number and quality of observations of horizontal structure in the upper ocean. Many of these data have been made possible by the widespread use of towed vehicles and shipboard Doppler sonars. We review what these observations have revealed about processes in the upper ocean, with special emphasis on mid-latitudes and horizontal scales of 1-100 km. Fronts are one of the most prominent features of the mixed layer on these scales. The vertical/across-front circulation created at a front may be quantified assuming quasigeostrophic dynamics. Typical of the mixed layer are fronts that are warm and salty on one side, and cool and fresh on the other such that the density contrast across the front is small. The ubiquitous existence of such compensated fronts suggests that horizontal mixing in the mixed layer is an increasing function of the horizontal density gradient. Arguments based on the conservation of potential vorticity indicate that 10-km-scale (roughly the Rossby radius of deformation) eddies in the mixed layer should be predominantly cyclonic. Recent observations of mixed-layer vorticity are positively skewed.