Horizontal Distribution and Characteristics of Shear Layers in the Seasonal Thermocline

Abstract

High-resolution velocity shear, CTD, and microstructure measurements were made simultaneously from the research submarine Dolphin in Monterey Bay in October 1984. During three consecutive dives, the Dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of Monterey. Inside the seasonal thermocline, the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally. The thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline. In contrast, no large shear layers were observed below a 50 m depth. The depth levels at which the shear layers were observed were nearly constant throughout each dive, suggesting that the shear layers, with some unknown degree of intermittency, might extend horizontally over several square kilometers. The shear vector inside the seasonal themocline (at σt = 25.5) rotated 360° over an inertial period, but did appear to propagate vertically over the 30-hour observation period. These observations suggest that the passage of a storm caused the upper thermocline to ring, creating a local jetlike flow below the mixed layer.