Oceanic fine structure

TLDR

Free‑fall instruments measured the vertical temperature gradient structure in the San Diego Trough, Florida Current, and central Pacific, and provided estimates of entropy generation. The measurements revealed meter‑scale, high‑gradient layers with gradients exceeding ten times the mean, horizontal extents over 750 m near the surface but only a few hundred meters at depth, sub‑centimeter fine fluctuations, five‑minute timescales, and entropy generation of 240–700 erg cm⁻² s⁻¹—much lower than the 3600 erg cm⁻² s⁻¹ implied by the mean gradient and an eddy coefficient of 1 cm² s⁻¹.

Abstract

Measurements by free fall instruments, in the San Diego Trough, the Florida Current, and the central Pacific, reveal the detailed structure of the vertical component of the oceanic temperature gradient. The temperature changes are concentrated into regions on the order of a meter thick wherein the measured gradients are often more than ten times the average gradient. The horizontal extent of the regions of high gradient is greater than 750 meters in the seasonal thermocline off San Diego, but is only a few hundred meters at depths greater than 400 meters.Fine scale measurements show that the layers of high gradient consist of even finer fluctuations in gradient which are only a few centimeters thick. Time scales of the thinnest of these regions of high gradient are of the order of five minutes. The data also yields an estimate of the entropy generation. According to the results of an idealized model relating entropy generation to the turbulent heat transport, only 240 to 700 ergs per cm.2 per sec were transported in a 25 meter vertical section measured in the San Diego Trough. This value compared with 3600 ergs per cm.2 per sec estimated from the mean gradient and an eddy coefficient of 1 cm.2 per sec.