Horizontal Variability of Microstructure in the Vicinity of a Sargasso Sea Front

Abstract

Temperature microstructure variability has been determined from measurements of electrical conductivity (?1.5 cm wavelength resolution) along two depths in the seasonal thermocline of the Sargasso Sea in July 1981. The microstructure sensors were attached to a thermistor chain, which was towed in and away from a frontal shear zone in the region of the Subtropical Convergence Zone. Averaged over the 170-km-long tow, the estimated dissipation rate of temperature variance, ?, was ?10?8 °C2 s?1, but ? values ranged from 10?11 (noise level) to 10?5 in the most energetic events. Cox numbers, C, were calculated by making use of a local temperature gradient calculated over a fixed ?1 m vertical spacing on the chain. Mean values of C were ?10, but values as high as 105 were observed. The signals wear highly intermittent, varying by as much as five orders of magnitude over scales of the order of 10 m. Probability distributions of ? and C appeared to resemble the lognormal form only in cases where the data were carefully drawn from energetic events. Low values of a large-scale Richardson number (7 m vertical by 450 m horizontal averages) hare no consistent relationship to the occurrence of an event.