Richardson Number and Ocean Mixing: Towed Chain Observations

Abstract

A 7.2-km tow in the upper seasonal thermocline of the Sargasso Sea with an 80-m aperture thermistor? conductivity chain and shipborne 150-kHz ADCP is examined to determine the relationship between patches of scalar activity and buoyancy frequency, vertical shear, and gradient Richardson number (Ri). The horizontal temperature gradient variance in the 0.5?3-m band normalized by the mean gradient squared is used as a measure of scalar activity. Two-dimensional images show qualitative coincidence between patches of activity and low Ri computed over 10-m vertical and 90-m horizontal scales. Hypothesis testing quantifies and supports these visual results. The null hypothesis, ?scalar activity is independent of Ri number,? fails at the 90% confidence interval for 10-m Ri < 0.8 for a threshold representing ?9% of the most active regions. The probability of scalar activity increases rapidly as the 10-m Ri approaches 0.25. The results are robust for vertical smoothing from 4 to 20 m, although the details of the probabilities are dependent upon the scalar activity threshold and vertical smoothing. It is shown that the most intense scalar activity occurs for 10-m Ri < 0.3 and that including less intense activity results in probabilities greater than the null hypothesis for Ri as high as 1.2?1.5. These results are strong evidence for the Kelvin?Helmholz shear instability but no single critical 10-m Ri, either 0.25 (Miles and Howard) or 1 (Abarbanel et al.), is identified. Increased scalar activity for part of the tow with enhanced short-wavelength (?125 m) internal waves and 10-m Ri near 1 is suggestive of the advective instability enhanced by low Ri.