Ocean Turbulence and Mixing Near the Shelf Break South-East of Nova Scotia

Abstract

Formation of coastal fog was observed near the southern tip of Nova Scotia when warm, humid air was advected towards the shore over an area of colder water. The sea-surface temperature in the colder and higher salinity patch near the coast was below 14 °C compared to that of the surrounding sea of ≈ 17–18 °C. Measurements of stratification, currents, and the dissipation rate of turbulence kinetic energy (ε) on the Nova Scotia shelf, shelf break, and continental slope revealed the frequent occurrence of shear instability in the pycnocline. The probability of the gradient Richardson number $(Ri)$ being less than a critical value of $Ri_{cr} = 0.25$ in the depth range 30–90 m exceeded 50% on the shelf and over the slope, and it was above 75% along the shelf break, while in the open ocean pycnocline the probability of $Ri < 0.25$ is below 5%. The cumulative distribution functions of the dissipation rate in the pycnocline south-east of Nova Scotia followed the Burr probability model with median values from $7.9 \times 10^{ - 9}$ W kg⁻¹ on the slope to $1.9 \times 10^{ - 8}$ W kg⁻¹ at the shelf break. The eddy diffusivity ($K_{N}$) estimates follow a generalized extreme value distribution with a high median value of $\left\langle {K_{N} } \right\rangle = 5.8 \times 10^{ - 5}$ m² s⁻¹ at the shelf break. The diffusivity $K_{N}$ depended on $Ri$ in general agreement with the parametrization previously suggested by Lozovatsky and Fernando (Philos Trans R Soc A 371:20120213, 2013) but with a higher level of mixing at $Ri > 1$. This could be relevant to the nature of turbulence generation near the shelf break by random wave instabilities in the pycnocline at high Richardson and high Reynolds numbers.