Detailed Observations of Centimetre-Scale Structures and Entrainment in the Transition Layer

A crucial region of the ocean surface boundary layer (OSBL) is the strongly-sheared and -stratified transition layer which connects the mixed layer to the upper pycnocline. There is a diverse range of waves and instabilities possible in the transition layer. Previous work has suggested that these different waves and instabilities will lead to different OSBL behaviours. Therefore, understanding which physical processes will occur is a key question for modelling turbulence and entrainment in the transition layer. Here we present observations of temperature, velocity, and salinity from a Lagrangian float deployed near Ocean Weather Station Papa (50°N, 145°W) during the fall 2018 mixed layer deepening season. During the 70-day deployment, high-resolution measurements of the transition layer were obtained via a float-mounted ADCP and cm-scale temperature chains in order to image the structures at the base of the mixed layer. The measured temperature profiles showed the frequent occurrence of very sharp interfaces, with temperature jumps of O(1°C) over 6cm or less. Temperature inversions were typically small (≈10cm or less), frequent, and strongly-stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles, consistent with Holmboe-like behaviour rather than Kelvin-Helmholtz-type overturning. Relating the observed temperature structure to an overall entrainment rate via a Thorpe-scale analysis suggests that these frequent small temperature inversions can account for the observed mixed layer deepening over the course of the float deployment. That is, the mixed layer entrainment is dominated by the dynamics of these 10cm-scale structures, rather than larger overturns.