High-Resolution Observations of the North Pacific Transition Layer from a Lagrangian Float

A crucial region of the ocean surface boundary layer (OSBL) is the strongly sheared and strongly stratified transition layer (TL) separating the mixed layer from the upper pycnocline, where a diverse range of waves and instabilities are possible. Previous work suggests that these different waves and instabilities will lead to different OSBL behaviors. Therefore, understanding which physical processes occur is key for modeling the TL. Here we present observations of the TL from a Lagrangian float deployed for 73 days near Ocean Weather Station Papa (50 degrees N, 145 degrees W) during fall 2018. The float followed the vertical motion of the TL, continuously measuring profiles across it using an ADCP, temperature chain, and salinity sensors. The temperature chain made depth-time images of TL structures with a resolution of 6 cm and 3 s. These showed the frequent occurrence of very sharp interfaces, dominated by temperature jumps of O(1)degrees C over 6 cm or less. Temperature inversions were typically small (less than or similar to 10 cm), frequent, and strongly stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles. These structures are consistent with scouring behavior rather than Kelvin-Helmholtz-type overturning. Their net effect, estimated via a Thorpe-scale analysis, suggests that these frequent small temperature inversions can account for the observed mixed layer deepening and entrainment flux. Corresponding estimates of dissipation, diffusivity, and heat fluxes also agree with previous TL studies, suggesting that the TL dynamics is dominated by these nearly continuous 10-cm-scale mixing structures, rather than by less frequent larger overturns.

Automated summary

Observations of the transition layer (TL) in the ocean surface boundary layer (OSBL) reveal the presence of frequent small scale mixing structures, which are consistent with frequent occurrences and can account for the deepening of the mixed layer and entrainment flux, supporting previous research findings.