The effects of surface forcing and mixed layer turbulence on transition layer dynamics

Abstract:

The ocean surface boundary layer (OSBL) plays an important role in the global climate system, and understanding how it evolves in response to forcing by wind, buoyancy fluxes, and waves at the sea surface is a key question. A crucial component of the OSBL is the strongly-sheared, strongly-stratified transition layer (TL) connecting the mixed layer to the upper pycnocline. The TL can theoretically support a diverse range of waves and instabilities which drive entrainment and therefore mixed layer deepening. To study the physics of the TL, a Lagrangian float was deployed near Ocean Weather Station Papa (50°N, 145°W) during the Fall 2018 mixed layer deepening season for 70 days. High-resolution measurements of the transition layer were obtained via a float-mounted ADCP and cm-scale thermistor chains. Using the TL temperature measurements, the dissipation and entrainment rate can be estimated and shown to agree well with the overall OSBL evolution during the deployment. Here we extend this analysis by considering how the observed entrainment relates to the surface wind, buoyancy, and wave forcing. Using the ADCP measurements, we further consider the response of the mixed layer turbulence to the surface forcing and its effects on the TL dynamics.