Surface Layer Structure and Turbulent Kinetic Energy Dissipation Across the East Australian Current

The East Australian Current (EAC) is a highly energetic and intricate mixing system in the western Pacific. Between variable wind forcing, and separate surface- and under-currents, a complex turbulent surface layer develops. A Turbulence Ocean Microstructure Acquisition Profiler (TurboMAP) was used to collect vertical profiles of shear and Conductivity-Temperature-Depth (CTD) at six stations transecting the EAC. Using the universal Nasmyth spectrum, the shear microstructure profiles provided estimates of turbulent kinetic energy (TKE). Temperature and salinity profiles typically revealed a three-layer structure: a well-mixed layer in the upper 30 m, an underlying, clearly defined subsurface layer, and an extensive thermocline region spanning a depth of 100 m or more. Shear data and TKE dissipation estimates were compared alongside the CTD profiles to clearly illustrate the three-layer structure. TKE dissipation trended to a local minimum in the upper mixed layer on the order of 10-9 W/kg, as wave action attenuated. The magnitude of dissipation then proceeded to fluctuate between 10-9 and 10-8 W/kg with subsequent stratification. Additionally, local peaks in shear and TKE dissipation were associated with the sharp transitions between layers and instabilities along the thermocline. Such observations of turbulence across the EAC provide a rare and unique insight into the dynamic nature of the upper ocean. The turbulent structure of these sublayers suggests the potential for rapid mixing and transport of nutrients between the mid- and upper waters along the EAC.