Lower atmosphere processes affecting the surface ocean: mixed and mixing layer depths

Valentina Giunta and Brian Ward, National University of Ireland, Galway (NUIG), School of Physics, Galway, Ireland
The ocean surface boundary layer (OSBL) is typically well mixed as a result of different turbulent processes. This layer controls relevant processes such as the transfer of heat, momentum, and trace gases between the ocean and atmosphere and represents an important element in the global climate system.

The base of the OSBL can be represented using the mixed layer depth (MLD) or the mixing layer depth (XLD). The MLD represents the limit of the layer at which variables are nearly constant with depth, and the XLD the layer where turbulent processes are actively mixing the ocean surface. Using data collected with an autonomous vertical profiler (the Air-Sea Interaction Profiler, ASIP) in the North Atlantic, both parameters were estimated using four deployments and then compared with local conditions.

The mixing in the OSBL is mainly produced by the contribution of three sources of turbulence: waves, wind, and buoyancy. Using the data obtained during ASIP deployments, it is possible to link changes in the XLD associated with changes in the mixing process. Under positive strong buoyancy flux (B0) convection is favorable and the mixing can penetrate deeper. In this case, both MLD and XLD are nearly equal and are controlled by changes in B0. Under strong winds, the XLD shows a strong positive correlation with the wind energy flux whereas the response of the MLD is much weaker. Typically the MLD is deeper than the XLD.

Since the XLD seems to be strictly linked to the mixing processes at the OSBL, it is possible to scale the XLD by dividing the data into different mixing regimes. Using the data recollected with ASIP and the rate between the MLD and the Monin-Obukhov length, three mixing regimes are proposed along with scalings for the XLD.