Turbulence measurements from the SPURS-2 mooring

The ocean surface boundary layer governs exchanges of momentum, heat, and gasses with the atmosphere. These fluxes are strongly influenced by turbulent processes forced at the air-sea interface. Here, we use a densely instrumented mooring to examine the role and response of turbulence to upper ocean salinity as part of the Salinity Processes Upper-ocean Regional Study 2 (SPURS-2) program. The mooring was deployed for 14 months at the edge of the Eastern Pacific Fresh Pool, characterized by frequent rain in the fall and early winter, and strongly influenced by trade winds in winter and spring. The mooring had dense measurements of temperature, salinity, and velocity over the upper 100 m. Surface fluxes of heat, buoyancy, and momentum, were measured with an IMET system, and the surface buoy had a directional wave package for measuring surface gravity waves. Turbulent Kinetic Energy (TKE) dissipation rates were measured at 7, 21, 41, and 61 meters depth from pulse-coherent ADCPs using a direct wavenumber spectra method. These dissipation rates are used to evaluate turbulent fluxes of salt through the mixed layer, and ocean surface boundary layer scalings.

We find that the turbulent flux of salt through mixed layer base can be large, especially during fall. Turbulence measurements nearest to the surface are well scaled by Monin-Obukhov similarity theory driven by shear and buoyancy, and also fairly well scaled separately by wave breaking turbulence. These scalings for turbulence are least effective in the rainy season, where stratified layers caused by rainfall modify turbulence at the surface. Some rain events cause short (1 hour) decreases in TKE dissipation rate relative to the surface forcing, while other rain events can cause moderate enhancement for hours. Better understanding of these modifications to boundary layer scaling may improve estimates of air-sea exchanges.