Diurnal restratification and turbulence in the oceanic surface mixed layer: 1. Observations

Abstract

By repeated microstructure profiling during PATCHEX, we observed 11 cycles of daytime stratification and turbulent decay within the surface mixed layer during a period of generally light winds. Absorption of solar radiation strongly stratified water near the surface, but also weakly stratified all depths in the remnant mixed layer below the diurnal thermocline. Advection due to relaxation of horizontal density gradients also routinely contributed to remnant layer restratification. Between 0.2 and 0.4 MPa stratification increased linearly with time from (N2) ≈ 5 × 10−7 s−2 at the end of nighttime convection to (N2) ≈ 3.5 × 10−6 s−2 by the end of the day. Throughout the day we observed high dissipation rates (〈ε〉 > 10−6 W kg−1) in the near‐surface zone, which extended down to 0.07–0.10 MPa. Within the remnant layer, dissipation remained nearly constant for the first half hour after the end of convective forcing and then decayed by a factor of 40 in 4 hours, reaching 5 × 10−10 W kg−1; throughout the decay, overturning scales were limited by the growing stratification. Within the seasonal thermocline there was a daily cycle in ε, with nightly maxima of about 3 × 10−8 W kg−1 and daytime minima around 2 × 10−9 W kg−1. The mean turbulent heat flux within the seasonal thermocline was comparable in magnitude to the mean heat flux through the surface.