DIURNAL RESTRATIFICATION AND TURBULENCE IN THE OCEANIC SURFACE MIXED-LAYER .1. OBSERVATIONS

By repeated microstructure profiling during PATCHEX, we observed 11 cy cles of daytime stratification and turbulent decay within the surface mixed layer during a period of generally light winds. Absorption of so lar radiation strongly stratified water near the surface, but also wea kly stratified all depths in the remnant mixed layer below the diurnal thermocline. Advection due to relaxation of horizontal density gradie nts also routinely contributed to remnant layer restratification. Betw een 0.2 and 0.4 MPa stratification increased linearly with time from ( N2) almost-equal-to 5 X 10(-7) s-2 at the end of nighttime convection to (N2) almost-equal-to 3.5 x 10(-6) s-2 by the end of the day. Throug hout the day we observed high dissipation rates ([epsilon] > 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 x 10(-10) W kg-1; throughout the decay, overturning scales were limited by the growing stratificati on. Within the seasonal thermocline there was a daily cycle in epsilon , with nightly maxima of about 3 x 10(-8) W kg-1 and daytime minima ar ound 2 x 10(-9) W kg-1. The mean turbulent heat flux within the season al thermocline was comparable in magnitude to the mean heat flux throu gh the surface.