Sub‐mesoscale lateral density structure in the oceanic surface mixed layer

We investigate the horizontal scales of density variability in the surface mixed layer (SML) in the North Pacific Subtropical Front (STF) during a period of highly variable atmospheric forcing. Wavelet analysis shows that horizontal density variability is not restricted to scales, L , ≥10 km as previously suggested but extends to L = 2 km. The limiting L varies strongly with location and corresponds to a local internal Rossby radius that accounts for transient stratification above the seasonal thermocline. Density compensation in the SML, achieved when temperature and salinity effects cancel in their effect on density, occurs at 30°N at the climatological front associated with the northern boundary of the STF where large thermohaline gradients are observed. At 28°N, however, temperature gradients within the SML are not compensated by salinity, and horizontal density gradients result in 2 ≤ L ≤ 10 km. Our observations suggest dynamic processes restratify the SML at scales rarely resolved by numerical models of the SML.