Seasonal evolution of upper‐ocean horizontal structure and the remnant mixed layer

Abstract

We discuss the seasonal evolution of upper‐ocean thermohaline structure at small horizontal scales. The upper 350 m of a 1000 km long section in the subtropical North Pacific was observed in winter, spring, and summer with 3–14 km horizontal resolution. Four vertical regions had distinct density and salinity structure: the mixed layer, remnant mixed layer, high‐stratification layer, and permanent thermocline. The remnant mixed layer consists of water from the winter mixed layer left over after restratification. The remnant mixed layer was most similar to the mixed layer in winter and spring, and most similar to the high‐stratification layer below in summer. The high‐stratification layer had elevated stratification that varied seasonally. The permanent thermocline varied little seasonally and was horizontally and vertically uniform in comparison. In all seasons, density ratios showed that mixed‐layer θ‐S differences tended to compensate in density with the strongest tendency toward compensation in winter. Density ratios were temperature dominated in the remnant mixed layer consistent with salt‐fingering. Salinity anomalies were largest at the surface and decayed with depth in all seasons. Spectra of isopycnal depth and θ‐S anomalies along isopycnals are compared between the three seasons and four vertical layers. Isopycnal depth variance at 30–46 km wavelengths decreased from winter to spring to summer by a factor of 2–10 in stratified regions. By treating salinity anomalies as a tracer, the effective isopycnal diffusivity in the remnant mixed layer was estimated to be 1.4 m2 s−1 over 30–46 km wavelengths.