Seasonal Observations of Upper Ocean Thermohaline Structure and the Relic Mixed Layer

The different processes that stir and mix the mixed layer and thermocline produce different relationships between temperature, salinity, and density. Observations of temperature and salinity from the surface to 300- 400 m depth along a 1000 km section in the North Pacific were taken in winter (March-April 2005), spring (June 2004), and summer (September 2004) with 3-14 km horizontal resolution. Horizontal temperature and salinity gradients were common on depth and isopycnal surfaces and were largest in the mixed layer and relic mixed layer, which is the layer that forms when deep winter mixed layers restratify. In all seasons, gradients along isopycnal surfaces decayed with depth below the winter mixed layer base. Stirring occurred at horizontal length scales of 20-40 km in the thermocline and 50-85 km above the winter mixed layer base. In the relic mixed layer between spring and summer, stratification increased, horizontal length scales increased, and the magnitude of horizontal gradients along isopycnals decreased. Horizontal diffusivity in the relic mixed layer between spring and summer was 2 m2s-1 on 6-8 km scales and 210 m2s-1 on 50-85 km scales. In the mixed layer, density compensation of temperature and salinity was observed in all seasons, and was more common in winter than in spring or summer. In the relic mixed layer, compensation was observed in winter and spring, but not in summer. In the thermocline, temperature gradients exceeded salinity gradients in all seasons. Larger thermohaline gradients had a stronger tendency towards compensation in the mixed layer and towards temperature dominance in the thermocline.