Subarctic and subtropical of the North Pacific surface

　Roughly, a density of less than 26.7σ is considered a surface layer. The surface layer is the combination of the surface mixed layer and the subduction layer. In the subtropics, Ekman transport causes near-surface water to converge and form a thick surface layer. Subarctic do not have a subduction layer, so they have a thin surface layer. The thick subtropical surface layer is about 700 m (surface mixed layer + subduction layer), while the thin subarctic surface layer is about 200 m (surface mixed layer only).

In the North Pacific, roughly, the subtropical is above 34 salinity and the subarctic is below 34 salinity.