Figure 1. The three-layer structure of the world ocean.

Schematic of an idealized meridional section across the world ocean illustrating the ocean’s three-layer structure. The upper seasonal mixed layer is stirred by a range of turbulent processes driven by wind and buoyancy forcings (see Suppl Mat. 3); the seasonal pycnocline emerges from the density contrast (i.e. stratification) between surface and deep waters, and acts as a barrier reducing communication between surface and deep waters; the deep ocean is largely insulated from the atmosphere, but climate signals propagate from and to the deep ocean through mixing across the seasonal pycnocline and / or through direct contact with the mixed layer as seasonal pycnocline stratification is eroded in winter. In this paper, we present 50-year trends in both mixed layer depth and pycnocline stratification, with impacts on upper-ocean structure and deep-ocean ventilation.