Abstract
The ocean’s permanent pycnocline is a layer of elevated stratification
that separates the well-ventilated upper ocean from the more
slowly-renewed deep ocean. Despite its pivotal role in organizing ocean
circulation, the processes governing the formation of the permanent
pycnocline remain little understood. Two factors in particular are
generally overlooked: the presence of a zonal channel in the Southern
Ocean, and the nonlinear interplay between temperature and salinity
distributions. Here we assess the mechanism generating the Southern
Ocean’s permanent pycnocline through the analysis of a high-resolution,
realistic, global sea ice-ocean model. We show that the permanent
pycnocline is formed by seasonal sea ice-ocean interactions in two
distinct ice-covered regions, fringing the Antarctic continental slope
and the winter sea ice edge. In both areas, persistent sea ice melt
leads to the formation of strong, salinity-based stratification at the
base of the surface mixed layer in winter. The resulting sheets of high
stratification subsequently descend into the ocean interior at fronts of
the Antarctic Circumpolar Current, and are projected equatorward into
the Southern Hemisphere basins along density surfaces. Our findings thus
highlight the crucial role of localized sea ice-ocean interactions in
configuring the vertical structure of the Southern Ocean.