Regarding the first part of your post, I'm well aware that dynamical forcing can shift the vertical position of isopycnals in the subtropical gyres. (realclimate.org)
The researchers used data from the «Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean» (DIMES) project, a UK / US field program aimed at measuring diapycnal and isopycnal mixing in the Southern Ocean. (sciencedaily.com)
The salinity differences are differences along isopycnals that have been mapped to pressure surfaces. (ipcc.ch)