This paper highlights a new implementation of density co-ordinate functionality within an adjoint ocean model, the MITgcm ECCO adjoint. This is illustrated accordingly with a science application investigating the variability of Subantarctic Mode Water. The use of adjoints helps to trace causal links within the ocean, and the density-following feature is of particular interest for the ocean community with many possible further applications.

This paper is of high quality, it exemplifies the complexity of detecting ocean eddies (eulerian versus lagrangian, importance of the code parameters of the regions considered), it acknowledges the fact that not all ocean eddies are the same (dynamically talking), and that their effect on plankton is complex. The method presented is well-described, powerful and would allow digging into these biophysical interactions in many different oceanographic settings. It shows that the common view following seminal papers by e.g. McGillicuddy and D. Chelton that consists of "cyclonic eddies -> isopycnals shaoling -> increase phytoplankton biomass" versus "anticyclonic eddies -> isopycnal deepening -> decreased phytoplankton biomass" is too simplistic and needed to be updated.