Marine heatwaves are of particular interest globally currently, both scientifically and for their impact on communities. While much work has been done on surface expressions of marine heatwaves, their subsurface expressions have been less well studied. This is particularly the case in the Southwest Indian Ocean. This region is climatically important and the ecological impacts of marine heatwaves (both surface and subsurface) in this area could be significant, with consequential negative socio-economic outcomes.

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.

The use of neural networks to emulate the dynamics of the Earth system (ocean, atmosphere) is of critical importance since it enables efficient and operational forecast, data assimilation, and affordable uncertainty quantification. This field has been very productive in numerical weather forecasts but relatively fewer results were demonstrated in the oceans. This paper could be a reference for mesoscale ocean dynamics emulation.