Mesoscale eddy parameterisation in numerical "grey zone" ocean models

It is known that ocean models at eddy permitting horizontal resolutions, while being the target for the next generation Earth System Models, suffer from several known deficiencies that lead to rather significant biases in the physical response particularly in the Southern Ocean. One cause of such deficiencies is attributed to the weak mesoscale eddies that are permitted, leading to weaker feedbacks onto the large-scale ocean circulation, with resulting consequences for other components of a Earth System Model. Without a parameterisation active, the eddy feedback is too weak and under-counted, but if a parameterisation is active then there is a double-counting issue, and explicit eddies may be severely damped by the parameterisation. Two existing approaches are to employ a resolution function, or to accept the damping introduced by the parameterisation but backscatter some energy/momentum into the modelled flow in some way.Here we propose an alternative approach that reduces the damping of explicit eddies by the parameterisation in the first place, and instead of a resolution function asks for a definition of the "large-scale" state. We test the procedure in an idealised channel and gyre model in NEMO 4.0.5, in combination with the GEOMETRIC parameterisation for the eddy induced advection. Impacts to the explicitly resolved variability, total (explicit and parameterised) eddy energy levels, modelled mean state and its sensitivity, as well as biogochemical responses are discussed.(Sample output: see https://imgur.com/ifM1SPQ for full resolution images. Showing surface relative vorticity, for (a) no parameterisation, (b) standard GM parameterisation applied as is, (c) GEOMETRIC applied as is, (d) GEOMETRIC applied in the new way.)