An adaptive vertical coordinate for ocean models

The ocean's aspect ratio means that the vertical discretisation of ocean models needs to be handled differently from the horizontal discretisation. For large-scale ocean models, vertical schemes have historically favoured either geopotential vertical coordinates (such as Z or Z*, which operate well near the ocean surface but produce excessive mixing in the interior) or isopycnal coordinates (good in the interior, but poorly behaved near the surface). Large-scale models are now simulating progressively finer scales, which exacerbates the problems associated with both geopotential and isopycnal coordinates.

Recent advances in ocean modelling have made the prospect of flexible or hybrid vertical coordinates more accessible. In this presentation we will outline a proposed adaptive scheme which evolves with the ocean stratification. We implement this adaptive grid scheme in several MOM6 ocean model configurations, from idealised to realistic. We compare the numerical performance, transient spinup and equilibrium mean state of the adaptive method with existing coordinate schemes.