On the implementation of Leith viscosities in NEMO: Results from a forced global ocean model

In CMIP6, eddy-permitting models (notably HadGEM3-GC3.1 N216ORCA025) were found to perform poorly
against their coarse-resolution counterparts, particularly in the Southern Ocean. Compared with N96ORCA1
(1 degree), ORCA025 exhibited an enhanced warm bias, weakened Antarctic Circumpolar Transport (~ 60 Sv),
overactive Antarctic gyres, and lower Antarctic sea-ice extent and volume. The poor performance of the eddy-
permitting model has been attributed to their difficulty in representing mesoscale processes at higher latitudes.
Despite these shortcomings, eddy-permitting models remain desirable for their capacity to resolve meridional
transports of heat and carbon, and ice-ocean interactions.

The objective of this work is to improve the representation of mesoscale processes in ORCA025 through the
implementation of two viscosity schemes: 2D Leith and Quasi-Geostrophic Leith. These viscosity schemes
have been shown to improve interior mixing by mesoscale eddies in eddy-rich models. Both schemes offer a
parameterised viscosity coefficient that is flow and scale-aware, in contrast to a typical constant biharmonic
viscosity employed in N216ORCA025.

In a forced ocean configuration (GOSI9p8.0 ORCA025), both Leith schemes demonstrate a marked impact on
the ocean’s circulation and its properties compared to biharmonic viscosity. Here, the Leith schemes dampen
the eddy kinetic energy field and reduce the Antarctic circumpolar transport, with corresponding changes in
temperature and salinity fields. Additional simulations, both forced and coupled, are ongoing and may provide
further insights into the different impacts of the Leith viscosity schemes on physical processes in eddy-permitting
Earth system models.