The ocean is the “heat engine” of the planet, storing and transporting the majority of the heat in the Earth system. It takes up more than 90% of the energy resulting from anthropogenic radiative forcing, and thus is critical to model accurately. The physical ocean model in UKESM is NEMO (Nucleus for European Modelling of the Ocean) (http://www.nemo-ocean.eu/), a state-of-the-art community ocean modelling framework , which is owned and maintained by a consortium of institutes including the Met Office and NERC. NEMO has a large user community in Europe and beyond and is used for research and operational applications.
The specific science configuration used in UKESM1 was developed by the Met Office, NOC and BAS as a component of the physical climate model HadGEM3-GC3.1. UKESM uses configurations with a 1° or 0.25° nominal horizontal resolution. This includes a non-linear free surface, in which the cell thicknesses throughout the water column are allowed to vary with time (the z∗ coordinate of Adcroft and Campin, 2004). This permits an exact representation of the surface freshwater flux, and therefore explicit modelling of sea-level rise.
The vertical mixing of tracers and momentum is parameterised using a modified version of the Gaspar et al. (1990) Turbulent Kinetic Energy (TKE) scheme. The horizontal viscosity is bilaplacian, while Lateral tracer mixing is laplacian along isoneutral surfaces. At 1° resolution, the Gent and McWilliams (1990) parameterisation of adiabatic eddy mixing is used, while at 0.25° resolution no such parameterisation is used despite the fact that eddies are only partially resolved.
In the ice sheet configuration of UKESM1, the ocean domain includes the cavities under Antarctic marine ice shelves. NEMO interactively simulates melt at the ocean-ice interface and freshwater flux associated with this drives the local circulation.
Adcroft, A. and Campin, J.-M.: Rescaled height coordinates for accurate representation of free-surface flows in ocean circulation models, Ocean Modelling, 7, 269 – 284, doi:http://dx.doi.org/10.1016/j.ocemod.2003.09.003, 2004.
Gaspar, P., Grégoris, Y., and Lefevre, J.-M.: A simple eddy kinetic energy model for simulations of the oceanic vertical mixing: tests at Station Papa and long-term upper ocean study site, J. Geophys. Res., 95, 16179–16193, doi:10.1029/JC095iC09p16179, 1990.
Gent, P. R. and J. C. Mcwilliams (1990, January). Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr. 20 (1), 150-155.