On the sensitivity of projected oceanic thermal expansion to the parameterisation of sub-grid scale ocean mixing

A coupled model of intermediate complexity is used to examine the importance of the parameterisation of sub‐grid scale ocean mixing on the global mean steric sea level rise in global warming simulations. It is shown that when mixing associated with mesoscale eddies is treated in a more physically realistic way than the commonly used horizontal/vertical scheme, quasi‐equilibrium projected steric sea level rise is more than two times lower in both 2 × CO2 and 4 × CO2 climates. This occurs despite the invariance of the coupled model climate sensitivity to the particular sub‐grid scale mixing scheme employed. During the early phase of the transient integrations thermal expansion differences are smaller, although experiments using the Gent and McWilliams parameterisation for mixing associated with mesoscale eddies approach equilibrium more rapidly once the radiative forcing is held fixed. This reduced expansion commitment reflects a greater decoupling of the surface ocean from the deep ocean, due to a reduction in spurious high latitude convection that occurs when a horizontal/vertical mixing scheme is used.