Available potential energy gain from mixing due to the nonlinearity of the equation of state in a global ocean model

Densification in the ocean interior upon mixing at high latitudes, due to the nonlinear equation of state (EoS) of seawater, enhances the meridional overturning circulation (MOC). However, recent calculations using numerical simulations of global ocean circulation have shown that the nonlinearity of the EoS leads to a sink of gravitational potential energy (PE), from which one might infer that there is less energy available to be released to the MOC. Here the available PE (APE) budget of the global ocean is investigated using a numerical model with a nonlinear EoS under a realistic configuration. The results show that, while the nonlinearity of the EoS leads to a loss of gravitational PE, it is a source of APE. For the model used in this study, nonlinearity of the EoS is as significant as surface buoyancy forcing in generating APE.