The Buoyancy Budget with a Nonlinear Equation of State

Abstract The nonlinear equation of state of seawater introduces a sink or source of buoyancy when water parcels of unequal salinities and temperatures are mixed. This article contains quantitative estimates of these nonlinear effects on the buoyancy budget of the global ocean. It is shown that the interior buoyancy sink can be determined from surface buoyancy fluxes. These surface buoyancy fluxes are calculated using two surface heat flux climatologies, one based on in situ measurements and the other on a reanalysis, in both cases using a nonlinear equation of state. It is also found that the buoyancy budget in the ocean general circulation model Nucleus for European Modeling of the Ocean (NEMO) is in good agreement with the buoyancy budgets based on the heat flux climatologies. Moreover, an examination of the vertically resolved buoyancy budget in NEMO shows that in large parts of the ocean the nonlinear buoyancy sink gives the largest contribution to this budget.