Available Potential Energy and the General Circulation: Partitioning Wind, Buoyancy Forcing, and Diapycnal Mixing

Abstract

he ocean energy cycle is calculated using a new available potential energy (APE) decomposition, which partitions adiabatic buoyancy fluxes from diapycnal mixing, applied to results from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), eddy-permitting ocean state estimate and observed surface buoyancy fluxes from the WHOI OAFlux project. Compared with the traditional Lorenz energy cycle, this framework provides a more accurate estimate of the background potential energy (PE) of the global oceans and the surface generation and interior fluxes of APE. Calculations of the global energy budget using 16 yr of ECCO2 output suggest that the adiabatic portion of the general circulation is maintained by a balance between the mean wind-driven upwelling that increases APE (+0.27 TW) and time-fluctuating processes, including mesoscale eddies, which release APE (?0.27 TW). The APE generated by surface buoyancy fluxes (0.46 TW) is comparable to the generation by the mean winds. The global rate of irreversible mixing (0.46 TW), which balances surface APE generation, is consistent with previous estimates of the diapycnal fluxes associated with maintaining deep stratification (see Munk and Wunsch) and a global diapycnal diffusivity of O(1 ? 10?4) m2 s?1. However, the net contribution of diapycnal mixing to the total potential energy is negligible, which suggests that mixing, contrary to one current paradigm, does not place a global demand on kinetic energy dissipation. However, there are regions where mixing is significant, for example, between 3000 and 5000 m (in ECCO2), where mixing increases PE by 0.1 TW. The work provides a new framework for separating adiabatic?diabatic fluxes and for monitoring the global rate of diapycnal mixing rate using measurable surface properties such as SST and heat flux.