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

Abstract

The 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 (10.27 TW) and time-fluctuating processes, including mesoscale eddies, which release APE (20.27 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 3 10 24 ) m 2 s 21 . 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.

Time-Dependent Eddy-Mean Energy Diagrams and Their Application to the Ocean

“…Therefore, we follow Chen et al (2014a) and use the cube 87 version of the ECCO2 state estimate for the energy diagnosis. Some other recent studies (e.g., Zemskova et al 2015) also indicate that the ECCO2 state estimate is suitable for evaluating ocean energetics. Details of the model configuration and fidelity are available in Chen et al (2014a).…”

“…Therefore, the source, sink, and transformation of energy in the global ocean has received much attention (e.g., Wunsch and Ferrari 2004;Ferrari and Wunsch 2009;Zemskova et al 2015). Many studies exist about energetics during eddy-mean flow interactions, and most of these focus on characterizing the spatial structures and magnitude of energy conversion rates between the mean and eddies from a long-term, time-mean perspective (e.g., von Storch et al 2012;Liang et al 2012;Chen et al 2014a;Kang and Curchitser 2015).…”

Heat‐Engine and Entropy‐Production Analyses of the World Ocean

“…The positive Carnot work also lends support to available potential energy (APE) theories for a large‐scale thermally driven ocean circulation. Theories (e.g., Tailleux, ) and analyses (e.g., Gregory & Tailleux, ; Oort et al, ; Zemskova et al, ) suggest the large‐scale ocean circulation is driven roughly equally by the wind and the thermal forcing. We note that APE theories can predict a Carnot efficiency.…”

On the Local View of Atmospheric Available Potential Energy

“…satisfying the properties for any scalar quantity Q, 1) Q = Q+Q , 2) Q = 0 and 3) Q = Q. In the context of studies of the atmospheric and oceanic energy cycles, zonal averaging has been primarily used for atmospheric studies (e.g., Lorenz 1955b), whereas temporal averaging is more characteristic of oceanic studies (e.g., von Storch et al 2012; Zemskova et al 2015).…”