Energetics of the Global Ocean: The Role of Mesoscale Eddies

Abstract

This article reviews the energy cycle of the global ocean circulation, focusing on the role of baroclinic mesoscale eddies. Two of the important effects of mesoscale eddies are: (i) the flattening of the slope of large-scale isopycnal surfaces by the eddy-induced overturning circulation, the basis for the Gent-McWilliams parametrization; and (ii) the vertical redistribution of the momentum of basic geostrophic currents by the eddy-induced form stress (the residual effect of pressure perturbations), the basis for the Greatbatch-Lamb parametrization. While only point (i) can be explained using the classical Lorenz energy diagram, both (i) and (ii) can be explained using the modified energy diagram of Bleck as in the following energy cycle. Wind forcing provides an input to the mean KE, which is then transferred to the available potential energy (APE) of the large-scale field by the wind-induced Ekman flow. Subsequently, the APE is extracted by the eddy-induced overturning circulation to feed the mean KE, indicating the enhancement of the vertical shear of the basic current. Meanwhile, the vertical shear of the basic current is relaxed by the eddy-induced form stress, taking the mean KE to endow the eddy field with an energy cascade. The above energy cycle is useful for understanding the dynamics of the Antarctic Circumpolar Current. On the other hand, while the source of the eddy field energy has become clearer, identifying the sink and flux of the eddy field energy in both physical and spectral space remains major challenges of present-day oceanography. A recent study using a combination of models, satellite altimetry, and climatological hydrographic data shows that the western boundary acts as a "graveyard" for the westward-propagating eddies.

Diagnosing the Thickness‐Weighted Averaged Eddy‐Mean Flow Interaction From an Eddying North Atlantic Ensemble: The Eliassen‐Palm Flux

“…This counteracting balance is consistent with what Aiki et al. (2016, the terms $${\partial }_{x}{R}^{x}$$ and $${\partial }_{z}\left({R}^{z}+{F}_{a}^{+}\right)$$ in their Figures 5a and 6) found in the Kuroshio extension region.…”

“…In this Appendix, we derive the energetics in a similar manner to Aiki et al (2016) but in a framework consistent with the ensemble formalism and a realistic equation of state (EOS). The thickness-weighted average (TWA) residual-mean horizontal momentum equation in geopotential coordinates neglecting dissipation is (Ringler et al, 2017;Young, 2012):…”

A New Assessment of Mesoscale Eddies in the South China Sea: Surface Features, Three‐Dimensional Structures, and Thermohaline Transports

“…Approximately 40% of the eddies dissipate west of 113°E, supporting that the western boundary acts as a “graveyard” for the westward propagating eddies (Aiki et al, ; Zhai et al, ). As suggested by Dewar and Hogg (), eddy energy that released at the western boundary is likely scattered into high‐wave‐number vertical modes, resulting in locally enhanced diapycnal mixing, which can have important implications for the abyssal overturning circulation (Aiki et al, ; Saenko et al, ). The dissipation of cyclonic eddies differs somewhat from that of anticyclonic eddies in the northern SCS (Figures h and i).…”

Topographic Influence on Baroclinic Instability and the Mesoscale Eddy Field in the Northern North Atlantic Ocean and the Nordic Seas

“…Here N 2 is the horizontally local time-mean buoyancy frequency. Most other studies have used some form of globally averaged N 2 in their definition of APE and hence in estimating T 2 (e.g., Chen et al 2014;Zhu et al 2014;Eden and Böning 2002;Aiki et al 2016;von Storch et al 2012).…”