A sea surface height perspective on El Niño diversity, ocean energetics and energy damping rates

<p>Ocean energetics is a useful framework for understanding El Niño development and diversity; however, its key element, available potential energy (APE), requires accurate ocean subsurface data that are hard to measure. However, sea surface heights (SSH) provide a useful alternative. In this study, we describe an SSH-based index, SSHI, that accurately captures APE variations and can be easily computed from satellite observations. Using SSHI we obtain an observation-based estimate of the APE damping timescale α<sup>-1</sup> of approximately 1.7 years, slightly longer than previous ocean reanalysis-based estimates. We further show that SSHI records the relative strength of the thermocline feedback, serving as an indicator for El Niño “flavors”. SSHI demonstrates a clear decadal shift in El Niño-Southern Oscillation (ENSO) properties that occurred in early 2000s, with a more tilted mean thermocline and weaker thermocline slope variations indicative of the dominance of “Central Pacific” El Niño activity during the past two decades.</p>

A Description of Local and Nonlocal Eddy–Mean Flow Interaction in a Global Eddy-Permitting State Estimate

The assumption that local baroclinic instability dominates eddy–mean flow interactions is tested on a global scale using a dynamically consistent eddy-permitting state estimate. Interactions are divided into local and nonlocal. If all the energy released from the mean flow through eddy–mean flow interaction is used to support eddy growth in the same region, or if all the energy released from eddies through eddy–mean flow interaction is used to feed back to the mean flow in the same region, eddy–mean flow interaction is local; otherwise, it is nonlocal. Different regions have different characters: in the subtropical region studied in detail, interactions are dominantly local. In the Southern Ocean and Kuroshio and Gulf Stream Extension regions, they are mainly nonlocal. Geographical variability of dominant eddy–eddy and eddy–mean flow processes is a dominant factor in understanding ocean energetics.

Dynamical Potential Energy: A New Approach to Ocean Energetics

The concept of available potential energy is supposed to indicate which part of the potential energy is available to transform into kinetic energy. Yet it is impossible to obtain a unique definition of available potential energy for the real ocean because of nonlinearities of the equation of state, rendering its usefulness largely hypothetical. In this paper, the conservation of energy is first reformulated in terms of horizontal anomalies of density and pressure for a simplified ocean model using the Boussinesq and hydrostatic approximations. This framework introduces the concept of “dynamical potential energy,” defined as the horizontal anomaly of potential energy, to replace available potential energy. Modified conservation equations are derived that make it much simpler to identify oceanic power input by buoyancy and mechanical forces. Closed budgets of energy are presented for idealized circulations obtained with a general circulation model, comparing spatial patterns of power inputs generated by wind and thermal forcings. Finally, a generalization of the framework to compressible fluids is presented, opening the way to applications in atmosphere energetics.