Deep Eddy Energy in the Southern Philippine Basin

The deep eddy activities, which contain a pronounced portion of kinetic energy, are the most energetic form of flow in the deep ocean. However, the dynamics of deep eddies are inadequately understood. Deep intraseasonal variability (ISV) and eddy kinetic energy (EKE), as detected in velocity measurements from four moorings in the Southern Philippine Basin, are generally separated from the upper ocean and vertically coherent below 1,500 m. Combined with the analysis of observed temperature and salinity and outputs from an eddy-resolving reanalysis product, it is confirmed that the events causing deep ISV are deep mesoscale eddies. The energetics of deep eddies are examined by calculating the time derivative of EKE and its potential contributors during the evolution of a fixed eddy. Our energetics analysis reveals that the generation of the deep eddy is predominantly facilitated by vertical energy redistribution through pressure work between upper and deep layers, with energy primarily sourced from upper-ocean perturbations. Dissipation, inverse kinetic energy cascade, and horizontal pressure fluxes ultimately contribute to the extinction of the deep eddy. During the mature stage of the deep eddy, energy fluxes from pressure work and conversions between EKE and eddy available potential energy are notably high, although the dominant contributors vary over time. This work highlights that the deep eddy energy comes from the upper-ocean eddy, and pressure work plays a crucial role in bridging the energetics of the upper and deep layers.