Geographical Distribution And Anisotropy Of The Inverse Kinetic Energy Cascade, And Its Role In The Eddy Equilibrium Processes

Satellite altimetric observations of the ocean surface reveal a circulation dominated by turbulent flow on scales from tens to hundreds of kilometers, the oceanic mesoscale.mesoscale eddies contains ninty per cent of the kinetic energy of the global ocean. But how these eddies are generated and what sets their equilibrated scale remain open questions. The oceanic inverse kinetic energy cascade from satellite altimetry may provide a new perspective to understand these processes deeply. In this study, we use different data sets to diagnose the geographic character, depth dependence, and anisotropy of the inverse kinetic energy cascade so that we can have a more detailed and comprehensive understanding of the inverse cascade. Meanwhile, it will deepen our understanding of the generation and evolution of the mesoscale eddy. Major findings are listed as follows:1. The geographic character of the inverse cascade are analyzed based on the spectral kinetic energy flux calculated in the global ocean, using sea surface height(SSH) data from satellites, reanalysis data, and model outputs, with focusing on the amplitude and characteristic scales. A ubiquitous inverse cascade was also found at about100–500 km scales outside the equatorial band, which varies greatly among different regions.2. There is a close agreement between and the spatial scale predicted by local linear baroclinic instability outside the tropical band, which indicates that the baroclinic instability is the major source of the upscale kinetic energy. Most of the eddy kinetic energy(EKE) comes from the available potential energy(APE), then the EKE is upscale transferred from scales near to the deformation radius to larger scales to maintain a statistical equilibrium state of the ocean. That is the inverse kinetic energy cascade is able to bridge the gap between the linear theory and the observations.3. We also diagnose the depth dependency and the anisotropy of the inverse linetic energy cascade to explore the possible “sinks” of the upscale kinetic energy. The results show that a part of the upscale energy is transferred down the water column, reducing the surface expression and decreasing the signal seen by the altimeter in highly energetic predominantly eastward-flowing currents. Meanwhile, a part of the upscale energy will finally induce zonal jets. However, the contribution of each possible sink is still unclear in our study.4. We further used above methods to explore the decadal modulation of the eddy field in the Kuroshio Extension. Our results show that he local linear baroclinic instability is not the dominant mechanism controlling the decadal modulation of the mesoscale eddy field in the upstream region. while, the barotropic instability makes a positive contribution to the decadal modulation of the mesoscale field. The observed eddy field is the result of an inverse cascade, and cannot be understood by linear theory alone.