Mesoscale Eddy

.Mesoscale is the smallest scale dominated by geostrophic balance, thus the dynamicfeatures of mesoscale eddies can be shared with a abroad range of rotating-fluidprocesses controlled by geostrophic balance. Due to the observational difficulties, areliable three-dimensional structure of mesoscale eddy is still unavailable. At the sametime, we also do not have a precise definition of mesoscale eddy, which lead toconceptual confusion and theories conflict with each other. The concept of mesoscaleeddy was even obscued by the Rossby wave. Although wave action can over a widerange of fluid phenomena, there is also a group of phenomena in rotating fluid can notexplained by wave theory. Indeed, the attempt to treat mesoscale eddies as Rossby wavelead to some serious conceptual difficulties. Thies paper is based on such an idea: themesoscale eddy have simple definition independent of Rossby wave, this simpledefinition will lead to univeral structure and univeral dynamical framework of mesoscaleeddy. We first reveal the global univeal three-dimensional (3D) strucuture of mesosclaeeddies through composite analysis by combining altimeter remote sensing data and Argofloats profiles. Then we define the mesoscale eddy as local potential-vorticity (PV)anomaly dominated by geostrophic balance. Based on this definition, we construct adynamical framwork of mesoscale eddies by Extremum Principle, and this framwork helpus solve a series of problems caused by the Rossby-wave explanation of mesoscale eddy:(1) The existence of mesoscale eddies on f plane, which do not permit the existence ofRossby wave.(2) Why mesoscale eddies have circular shape with limited horizontalscale?(3) The self-sustaining mechanism of the eddy vertical structure.(4) Whymesoscale eddies move westward on β plane.(5) Why the barotropic mode and thebaroclinic mode do not separate during the westward movements of mesoscale eddy on βplane.(6) Reveal the mechanism of mesoscale eddy trapping fluid parcels by closed PVcontour and transporting them during westward movement on β plane. Finally, we putour theorical results into practical use in two examples:(1) Based on the universal3Dstructure of mesoscale eddies, we develop an inverse method to predict the3-D structure of mesocale field from the sea surface imformation.(2) We calculate the volume trappedby mesoscale eddies in the real ocean, and estimated their global zonal mass transport upto30-40Sv.