Two typical merging events of oceanic mesoscale anticyclonic eddies
Abstract. The long-term theoretical energy paradox of whether the final state of two merging anticyclones contains more energy than the initial state is studied by observing two typical merging events of ocean mesoscale eddies. It is found that the total mass (volume), total circulation (area integration of vorticity) and total angular momentum (AM) are conserved if the orbital AM relative to the center of mass is taken into account as the eddies rotate around the center of mass before merging. For subsurface merging, the mass trapped by the Taylor–Proudman effect above the subsurface eddies should also be included. Both circulation conservation laws and orbital AM have been overlooked in previous theoretical studies. The total eddy kinetic energy slightly decreases after merging due to fusion. On the contrary, the total eddy potential energy (EPE) significantly increases after the merging. The increase of the EPE is mostly supported by the loss of gravitational potential energy (PE) via eddy sinking below the original level. This implies that the merging of eddies requires the background gravitational PE to convert to the EPE. In contrast, the vorticity and enstrophy consequently decrease after merging. Thus, the eddy merging effect behaves as a large-scale energy pump in an inverse energy cascade. It is noted that eddy conservation and conversion laws depend on laws of physical dynamics, even if additional degrees of freedom can be provided in a mathematical model.