The Horizontal And Vertical Merging Events Of Mesoscale Anticyclone Eddies From Satellite Observations

Mesoscale eddy which has an important influence on ocean circulation,material and heat transport,climate change and marine ecology,is ubiquitous in the ocean.An eddy will continuously interact with other eddies during its long life cycle.A common and important interaction is the merger of two like-sign eddies.In this paper,using sea surface height data and sea surface temperature reanalysis data,two typical horizontal and two vertical merging events in the vicinity of the Kuroshio extension were analyzed.The study used a SST-based identification method to determine the natures,surface or subsurface,of eddies during these events.In the horizontal type merging two eddies have the same natures,while in the vertical type have different natures.Then,a Gaussian vortex surface fitting method was used to estimate 2D flow field model,and calculate dynamic properties.Horizontal type and vertical type merging have different surface features.In a typical horizontal merging event:the nature of pre-merged and post-merged eddies were always the same.The area of the post-merged eddy would be bigger than pre-merged ones,but the total area was almost conserved during the merging.The relative vorticity of post-merged eddy was significantly lower than that the pre-merged ones.In a typical vertical merging event,the two pre-merged eddies had different natures.One is surface eddy and the other is subsurface.After merging,only surface eddy was identified since the SSTA signals of subsurface were covered by the surface one.The area was not conserved in vertical type mergers.The area of the post-merged eddy is more like that of the pre-merged larger one.As to the relative vorticity,the post-merged eddy didn't show a significant reduction,but look more like the value of the pre-merged surface one.In this paper,two vertical structural models of vortices,cylinder model and two-layer model,were established to verify the conservation laws proposed by the classic horizontal merging theory.The results show that:(1)Two typical horizontal merge cases approximately satisfy the conservation of total mass;(2)the total relative angular momentum increases after merging;(3)the total energy after merging increases.The increase in the total relative angular momentum before merging is mainly contributed by the systematic rotation of the two eddies around their center,but the system rotation cannot explain the increase in total energy.In the results of the two-layer model,the total integral potential vorticity before and after the merger appear to be conserved.The increased energy is mainly contributed by the potential energy.This result is quite consistent with the "energy paradox"merging pattern that proposed by Gill and Griffiths in 1981 which satisfy the conservetion of mass and potential vorticity and cause the increase of potential energy and total energy.