Mesoscale eddies observed in coarse-resolution data typically appear with a single center. However, recent altimetry observations of Surface Water and Ocean Topography (SWOT) reveal many eddies own multicore structures. This study investigates the differences between these multicore eddies and single-core eddies and how the multicore eddies are generated. SWOT observation shows that about two-thirds of anticyclonic and cyclonic eddies have multiple centers, mainly found in mid-latitudes in the Northern Hemisphere and high-latitudes in the Southern Hemisphere. Additionally, the geostrophic velocity in multicore eddies is generally higher, through inverse barotropic energy, than in single-core ones. Using LLC4320 simulations, we found that the baroclinic (BC) instability plays a key role in forming these subcenters, and the probability of eddies generating multiple centers decreases during the eddy lifecycle. There are more multicore eddies observed in winter, corresponding to a larger BC growth rate, than in summer. Our study provides new insights into the structure of eddies.

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