Cold-core rings

From Glossary of Meteorology

cold-core rings

Large (roughly 300-km diameter) cyclonically rotating eddies found in the Sargasso Sea, containing cold Slope Water in their cores.

They persist for several months, occasionally interacting with the Gulf Stream and getting destroyed in the process. A cold-core ring is formed from a large-amplitude Gulf Stream meander that pinches off to the south, trapping relatively cold Slope Water from north of the Gulf Stream within its circumferential current.
See also warm-core rings, Gulf Stream rings.