Observational evidence of a new type of cyclonic eddy of the Gulf Stream

Based on over four decades of satellite and in-situ observations, we present evidence that there are two types of cyclonic Gulf Stream eddies formed to the south of the Gulf Stream. One of these types is the well-known pinch-off rings generally formed over and to the east of the New England Seamount Chain (NESC) when a large amplitude meander trough of the Gulf Stream occludes and traps cold slope water in the eddy core. A large number of cyclonic eddies formed across the entire Gulf Stream follow a "hook-type" formation process, in which an offshore filament from the southern flank of the Gulf Stream elongates by extracting flow from the Stream, eventually acquiring cyclonic rotation and capturing Sargasso water (colder than the Gulf Stream-derived annulus) in its core. The hook-type cyclonic eddies have a distinct seasonality with formation peaking in spring, while the pinch-off rings do not show any discernible seasonal pattern. The pinch-off rings form predominantly on and east of the NESC, whereas hook-type eddies form across the entire Stream, possibly resulting from trapped and radiating instabilities and have shallower thermoclines. Shifts in the longitude of the Gulf Stream destabilization point relate to the pinch-off rings on both sides of the Stream. The shifts are not associated with the flank-generated aneurysm and hook-type eddies. The abundance of smaller and shallower aneurysm-type anticyclonic eddies to the north and the newly observed hook-type cyclonic eddies to the south suggests Gulf Stream barrier characteristics west of the NESC, while pinch-off rings appearing mostly on and east of the NESC seem to explain the blender nature of cross-stream exchange of the Gulf Stream on and east of the NESC.