The Critical Role of Cold Eddies in the Decay of the 2004 Kuroshio Large Meander and the Persistence of the 2017 Event

The Kuroshio Large Meander (KLM) is a unique structure of the Kuroshio south of Japan. By analyzing eddy-resolving altimeter data and reanalysis datasets, we found that as the KLM develops, its vorticity increases, creating a subsurface potential vorticity (PV) gradient on both sides of the Izu Ridge. This gradient hinders the downstream advection of its vorticity, thereby maintaining its stable structure. A strong cold eddy with an amplitude exceeding 1 m, detached from the Kuroshio Extension (KE), was identified as a key factor in the decay of the 2004 KLM. The cold eddy carried high-PV water as it propagated westward, resulting in the gradual flattening of the subsurface PV gradient on both sides of the Izu Ridge. This process facilitated the outward advection of high-PV water, leading to the rapid decay of the KLM. The systematic northward migration of the KE and the westward shift of the KLM since 2017 prevents such eddies from decaying the KLM and plays significant roles in its maintenance.