Shuiming Chen (University of Hawaii, USA); Eitarou Oka (The University of Tokyo, Japan)

Since the 1976/77 climate regime shift in the North Pacific Ocean, the Kuroshio Extension (KE) system has been observed to vacillate between a stable and an unstable dynamic state with a preferred decadal timescale. Physically, this decadal vacillation is induced by a delayed negative feedback mechanism in which the wind-forced upper ocean heat content anomalies in the eastern North Pacific basin propagate westward and modify the stability of the pre-existing KE dynamic state, that in turn feeds back to the overlying wind forcing pattern through migration by extra-tropical stormtracks. The decadal KE system vacillation was disrupted in late 2017 when the Kuroshio path south of Japan developed into an offshore-detouring large meander (LM) pattern. Persisting now for > 6 years, the on-going Kuroshio LM has become the longest event in the past 70-year observational record. In concert with this LM event, the KE system after 2018 has transitioned to an unprecedentedly stable state, where the KE axis is observed to extend to as north as 40N east of Japan (cf. the climatological path is around 36N). Poleward shifting by the subtropical/subarctic wind pattern over the North Pacific basin is found to be responsible for both the long-lasting LM formation and the altered baroclinic responses by the KE system. These two changes have likely switched the here-to-fore vacillating KE system into a new superly stable dynamical regime.