Revisit of the Occurrence of the Kuroshio Large Meander South of Japan

A unique characteristic by the Kuroshio off the southern coast of Japan is its bimodal path variations. In contrast to its straight path that follows the coastline, the Kuroshio takes a large meander (LM) path when its axis detours southward by as much as 300 km. Since 1950, eight Kuroshio LM events took place and their occurrences appeared random. By synthesizing available in situ/satellite observations and atmospheric reanalysis product, this study seeks to elucidate processes conducive for the LM occurrence. We find neither changes in the inflow Kuroshio transport from the East China Sea nor in the downstream Kuroshio Extension dynamic state are determinant factors. Instead, intense anticyclonic eddies with transport > 20 Sv (1 Sv (math)10(6) m(3) s(-1)) emanated from the Subtropical Countercurrent (STCC) are found to play critical roles in interacting with Kuroshio path perturbations southeast of Kyushu that generate positive relative vorticities along the coast and lead the nascent path perturbation to form a LM. Occurrence of this intense cyclonic-anticyclonic eddy interaction is favored when surface wind forcing over the STCC is anticyclonic during the positive phasing of Pacific decadal oscillations (PD0s). Such wind forcing strengthens the meridional Ekman flux convergence and enhances eddy generation by the STCC, and seven of the past eight LM events are found to be preceded by 1-2 years by the persistent anticyclonic wind forcings over the STCC. Rather than a fully random phenomenon, we posit that the LM occurrence is regulated by regional wind forcing with a positive PDO imprint.

Automated summary

The study suggests that the occurrence of the Kuroshio large meander path is regulated by intense anticyclonic eddies, especially during the positive phase of the Pacific decadal oscillations (PDO). Surface wind forcing over the Subtropical Countercurrent (STCC) is found to enhance eddy generation and facilitate the formation of the meander path. Most of the past eight events were preceded by persistent anticyclonic wind forcings over the STCC.