Temporal variation of the 2017 Kuroshio large meander based on repeated surveys along 138°E

Abstract

During August 2017, the Kuroshio began to follow its large meander (LM) path, and this was 12 years after the last LM event. Such LM events lead to the formation of an inshore cyclonic eddy (ICE) within the Kuroshio inshore region between the Kuroshio and the southern coast of Japan. We analyzed data from repeated seasonal surveys along a transect that followed 138°E, and from additional surveys, satellite observations, Argo floats, and a high-resolution reanalysis dataset to describe the temporal variation of the LM and ICE between 2017 and 2021. Cross sections from the repeated surveys highlighted the variations in the water mass structures. The time series of the indicators of the ICE intensity such as the Kuroshio volume transport and the planetary contribution of potential vorticity (Q) in the ICE revealed a maintenance trend in their intensity during the observation period. High-Q water was distributed in the ICE during the LM period compared to the non-LM period, and showed a clear seasonality within the shallower layers, suggesting the injection of Q via the advection from the upstream region in summer. Additional surveys captured an eddy that became detached from the ICE during summer 2020. This detached eddy had a Q value greater than the ICE and velocity similar to the Kuroshio, suggesting that the main stream had become temporarily separated. The structures and volume transport of warm water intrusions into the inshore region were also examined, and centrifugal instability was considered to be one of their generation mechanism.