Synchronized decadal variabilities in the Kuroshio and Kuroshio Extension system

Abstract

Using satellite altimetry and wind stress data, the spatially coherent decadal variabilities in current intensity and current path during 1993–2018 were investigated for a western boundary current system, namely, the Kuroshio from the east of Luzon Island to the south of Japan and the Kuroshio Extension (KE). Analyses based on Hovmöller diagrams showing decadal variabilities along the entire current demonstrated three types of synchronic relationship between different regions: 1) an out-of-phase relationship in current intensity between the Kuroshio south of Japan and the KE during 1993–2018; 2) an out-of-phase relationship in current intensity between the Kuroshio from the east of Taiwan (ETW) to the East China Sea (ECS) and the KE during 2006–2014; 3) an out-of-phase relationship in current position between the Kuroshio paths in the ETW to ECS and the Luzon Strait during 1993–2018. These patterns were dynamically regulated by the Pacific Decadal Oscillation (PDO). The synchronized current-intensity variability in 2006–2014, which governed almost the entire current system, was caused by coherent phenomena comprising two kinds of baroclinic Rossby wave propagations along the KE and the subtropical countercurrent (STCC), and a regional sea surface height (SSH) anomaly advection from the South China Sea (SCS) to the ECS via the Taiwan Strait. However, the synchronized current-path variability between 1993 and 2018 was caused by an SSH anomaly migration advected by the Kuroshio from the western North Equatorial Current zone to the ECS and an SSH variability localized in the SCS.In this study, we paid particular attention to that the current intensity of the Kuroshio in the ETW to ECS had a positive no-lag correlation with the SSH-based PDO index in 1996–2005 while a positive 3–4 years lag correlation with that index during 2006–2014. It was emphasized that this transition was due to the following mechanism: the current-intensity variability in 1996–2005 was mainly caused by the variability of the eddy activity in the western STCC zone, while that variability in 2006–2014 was mainly caused by the amplification of baroclinic Rossby waves propagating along the STCC zone; the shift from the former to the latter was generated by a rapid phase shift with a relatively large amplitude for the wind stress curl anomaly and a timescale change of the eddy kinetic energy over the STCC zone around 2006.