Causal Forcing Analysis on the Low-Frequency Variations of Eddy Kinetic Energy in the Kuroshio Extension Region

Abstract

Abstract The eddy kinetic energy (EKE) in the Kuroshio Extension (KE) region may be affected by two factors: EKE in the Kuroshio large meander (KLM) region and the North Pacific Gyre Oscillation (NPGO). Previous studies reported that the Kuroshio path variations south of Japan may affect the low-frequency variability of the KE jet and related EKE, but the linear correlation between these phenomena derived from long time series is low and not significant, implying that the linkage between EKE in the KLM and KE regions is still unclear. Besides, whether NPGO has a causal effect on the KE EKE remains under debate. In this study, we investigate the causal forcing of the KLM EKE and NPGO on the KE EKE using the convergent cross mapping (CCM) approach based on satellite sea surface height observations. The analysis shows that the KLM EKE affects the EKE only in the KE upstream area (west of 146°E), with no significant causal effect on the EKE in the downstream area; the NPGO plays, instead, a remarkable role on the EKE in both areas. The effect of the KLM EKE on the KE EKE is found to depend on the Kuroshio latitudinal position over the Izu Ridge. Changes in the KLM EKE affect the downstream advection of eddies and induce changes in the Kuroshio position over the ridge, which cause different EKE levels in the KE upstream region. The NPGO affects the KE EKE through the westward propagation of sea surface height anomalies remotely forced by wind stress anomalies associated with the North Pacific Oscillation.