The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

The transfer of energy from the upper to deep oceans is a well-known and challenging subject in physical oceanography. This research investigates the intricate relationship between surface and deep ocean currents. Utilizing nearly two years of observations from the Kuroshio Extension System Study (KESS), our study unveils the deep-ocean kinetic energy response to the Kuroshio Extension (KE) variability. We introduce the Kuroshio Extension Jet Path Index (KEJPI), which identifies two distinct modes of the KE jet on an intra-seasonal timescale. Our findings reveal a strong correlation (0.73) between KEJPI and the mean kinetic energy of deep-ocean geostrophic circulation, suggesting that the KE jet's large amplitude meanders have a significant impact on deep-ocean kinetic energy. Singular Value Decomposition (SVD) analysis further unveils a co-evolving spatial pattern between upper and lower ocean kinetic energy. We investigate the dynamic vertical coupling (DVC) mechanism by examining the coherent variation among the sea surface height (SSH), 15 ˚C isotherm (Z15), deep pressure anomaly, and abyssal flow. The KE jet migration can induce net divergence and convergence within the water column, which in turn generates deep-ocean quasi-geostrophic currents. These currents show a marked increase in kinetic energy, reaching levels three times higher than the background. This DVC-driven kinetic energy can further cascade into near-inertial and high-frequency internal waves, contributing to abyssal mixing. Our study underscores the role of large current system instabilities in transferring energy to the deep ocean and facilitating deep mixing processes.