The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

The tropical western Pacific Ocean, characterized by crucial boundary currents, such as the Kuroshio, Mindanao Current (MC), and New Guinea Coastal Current (NGCC), exhibits significant multiscale variability, particularly on intra-seasonal time scales; however, the cross-scale interactions remain unclear. Here we investigate the spatiotemporal characteristics of energy transfer of kinetic energy and available potential energy between intra-seasonal variability (ISV) and low-frequency variability, namely seasonal-biennial and background flow, in the tropical western Pacific Ocean. The multiscale windows transform, canonical transfer, and a localized multiscale energy analysis framework are applied with numerical assimilations. In the upper layer, the interaction between the ISV and background flow is very strong but possesses regional features: Evident inverse scale cascades of kinetic energy are found in the MC region, while barotropic instability dominates the cross-scale energy transfer of ISV in the regions of Kuroshio, North Equatorial Countercurrent (NECC) and NGCC. The energy transfer in the sub-thermocline layer is quite different from that in the upper layer with the exception of the NGCC region. Firstly, nonlocal energy transfer exerts a more significant influence. In the region of Kuroshio and NECC, the contribution of nonlocal energy transfer to ISV energy even exceeds 90%. Secondly, seasonal-biennial variability appears to have a slightly greater impact than the background flow for the local energy transfer, especially for the Kuroshio and MC. Thereinto, intra-seasonal available potential energy is the major energy source of subsurface ISV kinetic energy in the MC and hence may play an important role in the generation of sub-thermocline eddies there.

Multiscale interactions,intra-seasonal variability,tropical western Pacific Ocean