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