Energetics and variability of semidiurnal internal tides across the East China Sea shelf: Local generation versus remote forcing

Abstract

Internal tides (ITs) represent a crucial energy source driving turbulent mixing in stratified oceans. The East China Sea (ECS) continental margin is a major IT generation hotspot, yet IT variability across its broad shelf remains poorly constrained. Using high-resolution modeling with realistic forcing, we investigate semidiurnal IT energetics and variability in the ECS. Results show strong seasonality with peak IT activity in summer, primarily driven by stratification changes. Two distinct mechanisms contribute to the on-shelf ITs: (1) local generation at isolated topographic features and (2) onshore propagation from the ECS shelf break. Analysis demonstrates that local generation dominates the IT energy budget across the ECS shelf (1.32 GW integrated over depths shallower than 200 m), particularly at the prominent topographic rises including the Changjiang Estuary area and middle-shelf seamounts. Although the onshore-propagating IT energy fluxes from this region contribute relatively little to the total shelf energy budget, the shoreward IT propagation seriously regulate the local IT dynamics, especially in the outer shelf regions (100 – 200 m depth range). It has generated a pronounced cross-isobath asymmetry in IT energy transport, with onshore fluxes surpassing offshore fluxes by a factor of 2.3 at the 200-m isobath (0.25 GW vs. 0.11 GW) and 1.9 at the 100-m isobath (19.22 MW vs. 10.04 MW). These onshore energy fluxes, regulated by both shelf-break generation intensity and topographic criticality, peak northeast of Taiwan Island and along the northern ECS shelf break. These fluxes play a substantial role in modulating on-shelf IT variability in adjacent areas.