Using the mooring ADCP datasets obtained near the diurnal critical latitude (28.9N) of the East China Sea Shelf, this study explores the temporal and spatial variation of diurnal (D1) and semi-diurnal (D2) tide and the underlying mechanisms.

Our rotary spectral analysis indicates that the energy of D1 and D2 experiences significant seasonal variations; specifically, it is the strongest in summer, which then attenuates in spring, and is the weakest in winter. Although the Clockwise D2 energy is almost the same in summer and spring, the energy within the D1 band is an order of magnitude worse in summer. Meanwhile, the energy measured in the mooring site near the shelf break is stronger than the one measured near the coast.

Further analysis shows that there exists a dramatic change in the topography on the southwest side of ECS, which suggests its role in D2 internal tides generation of. The result, together with the ray path theory, suggests that the D2 internal tide generated there can propagate to the mooring site, though the energy decreases gradually due to bottom friction.

Besides, we also find that the above-mentioned energy distribution pattern shall be mainly caused by the seasonal variation of stratification, while the energy input from the rotary wind and energy transaction from parametric subharmonic instability (PSI) also play a role. With several typhoons passing by, the stratification in ECS continental shelf is known to be the strongest in summer.

Moreover, since the mooring sites were deployed near the diurnal critical latitude, we noticed that, in most situations, energy can be transferred from D2 to D1 through the PSI. However, weakening or cessation of the PSI would also occur in some situation, specifically when the slope intrusion mesoscale eddies derived from Kuroshio changes the background vorticity or when there exists a Doppler effect caused by the background flow