Observations of Internal Tides in the East China Sea around the Diurnal Critical Latitude

Abstract Parametric subharmonic instability (PSI) of internal tides (ITs) in the East China Sea (ECS) shelf near the diurnal critical latitude is investigated for the first time using in situ observations in different seasons. It is found that ITs in diurnal (D 1 ) and semidiurnal (D 2 ) bands exhibit apparent seasonal variability, with their magnitude being the highest in summer and lowest in winter. In contrast to previous studies suggesting that D 2 ITs are overwhelmingly dominant in the ECS shelf, in our case, the kinetic energy of D 1 band ITs is found to dominate over that of the D 2 band during summer when ITs are energetic. Additionally, D 1 ITs are more stationary than D 2 ITs with their stationary components accounting for 57%–97% (46%–72%) of D 1 (D 2 ) motions. In summer, high fraction (>90%) of the stationary component of D 1 ITs is observed, which is suggested to be generated by the D 2 ITs through PSI. Moreover, remarkable temporal variability in PSI efficiency is observed. Overall, PSI is strong in summer and negligible in winter; besides, it also changes dramatically between adjacent spring–neap tidal cycles, which is mainly induced by the joint effect of local vorticity field and ambient-flow-relevant Doppler shift. In this work, an efficiency rate index R eff is proposed, which enables the quantitative evaluation of both the effect of vorticity on the subharmonic waves (D 1 ITs), the role of ambient flow on the primary wave (D 2 ITs), and their joint effect; when R eff is larger than 1, PSI is more efficient. Significance Statement Parametric subharmonic instability (PSI), a specific type of nonlinear wave–wave interaction, can transfer energy from an internal tide (IT) to smaller-scale waves with half of its frequency. PSI-induced internal tide energy transfer has been reported in many sea areas. However, the observed dissipation rate is not as significant as suggested by numerical simulations. Based on observations, we present the seasonal variation of PSI. Besides, the variation in PSI efficiency between adjacent spring–neap tidal cycles is analyzed. We find evidence that the joint effect of the Doppler shift and vorticity can modulate PSI. This finding can improve our understanding on the effectiveness of PSI in continental shelf seas.