Observation of internal tides and near-inertial motions in the upper 450 m layer of the northern South China Sea

Mooring ADCP current observations from August to November are used to study the barotropic tides, baroclinic tides and near-inertial motions in the upper 450 m layer of the northern South China Sea. The barotropic and baroclinic tides at the mooring station are all dominated by M₂, K₁, O₁, P₁, with the barotropic amplitudes being 7.8 cms·s⁻¹, 7.0 cm·s⁻¹, 5.4 cm·s⁻¹ and 3.5 cm·s⁻¹ respectively. The amplitudes of M₂, K₁ O₁, P₁ internal tides vary greatly, which are 12–15 cm·s⁻¹ in the thermocline, and then decrease with increasing depth. The amplitude of the barotropic near-inertial motions is less than 1 cm·s⁻¹, contributing little to the barotropic currents. However, that of the baroclinic near-inertial motions can be as large as 5 cm·s⁻¹. The inclination of tidal ellipse tends to increase with increasing depth, implying upward propagation of energy, while that of the near-inertial ellipse tends to decrease with increasing depth, implying downward propagation of energy.