LAUSR

Nonlinear interaction between near-inertial waves (NIWs) and diurnal tides (DTs) after nine typhoons near the Xisha Islands of the northwestern South China Sea (SCS) were investigated using three-year in situ mooring observation data. It was found that a harmonic wave (f+D-1, hereafter referred to as fD(1) wave), with a frequency equal to the sum of frequencies of NIWs and DTs (hereafter referred to as f and D-1, respectively), was generated via nonlinear interaction between typhoon-induced NIWs and DTs after each typhoon. The fD(1) wave mainly concentrates in the subsurface layer, and is mainly induced by the first component of the vertical nonlinear momentum term, the product of the vertical velocity of DT and vertical shear of near-inertial current (hereafter referred to as Component 1), in which the vertical shear of the near-inertial current greatly affects the strength of the fD(1) current. The larger the Component 1, the stronger the fD(1) currents. The background preexisting mesoscale anticyclonic eddy near the mooring site may also enhance the vertical velocity of DT and thus Component 1, which subsequently facilitates the nonlinear interaction-induced energy transfer to the fD(1) wave and enhances the fD(1) currents after the passage of a typhoon.