Internal Tides Modulated Typhoon-Ocean Interaction in the Northern South China Sea

Previous studies have demonstrated that oceanic background environments could modulate the typhoon-ocean interaction processes, such as mesoscale eddies (Lin et al. 2005), barrier layers (Balaguru et al. 2012), etc. Based on abundant current and temperature data from mooring arrays in the Northern South China Sea (NSCS), we found that the background Internal Tides could also significantly influence the upper ocean response and hence negative feedback to typhoons. During super typhoon Megi (2010), a relatively weak (maximum amplitude of 0.4 m s^-1) and quickly damped (e-folding timescale of 2 inertial periods) near-inertial oscillation (NIO) was observed in the NSCS. Power spectrum and wavelet analyses both indicated an energy peak appearing at exactly the sum frequency fD1 (with maximum amplitude up to 0.2 m s^-1) of NIO (f) and diurnal tide (D1), indicating enhanced nonlinear wave-wave interaction between f and D1 during and after typhoon. Numerical experiments suggested that energy transfer from NIO to fD1 via nonlinear interaction between f and D1 may have limited the growth and accelerated the damping of mixed layer NIO generated by Megi. Further analysis under other typhoon cases indicated the nonlinear coupling between NIO and semidiurnal internal tides (D2) and generating of the secondary wave fD2. Due to the nonlinear wave-wave interaction, near inertial and tidal energy was transferred to secondary waves, elevating the vertical shear and contributing to a stronger vertical mixing and entrainment under typhoons in the NSCS. Based on satellite remote sensing SST data, a much larger (>50%) SST cooling was validated in the NSCS than that in the north western Pacific with the similar UOHC (Leipper and Volgenau 1972) or T100 (Price 2009) as NSCS. A larger SST cooling implies less enthalpy flux from ocean to typhoons and hence suppress the intensification of typhoons.