Comparison of typhoon-induced near-inertial oscillations in shear flow in the northern South China Sea

With moorings equipped with Acoustic Doppler Current Profilers (ADCP) in the northern South China Sea (SCS) in 2008 and 2009, we observed three near-inertial oscillation (NIO) events coded 2008a, 2009a and 2009b induced by passages of typhoons or tropical storms. This study compares characteristics of the three NIO events. Event 2008a was the strongest one among the three, and had the longest sustaining period (15 d), while events 2009a and 2009b sustained for only 4 and 8 d, respectively. The three events were distinguished by vertical energy distribution and phase propagation. As for the frequency shift of the NIO, event 2008a had a peak frequency lower than the local Coriolis frequency (red-shift), while events 2009a and 2009b showed blue-shift. The behavior of individual NIO event is jointly decided by the typhoon disturbance and the background ocean condition. Especially the background flow plays an important role by effects of advection and modulation. The results in this study provide observational evidence of variational NIO response to background flow field. As indicated by the distribution of vorticity and effective Coriolis frequency derived from numerical modeling, the large amplitude and elongated sustaining period of event 2008a were attributed to the waveguide effect of the background shear flow. This effect redistributed the NIO energy after the typhoon passage, absorbed incident waves and trapped energy in the area of the negative vorticity. While the background flow during events 2009a and 2009b did not have such effects due to the near-zero vorticity in the mooring area.