Observed Near Inertial Waves in the Wake of Typhoon Linfa (2015) in the Northern South China Sea

Abstract

This study examined the characteristics and vertical propagation of near inertial waves (NIWs) induced by Typhoon Linfa (2015), based on in situ observations conducted southeast of Dongsha Islands in the South China Sea. The results demonstrate that the near inertial currents induced by Linfa had velocities up to 35cm s−1 in the mixed layer and 20cm s−1 in the ocean interior. The near inertial currents were polarized with predominantly clockwise-rotating components, the magnitudes of which were about 10 times larger than the counter-clockwise rotating components. The energy density spectrum showed that the emergence of NIWs resulted in energy redistribution from the diurnal band to the near inertial band. The wavenumber spectrum and the downward/ upward current decomposition demonstrated that the NIWs and energy flux propagated mainly downward. The estimated vertical phase velocity and group velocity are 1.44 and 0.48m h−1, respectively, corresponding to a vertical wavelength of 49.7m. The e-folding time scale was 7.5 d based on the near inertial kinetic energy in the ocean interior. We found no obvious wave-wave interaction during the decay process of the NIWs. The frequency was blue-shifted, being 0.03f0 higher than the local inertial frequency, which was caused by the background vorticity. The normal mode analysis suggests that the higher mode plays a dominant role in the propagation stage of the NIWs.