Near- and super-inertial internal wave responses and the associated energy transfer after the passage of tropical cyclone Fitow at a midlatitude shelf slope

Abstract

Observations from a mooring station at the East China Sea (ECS) shelf slope revealed both near- and super-inertial dynamic responses to tropical cyclone (TC) Fitow. Different from the typical near-inertial response, near-inertial internal waves (NIWs) after TC Fitow showed a complicated phase pattern due to its superposition with parametric subharmonic instability (PSI)-generated M1 subharmonic waves. The wind-injected near-inertial kinetic energy (NIKE) was largely restrained to the upper 250 m. Wave-packet analysis revealed the cooccurrence of enhanced NIKE, circularly polarized near-inertial currents, veering NIW propagation direction, and shrinking NIW vertical wavenumber at the base of the Kuroshio Current (~ 180 m). This indicated the trapping and stalling of the TC-generated NIWs. Intense high-frequency internal waves (HFIWs) appeared immediately after TC Fitow which had an average period of ~ 24 minutes and lasted ~ 8 hours. HFIWs also existed before the arrival of TC Fitow with a regular semidiurnal cycle. However, the HFIW after TC did not follow the semidiurnal cycle and had much larger amplitudes and longer-lasting periods. Local generation of supercritical flow over a slope or evolution from propagating internal tide as modified by TC may have induced these HFIWs. Along with the occurrence of intense HFIWs after TC Fitow, intense energy transfers from NIWs to HFIWs were identified. Due to the limited vertical propagation of TC-induced NIWs, it was the PSI-generated M1 subharmonic wave rather than the wind-induced NIW that contributed most to the energy transfer.