Observation of Enhanced Nonlinear Interactions After Severe Tropical Storm Chanchu (2004) in the Western South China Sea

Abstract

AbstractSevere tropical storm Chanchu passed the subsurface mooring in the western South China Sea in June 2004, and its effects on the internal wavefield and nonlinear interactions were examined. The internal wavefield was characterized by diurnal (D1) and semidiurnal (D2) tides before Chanchu. However, after the passage of Chanchu, the internal wavefield was not only characterized by diurnal and semidiurnal tides but also by near‐inertial (f) waves, internal waves with higher harmonic frequency (fD2 = f+D2 and D3 = D1+D2), and enhanced diurnal currents. Analysis of the rotary coefficient and bicoherence suggested that the observed fD2 waves, D3 waves, and enhanced diurnal currents originated from the nonlinear interactions of f+D2 = fD2, D1+D2 = D3, and f+f = 2f, respectively. Previous studies indicated that the nonlinear interactions induced by tropical cyclones mainly occur between the near‐inertial waves and internal tides (f+D1 = fD1 or f+D2 = fD2). Our observations indicate that tropical cyclones can also induce the self‐interaction of near‐inertial waves (f+f = 2f) and enhance nonlinear interactions between diurnal and semidiurnal tides (D1+D2 = D3). Critical diurnal latitudes (where 2f ≈ D1) play an important role in the occurrence of these nonlinear interactions. Near‐inertial waves undergo self‐interactions (f+f = 2f) near the critical diurnal latitudes, and near‐inertial energy is transferred to diurnal waves. Enhanced diurnal waves further contribute to the nonlinear interactions of D1+D2 = D3. These nonlinear processes might promote the energy cascade of the near‐inertial waves induced by tropical cyclones near the critical diurnal latitudes and play an important role in ocean mixing.