Generation of second-mode internal solitary waves during the winter by a shoaling internal tide in the northern South China Sea

TL;DR

This study investigates how shoaling mode-2 internal tides from the Luzon Strait generate second-mode internal solitary waves in the northern South China Sea during winter, using modeling and theoretical analysis.

Contribution

It reveals the generation mechanism of mode-2 ISWs by shoaling mode-2 semidiurnal internal tides and highlights the role of intermodal coupling in internal tide evolution.

Findings

Mode-2 ISWs appear in pairs within a tidal cycle.

Larger internal tides produce larger ISWs.

Intermodal coupling influences internal tide propagation.

Abstract

Field measurements of second-mode internal solitary waves (mode-2 ISWs) during the winter on the upper continental slope of the northern South China Sea were reported in Yang et al. (2009), but their generation mechanism remains elusive. We investigated this issue with a multi-modal evolution model and theoretical analysis, which suggest that the observed mode-2 ISWs were generated by a shoaling mode-2 semidiurnal internal tide (IT) that emanated from the Luzon Strait. The results show that two groups of mode-2 ISWs usually appear within one semidiurnal tidal period, successively riding on expanded and subsequently compressed pycnoclines. The number of wave groups largely depends on the amplitudes of the ITs; that is, a larger IT produces larger mode-2 ISWs. Furthermore, intermodal coupling dominates the evolution of a mode-1 IT, highlighting the importance of considering mode scattering in the propagation of low-mode ITs.