Bizarre Love Triangle: Triad Interactions between the M2, K1, and O1 tides in the Western Pacific

Momentum exchange between ocean tidal waves of different frequencies facilitates energy transfer and dissipation. However, such mechanisms are poorly understood, and how such processes will change in a warmer ocean is unclear. Inviscid triads are nonlinear interactions involving three waves whose frequencies sum to zero; at least one of the waves is always internal. Triads exchange energy on a time scale slower than the frequencies of the original waves and can occur in resonant and non-resonant forms. Triad interactions are possible in a variety of media (e.g., light waves), and also occur in the ocean, especially for ocean tides. Here, we detail triad interactions between the M2, K1, and O1 tides in three complex regions of the Western Pacific Ocean: The South China Sea, The Solomon/Bismarck Sea region, and the Coral Sea along the eastern coast of Australia. We develop simplified diagnostic versions of the triad interaction equations and examine the amplitude, phase-lock, energy balance, and wavenumber match of each triad. Strongly resonant triads are prevalent in the Gulf of Thailand/Malacca Strait, in the Solomon Sea/Bismarck Sea, and along the eastern coast of Australia, with isolated triads seen in the South China Sea. All triads are found near regions of complex bathymetry and intense internal tide generation and activity, which provide the most likely explanation for their occurrence. However, triad properties are strongly dependent on latitude, local stratification, and depth; the latter two will change as the ocean warms and sea level rises.