Analysis of multi-decadal tide records, satellite altimetry, and high-resolution oceanic reanalysis around the Hawaiian Ridge identifies correlations between offshore and onshore mean sea level (MSL), M2 tidal amplitudes, and ocean stratification; these are linked to Pacific decadal climate variability. Empirical orthogonal function analyses reveal strongly correlated quasi-decadal variability in onshore and offshore tides and MSL, and all three factors are highly correlated with regional density stratification. This decadal variability is highly correlated with multiple Pacific climate indices, suggesting that this climate variability influences internal tides via coupled ocean-atmosphere mechanisms. The surface expression of variations in the M2 internal tide yield correlated variability between MSL and M2 offshore and onshore. The M2 signals at all tide gauges have stronger relationships to MSL in the altimetry era (1992-2023) than their respective full records, and both factors show stronger connections to climate variations in recent years. The M2 signal at Hilo is most clearly connected to climate variability over its full record, stronger even than the MSL-climate connections at all tide gauges. The amplitudes of the climate-induced tidal variations are on are on the order of 10% on top of MSL variability and long-term steric sea level rise. This amplification may exacerbate the frequency of high-tide flooding (also known as “sunny-day flooding”) in harbors and other low-lying areas of Hawai’i, highlighting the need for dynamic coastal management strategies that integrate astronomical, non-astronomical, and climatic factors, in sea level projections.

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.