On the spring‐neap variability and age of the internal tide at the Hawaiian Ridge

Observations and modeling have shown that an energetic M2 internal tide is generated by tidal flow over the Hawaiian Ridge. This paper presents numerical results that explore the variability of the internal tide over the spring‐neap cycle resulting from the combination of M 2 and S 2 constituents. The spring‐neap cycle is simulated from a superposition of separate model runs for M 2 and S 2 , and from longer model runs with combined M 2 and S 2 forcing. There are only small differences between the two methods, indicating that at tidal frequencies and the 4‐km model resolution, the system is approximately linear. Energy fluxes at spring tides are substantially stronger than the sum of M 2 and S 2 fluxes, suggesting strong contrast from spring to neap tides in energy available for mixing. The baroclinic currents vary in amplitude over the spring‐neap cycle by approximately 50%, but tidal beams maintain their overall structure. The time of maximum spring amplitudes relative to the astronomical forcing is examined in terms of the age of the tide. Near the ridge where the internal tide is generated, the age is the same as that of the barotropic tidal currents, which is approximately 1 day older than the barotropic surface elevation. The age of the internal tide is approximately 1 day along the ridge and increases moving away from the ridge, consistent with the group velocity of the internal waves. Hence, away from the ridge, spring internal tides may occur 2 or 3 days after spring tidal elevations at the ridge.