Global Observations of Open-Ocean Mode-1 M2 Internal Tides

A global map of open-ocean mode-1 M-2 internal tides is constructed using sea surface height (SSH) measurements from multiple satellite altimeters during 1992-2012, representing a 20-yr coherent internal tide field. A two-dimensional plane wave fit method is employed to 1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence and 2) separately resolve multiple internal tidal waves. Global maps of amplitude, phase, energy, and flux of mode-1 M-2 internal tides are presented. The M-2 internal tides are mainly generated over topographic features, including continental slopes, midocean ridges, and sea-mounts. Internal tidal beams of 100-300 km width are observed to propagate hundreds to thousands of kilometers. Multiwave interference of some degree is widespread because of the M-2 internal tide's numerous generation sites and long-range propagation. The M-2 internal tide propagates across the critical latitudes for parametric subharmonic instability (28.88S/N) with little energy loss, consistent with the 2006 Internal Waves across the Pacific (IWAP) field measurements. In the eastern Pacific Ocean, the M-2 internal tide loses significant energy in propagating across the equator; in contrast, little energy loss is observed in the equatorial zones of the Atlantic, Indian, and western Pacific Oceans. Global integration of the satellite observations yields a total energy of 36 PJ (1 PJ = 10(15) J) for all the coherent mode-1 M-2 internal tides. Finally, satellite observed M-2 internal tides compare favorably with field mooring measurements and a global eddy-resolving numerical model.