The Global Mode‐2 M2 Internal Tide

Abstract

AbstractThe mode‐2 M2 internal tide is observed from satellite altimetry. It is extracted in two steps: First, the mode‐2 component is separated from modes 1 and 3 by a bandpass filter with cutoff wavelengths of [0.85 1.35]×λ2, where λ2 is the mode‐2 wavelength; and second, three mode‐2 internal tidal waves are extracted by fitting plane waves in each 120 km by 120 km window. The satellite‐observed mode‐2 M2 internal tide underestimates its strength: It contains the phase‐locked component only, missing the time‐varying component; it contains the southbound/northbound component only, missing the eastbound/westbound component. The satellite results provide rich information on the global mode‐2 M2 internal tide. The results show that the mode‐2 M2 internal tide is ubiquitous in the ocean, and its sea surface height amplitudes are O (1 mm). The spatial patterns of mode‐1 and mode‐2 internal tides are very different. Mode 1 mainly originates at steep topographic features such as submarine ridges, but mode 2 is also generated at gentler topographic features such as abyssal seamounts and fracture zones. Mode‐1 beams propagate O (1,000 km), while mode‐2 beams can be tracked for O (100 km). Depth‐integrated energy and flux are calculated using sea surface height amplitudes and conversion functions built from the World Ocean Atlas 2013. The globally integrated energy of the mode‐2 M2 internal tide approximates to 8 PJ, about 22% of mode 1 (36.4 PJ). This work suggests that higher‐mode internal tides play an important role in the tidal energetics, in particular, over abyssal seamounts and fracture zones.