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The Global Mode‐2 M 2 Internal Tide
Author(s) -
Zhao Zhongxiang
Publication year - 2018
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2018jc014475
Subject(s) - internal tide , mode (computer interface) , internal wave , geology , amplitude , satellite , geodesy , wavelength , physics , optics , oceanography , computer science , operating system , astronomy
The mode‐2 M 2 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]× λ 2 , where λ 2 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 M 2 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 M 2 internal tide. The results show that the mode‐2 M 2 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 M 2 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.