The mode-2 M2 internal tide
Event: 2018 Ocean Surface Topography Science Team Meeting
Session: Tides, internal tides and high-frequency processes
Presentation type: Type Poster
Contribution: PDF file
The global mode-2 M2 internal tide is observed using sea surface height (SSH) measurements made by multisatellite altimetry. It is extracted in two steps: First, the mode-2 component is separated from modes 1 and 3 by an along-track bandpass filter with cutoff wavelengths of [0.85 1.35] times lambda, where lambda is the wavelength of the mode-2 M2 internal tide; second, three mode-2 M2 internal tidal waves are extracted by fitting plane waves in each 120 km by 120 km window. The altimeter-derived mode-2 M2 internal tide field underestimates its strength, due to the following reasons: (1) It contains the phase-locked component only, missing the time varying component; (2) it contains the southbound/northbound component only, missing the eastbound/westbound component. The satellite results reveal that the mode-2 M2 internal tide is ubiquitous and inhomogeneous in the global ocean. Its SSH amplitudes are a few mm. The spatial patterns of mode-1 and -2 M2 internal tides are very different. Mode 1 mainly originates at steep topographic features such as submarine ridges and continental slopes, but mode 2 is also generated at some gentler topographic features such as abyssal seamounts and fracture zones. Mode 1 can propagate thousands of km in the open ocean, but mode 2 can only be tracked for hundreds of km. Its depth-integrated energy and flux are calculated using SSH amplitudes and conversion functions built from the World Ocean Atlas 2013. The globally integrated energy of the mode-2 M2 internal tide is 8 PJ, about 22% that of mode 1 (36.4 PJ). This study suggests that mode 2 plays an important role in the SSH variance and energy budget of internal tides.