Gravity waves are what we normally think of as “wind waves”, such as the ocean swell generated by storms at sea that propagate to our beaches with periods of 10-30 seconds. Infragravity (IG) waves are generated when swell hits the coast and is transformed into sub-harmonics of the original wave [Herbers et al., 1995]. For example, IG waves are continuously generated as swell propagates southward along the west coast of North America [Bromirski and Gerstoft, 2009]. Some of the IG-wave energy leaks off the continental shelf and becomes “free waves” that can propagate across the Pacific Ocean from North America to the Ross Ice Shelf in Antarctica [Bromirski et al., 2010]. IG waves have longer periods (50-300 seconds), long wavelengths (over 15 km), and propagate rapidly in the open ocean.
Stress from Gravity Wave Flexing
In the ice-free austral summer, swell impacting the shelf front generally causes calving of relatively small icebergs, although periodically large tabular icebergs are calved. However, during colder winter months, free-floating sea-ice effectively damps virtually all ocean swell, eliminating swell impacts at the shelf front. The much longer wavelength IG waves, in contrast, are apparently undamped by sea-ice, and induce an ice shelf response even in winter.
The flexing of ice shelves by infragravity waves could provide the additional external forcing needed to extend existing fractures (rifts), thereby triggering sudden ice shelf collapse. The seismic measurements across the Ross Ice Shelf will provide the first comprehensive data on IG-wave amplitudes and the spatial, mechanical and elastic response of ice shelves to gravity wave forcing, both important to understand the ice-shelf disintegration process.
References & Further Information
Bromirski, P.D. and P. Gerstoft (2009).
Dominant source regions of the Earth’s “hum” are coastal.
Geophysical Research Letters. 36 doi: 10.1029/2009gl038903
Bromirski, P.D., O.V. Sergienko, D.R. MacAyeal (2010).
Transoceanic infragravity waves impacting Antarctic ice shelves,
Geophys. Res. Lett., 37, L02502, doi:10.1029/2009GL041488.
Herbers, T.H.C. , Steve Elgar, R.T. Guza, and W.C. O’Reilly (1995).
Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part II: Free Waves. J. Phys. Oceanogr., 25, 1063–1079.