Infra-Gravity waves across the oceans

Tuesday, 16 December 2014
Rawat Arshad, IFREMER, Plouzané, France and Fabrice Ardhuin, IFREMER, LOS, Plouzané, France; IUEM Institut Universitaire Européen de la Mer, Laboratoire de Physique des Océans, Plouzané, France
Ocean infragravity (IG) waves are long surface gravity waves generated along shorelines by incident swell and wind-waves. They have been studied mostly in the context of nearshore morphodynamics and harbor agitation, where their heights often exceeds 10 cm in shallow water. In the open ocean, infragravity wave properties are only known from sparse bottom pressure records, with heights of the order of 1 cm. In spite of these small amplitudes, they can be of much importance for ice shelf break up and errors in measurements of sea level by future satellite altimeters. Here we show how a recently developed numerical model can be used to interpret bottom pressure records from stations scattered across ocean basins. We show that bottom pressure signals in the infragravity frequency band can be dominated by bursts of energy that travel across ocean basins from the west coasts, and last several days. Two particularly strong events recorded in 2008 are studied, one in the North Pacific and the other in the North Atlantic. It was shown that infragravity waves are able to travel long distances without significant dissipation, so that the energy recorded in depths beyond 3000 m in the western part of an ocean basin is often dominated by waves coming from the other side of the same ocean basin.

The figures below shows (a) the modelled instantaneous IG wave field on 06/01/2008 over the North-Atlantic Ocean with locations of pressure sensors used (red squares). (b) IG levels measured (black lines) and modelled values (red lines and circles) for the corresponding station. Pressure values were translated into surface elevation for the frequency range 0.005Hz to 0.012Hz