Acoustic-gravity waves in the atmosphere generated by infragravity waves in the ocean

Abstract

Infragravity waves are surface gravity waves in the ocean with periods longer than approximately 30 s. Infragravity waves propagate transoceanic distances and, because of their long wavelengths, provide a mechanism for coupling wave processes in the ocean, atmosphere, and the solid Earth. Here, we present a strict physical justification for the hypothesis that background ocean waves may generate waves in the upper atmosphere. We show that, at frequencies below a certain transition frequency of about 3 mHz, infragravity waves continuously radiate their energy into the upper atmosphere in the form of acoustic-gravity waves. Based on ionospheric observations and estimates of the fluxes of the mechanical energy and momentum from the deep ocean, we conclude that acoustic-gravity waves of oceanic origin may have an observable impact on the upper atmosphere. We anticipate our work to be a starting point for a detailed analysis of global manifestations of the ocean-generated background acoustic-gravity waves.

Oceans are a major source of waves in the thermosphere

“…Away from shore, IGWs have very long wavelengths, on the order of tens to hundreds of kilometers, and can propagate thousands of kilometers without significant attenuation [Webb et al, 1991;Crawford et al, 2015], being efficiently reflected at ocean boundaries [Neale et al, 2015]. From theoretically predicted directions of propagation of AGWs generated by IGWs it follows that waves observed in the thermosphere would have originated on the ocean surface at horizontal separations of several hundred kilometers from the site of the thermospheric observations [Godin et al, 2015]. For example, as discussed in supporting information, the Pearson (linear) correlation coefficient of spectral amplitudes of IGWs for two Deep-ocean Assessment and Reporting of Tsunamis (DART) stations (44402 and 41424, separated by 731 km along the great circle arc) as determined over a 9 month interval, varies between~0.2 and~0.65 in the frequency band 0.1-3.4 mHz.…”

“…The frequency band covered (from 0.04 to 3.5 mHz) includes the range of frequencies where radiation of AGWs by IGWs is theoretically predicted [Godin et al, 2015] to play the most important role in coupling wave processes in the ocean and atmosphere. The frequency band covered (from 0.04 to 3.5 mHz) includes the range of frequencies where radiation of AGWs by IGWs is theoretically predicted [Godin et al, 2015] to play the most important role in coupling wave processes in the ocean and atmosphere.…”

“…Compared to convectively and orographically generated AGWs, those of oceanic origin typically have considerably higher horizontal phase speeds, which can exceed the maximum wind speeds. Therefore, while hardly observable in the troposphere and stratosphere, AGWs of oceanic origin were suggested to play a much larger role at ionospheric heights and potentially have a significant effect on thermosphere dynamics [Godin et al, 2015]. Therefore, while hardly observable in the troposphere and stratosphere, AGWs of oceanic origin were suggested to play a much larger role at ionospheric heights and potentially have a significant effect on thermosphere dynamics [Godin et al, 2015].…”

Source regions and reflection of infragravity waves offshore of the U.S.s Pacific Northwest

“…Bromirski et al [2010] have recently shown that infragravity waves generated along the Pacific coast propagate transoceanic distances and can be implicated in the flexure and subsequent breakup of Antarctic ice shelves. Infragravity waves at frequencies below 0.004 Hz may transfer energy from the ocean to the atmosphere [Livneh et al, 2007;Godin et al, 2015]. The deformation of the seafloor under the pressure of infragravity waves is used in measurements of seafloor compliance to determine the shear velocity structure of the shallow oceanic crust [Crawford et al, 1998], and the propagation of infragravity waves over a sloping seabed are thought to create low-frequency seismic noise known as Earth's seismic hum [Rhie and Romanowicz, 2006;Ardhuin et al, 2015].…”

Wentzel–Kramers–Brillouin approximation for atmospheric waves

“…Ground-breaking observational techniques (Nishida, Kobayashi & Fukao 2013;Garcia et al 2014) have provided new insights into atmospheric waves. AGWs couple wave processes in the solid earth and the ocean with those in the ionosphere and thermosphere (Watada 2009;Godin & Fuks 2012;Ardhuin & Herbers 2013;Godin, Zabotin & Bullett 2015) and, thus, underlie radar observations of earthquakes (Maruyama et al 2012;Astafyeva et al 2013) and satellite detection of tsunamis (Makela et al 2011;Occhipinti et al 2013;Garcia et al 2014;Coïsson et al 2015). Vertical transport of horizontal momentum by atmospheric waves plays a crucial role in large-scale circulation of the middle and upper atmosphere, and the development of climate models requires vastly improved parameterizations of the momentum flux (Vadas & Liu 2009;Geller et al 2013;Jia et al 2014 andSchirber et al 2014).…”