Ocean is a Major Source of Waves in the Thermosphere

Abstract:

Acoustic-gravity waves (AGWs) are an important component of atmospheric dynamics. AGW forcing plays a key role in large-scale circulation of the middle and upper atmosphere. From troposphere to mesosphere, wave processes are apparently dominated by those AGWs that are generated by moist convection, shear flow instabilities, jets, atmospheric fronts, and interaction of winds with the Earth’s topography. Recent theoretical analysis of the effects of atmospheric loading on oceanic waves led to suggestion that infragravity waves (IGW), i.e., long surface gravity waves in the ocean with periods longer than 30 s, can radiate AGWs and account for significant part of the wave activity observed in the upper atmosphere at frequencies between about 0.1 and 3.5 mHz, or wave periods between about 5 min and 3 h (Godin et al., Earth, Planets and Space, 2015). AGWs of oceanic origin typically have high horizontal phase speeds and can escape the critical level filtering. While hardly observable in the troposphere, stratosphere and mesosphere, AGWs of oceanic origin were suggested to play a much larger role at thermospheric heights and potentially have a significant effect on thermosphere dynamics (Godin et al., Earth, Planets and Space, 2015). In this paper, we report a definitive experimental demonstration of thermospheric waves being driven by the ocean. Bottom pressure recorders of two Deep-ocean Assessment and Reporting of Tsunamis (DART) stations located off the US East Coast have been used to measure IGW spectra. Wave activity in the thermosphere has been quantified using Dynasonde radar system located at Wallops Island, Virginia. Over a 9-month observation period, variations of IGW and AGW spectral amplitudes demonstrate large, statistically significant correlation in a broad range of frequencies (0.2–3.4 mHz) and altitudes (140–290 km). Peak correlation values (~0.55) indicate that waves radiated by the ocean represent a major constituent of the thermospheric wave activity.