Dynamics of Orographic Gravity Waves from the Surface to the Mesosphere As Observed and Modeled Over the Auckland Islands During the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

On 14 July 2014 during DEEPWAVE, aircraft remote-sensing instruments detected large-amplitude gravity-wave oscillations within mesospheric airglow and sodium layers at altitudes z∼78–83 km downstream of the Auckland Islands, located ~1000 km south of New Zealand in the Southern Ocean. A high-altitude reanalysis and a three-dimensional Fourier gravity wave model are used to investigate the dynamics of this event from the surface to the mesosphere. At 0700 UTC when first observations were made, surface flow across the islands' terrain generated linear three-dimensional wavefields that propagated rapidly to z∼78 km where intense breaking occurred in a narrow layer beneath a zero-wind region at z∼83 km. In the following hours, the altitude of weak winds descended under the influence of a large-amplitude migrating semidiurnal tide, leading to intense breaking of these wavefields in subsequent observations starting at 1000 UTC. The linear Fourier model constrained by upstream reanalysis reproduces the salient aspects of observed wavefields, including horizontal wavelengths, phase orientations, temperature and vertical displacement amplitudes, heights and locations of incipient wave breaking, and momentum fluxes. Wave breaking has huge effects on local circulations, with inferred layer-averaged westward mean-flow accelerations of ∼350 m s^-1 hour^-1 and dynamical heating rates of ∼8 K hour^-1, supporting recent speculation of important impacts of orographic gravity waves from subantarctic islands on the mean circulation and climate of the middle atmosphere during austral winter. We also discuss relevance of this event for parameterizations of subgrid-scale orographic gravity-wave drag within weather and climate prediction models.