PAOC Colloquium - Brian Arbic (University of Michigan)

Title:  Global modeling of oceanic internal tides and internal gravity waves

Abstract: This talk is about modeling oceanic internal gravity waves (IGWs) on a global scale. IGWs are waves having gravity as a restoring force, with their largest displacement  signals at depth, but with a measurable sea surface height (SSH) signal.  IGWs are important for several reasons.  IGW breaking controls most of the mixing in theopen-ocean beneath the mixed layer, and thus impacts the large-scale oceanic circulation, heat transport, and carbon transport.  IGWs impact the speed of sound and are therefore important for operational oceanography.  Finally, because IGWs yield a measurable SSH signal, they are important for satellite altimetry missions, including the upcoming Surface Water Ocean Topography (SWOT) mission that will measure SSH in two dimensions, and at smaller scales than current altimeters do.  We will discuss three important "flavors" of IGWs, near-inertial motions (generated by the high-frequency component of wind stress), internal tides (generated by tidal currents impinging upon topographic features), and IGW continuum spectrum, also called the Garrett-Munk spectrum.  The IGW continuum spectrum consists of high-frequency (supertidal) IGWs.  The classical paradigm is that the IGW continuum spectrum is generated by nonlinear interactions amongst internal waves, with the near-inertial waves and internal tides serving as primary energy sources. Based upon this paradigm, we expect that generating an IGW continuum spectrum in global models will require high-frequency atmospheric forcing (to generate near-inertial waves), tidal forcing (to generate internal tides), and high vertical and horizontal resolution (to facilitate nonlinear wave-wave interactions).  We will describe our work with the US Navy Hybrid Coordinate Ocean Model (HYCOM), in which we introduced high-resolution global ocean models simultaneously forced by atmospheric fields and the astronomical tidal potential.  We will also show newer simulations performed under similar conditions with the Massachusetts Institute of Technology general circulation model (MITgcm) and the Nucleus for European Modeling of the Oceans (NEMO). We summarize several papers on comparison of the modeled internal tides and the IGW continuum spectrum to altimetry and observations from moorings. We briefly discuss the generation of the continuum spectrum and the potential implications for a better understanding of ocean mixing. We also briefly mention other applications of global IGW models, including interpretation of satellite altimeter missions.