Instabilities in internal gravity waves

<abstract><p>Internal gravity waves are propagating disturbances in stably stratified fluids, and can transport momentum energy over large spatial extents. From a fundamental viewpoint, internal interesting due to the nature of their dispersion relation, linear dynamics reasonably well-understood. an oceanographic qualitative quantitative understanding significant wave generation ocean is emerging, while dissipation mechanisms being debated. This paper reviews current knowledge on instabilities waves, primarily focusing growth small-amplitude disturbances. Historically, wave-wave interactions based weakly nonlinear expansions have driven progress this field, investigate spontaneous transfer various temporal scales. Recent advances numerical/experimental modeling field observations further revealed noticeable differences between forms terms instability characteristics; turn has motivated theoretical calculations appropriately chosen fields settings. After brief introduction, we present pedagogical discussion different two-dimensional forms. The general ideas concerning triadic resonance then introduced, before proceeding towards characteristics plane beams modes. Results from theoretical, experimental numerical studies summarized provide overall picture gaps our understanding. An perspective given, both relevant outstanding questions additional factors at play. While applications review focused ocean, several atmospheric astrophysical systems too.</p></abstract>