Spontaneous Emission of Gravity Waves by Interacting Vortex Dipoles in a Stratified Fluid: Laboratory Experiments

Abstract

Results from a new series of experiments on the geophysically important issue of spontaneous emission of internal gravity waves during unsteady interactions of vortical structures are presented. Vortex dipoles are a common element of a quasi-two-dimensional turbulent flow. Vortex dipoles perform translational motion and can collide with other vortices. During collision events the flow is unsteady and unbalanced and a further adjustment process associated with these events can therefore result in the spontaneous emission of gravity waves. Our laboratory experiments demonstrate that gravity waves are emitted when two translating vortex dipoles interact (collide) in a layered fluid, in accord with the current theoretical results. The emission was evident both in a two-layer system and in a fluid with a linear distribution of density with depth. The waves were generated during the period of deceleration of the secondary dipoles which constitute a vortex quadrupole emerging immediately after the collision of the primary dipoles.

Spontaneous Generation of Inertia–Gravity Waves during the Merging of Two Baroclinic Anticyclones

“…In this case the energy transfer is (100 ϫ ⌬E T )/(⌬tE T ) Ӎ 0.7%, or 7% per rotation period. The order of magnitude of these values are in agreement with the 4% estimated by Afanasyev (2003) after a dipole collision in laboratory experiments, and the 1% obtained by Williams et al (2007, manuscript submitted to J. Atmos. Sci.).…”

“…IGWs easily develop also when unbalanced initial conditions are used in numerical simulations (e.g., Daley 1991, chapter 6); meanwhile the unbalanced mass and momentum fields slowly adjust through geostrophic adjustment (Rossby 1938) or, more generally, any balance adjustment (Zhang 2004). Recently, several oceanographic and atmospheric studies (Ford 1994;Ford et al 2000;Afanasyev 2003;Lovegrove et al 2000;Plougonven and Zeitlin 2002;Williams et al 2003Williams et al , 2005Lane et al 2004;Viúdez and Dritschel 2006;Viúdez 2006) have shown that weak (in the horizontal velocity field) IGWs can be spontaneously emitted (i.e., in the absence of any external forcing like wind stress or bottom topography) from balanced or near-balanced, nonstationary flows.…”

“…The spontaneous IGWs can be energetic enough to contribute as an interior source (far from the ocean's boundaries) to the energy budget of the gravity wave spectra in the ocean. Afanasyev (2003) found that approximately 4% of the total flow energy is radiated during the adjustment after a secondary vortex dipole collision in a rotating stratified tank. SAE represents a new source of IGWs that could explain the gap of the total amount of energy going into the IGWs' pool, usually computed only considering the wind stress and internal tides' inputs.…”

Gravity wave emission in an atmosphere-like configuration of the differentially heated rotating annulus experiment

“…A survey of the flow regimes observed in the experiment is found in Hide & Mason (1975). Promising laboratory experiments on wave generation observed at the interface between two superposed fluids of differing density have been done by Lovegrove, Read & Richards (1999, Williams, Read & Haine (2003) and Williams, Haine & Read (2005 for a baroclinically unstable flow in the rotating annulus, and by Afanasyev (2003) for colliding vortex dipoles in a non-rotating experimental set-up. A variant of the conventional differentially heated rotating annulus, where, in addition to the horizontal temperature gradient, an external vertical temperature gradient is applied, is discussed by Hathaway & Fowlis (1986) and Miller & Fowlis (1986) and modelled by Kwak & Hyun (1992).…”

Inertia–Gravity Waves Emitted from Balanced Flow: Observations, Properties, and Consequences

“…We discuss the consequences of this energy flux for the atmosphere and ocean in section 6. For comparison, Afanasyev (2003) estimates that approximately 4% of the energy is radiated as inertia-gravity waves in a nonrotating, linearly stratified laboratory fluid during the adjustment after the collision of two translating vortex dipoles.…”