Abstract
The emission of inertia-gravity waves (IGWs) from an exact geostrophic vortex in a rotating and stratified fluid is investigated by three-dimensional numerical modeling. An initially balanced geostrophic vortex inevitably generates IGWs with spiral patterns within a short transient time period through an instability mechanism. This result reinforces the nonexistence of exactly invariant slow manifolds. The direction of the rotation of spiral IGWs is clockwise for both anticyclonic and cyclonic geostrophic vortices, which is consistent with the theoretical prediction. Spiral patterning can be regarded as a universal feature of IGWs, which occurs in the transient generation process. In the vertical direction, the energy of IGWs is dominated by mode-1 in the generation and propagation processes, leading to weak dissipation and long-distance propagation. A comparison of barotropic and baroclinic vortices suggests that horizontal nonzero strain and vorticity are essential for the occurrence of this instability mechanism, while the presence of vortex baroclinicity increases the intensity of the IGWs. The amplitude of the IGWs increases linearly with the Rossby number in the range of 0.04–0.1. Additionally, the IGWs emitted from an anticyclonic vortex are stronger than those radiated from a cyclonic vortex. Anticyclonic and cyclonic geostrophic vortices transfer approximately 0.54% and 0.41% of their kinetic energy to IGWs in this transient generation process, respectively. This transient generation of IGWs can supply an energy pathway from mesoscale eddies to diapycnal mixing processes in the interior of the oceans.
Highlights
Inertia-gravity waves (IGWs), which are between the inertial and buoyancy frequency bands in the oceanic energy spectrum, are ubiquitous in the stratified ocean.1 inertia-gravity waves (IGWs) are thought to play an important role in oceanic energy cascades as a link between large-scale climatological forcing and small-scale turbulent dissipation.2 Nonlinear wave–wave interaction and the influence of background flows induce cascades of IGWs energy, including internal tides and near-inertial waves, to influence dynamics at small scales, subsequently resulting in the breaking and dissipation of IGWs.3,4 In the ocean interior, ocean mixing, which occurs mainly due to the breaking of IGWs, supplies potential energy to the meridional overturning circulation.5 Multiple mechanisms can lead to energy transfer from large-scale motions to the IGW fields
Spiral patterning can be regarded as a universal feature of IGWs, which occurs in the transient generation process
The theoretical results predict that the clockwise spiral patterns of IGWs are universal for exactly balanced flows with any specific form
Summary
Inertia-gravity waves (IGWs), which are between the inertial and buoyancy frequency bands in the oceanic energy spectrum, are ubiquitous in the stratified ocean. IGWs are thought to play an important role in oceanic energy cascades as a link between large-scale climatological forcing and small-scale turbulent dissipation. Nonlinear wave–wave interaction and the influence of background flows induce cascades of IGWs energy, including internal tides and near-inertial waves, to influence dynamics at small scales, subsequently resulting in the breaking and dissipation of IGWs. In the ocean interior, ocean mixing, which occurs mainly due to the breaking of IGWs, supplies potential energy to the meridional overturning circulation. Multiple mechanisms can lead to energy transfer from large-scale motions to the IGW fields. The initially imbalanced flow returns to near geostrophic balance by radiating IGWs to the far field.. To the near-balanced flows, the life cycle, and the dipole as mentioned before, a transient geostrophic flow generates IGWs spontaneously. Except for the dependence of IGW amplitudes on the Rossby number, the IGW characteristics, including their patterns, vertical structures, and horizontal wavelengths, are not unequivocal in the spontaneous adjustment of balanced flows. Focus is given in the spontaneous adjustment process to the horizontal patterns and vertical structures of the IGWs, as well as the dependence of the amplitude of IGWs on the Rossby number.
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