Gravity-wave emission and propagation in a vortex dipole

Vortex dipoles provide a simple representation of localized atmospheric jets. We consider a dipole in surface potential temperature in a rotating, stratified fluid with uniform potential vorticity. Following an initial period of adjustment, the dipole propagates along a slightly curved trajectory at a nearly steady rate and with nearly fixed structure for more than 20 days. The flow also contains upward propagating inertia-gravity waves that are stationary with respect to the dipole and form elongated bows concentrated near and roughly aligned with the leading edge of the dipole. An even more nearly steady solution can be obtained from initial conditions given by a 5-day time average in the reference frame moving with the dipole. This solution still exhibits the same gravity waves near its leading edge, thus confirming that the waves are emitted by the dipole rather than being remnants of imbalances in the initial conditions. The wavelength of the waves decreases and their vertical velocity increases as they near the stagnation point at the leading edge of the dipole, and we show that the wave vector tends to assume a preferred orientation determined by the local horizontal deformation and vertical shear. These simulations support the view that wave emission follows from the continuous forcing of gravity waves by the balanced flow.