Inertia-gravity waves generated within a vortex dipole

Chris Snyder (NCAR), David Muraki (Simon Fraser), Riwal Plougonven (LMD), and Fuqing Zhang (Texas A&M)

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 and a Rossby number of 0.2 based on the dipole radius and the maximum speed of the jet. 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 embedded within and stationary relative to the dipole. The persistence of the waves for tens of days strongly suggests the waves are inherent features of the dipole itself, rather than being remnants of imbalances in the initial conditions. The waves are most apparent near the surface along and downstream of the dipole's jet. The vertical velocity associated with the waves is symmetric across the jet to a first approximation and forms elongated bows that are aligned with the leading edge of the dipole and whose horizontal scale shrinks as the leading stagnation point in the dipole's flow is approached. This structure and its vertical and horizontal variation are consistent with inertia-gravity waves propagating in horizontal and vertical shear. The evolution of both the dipole and the waves occurs on a time scale of tens of days. Since this time scale is longer than the advective time scale that characterizes the dipole's propagation and much longer than the 1/f timescales for wave propagation, it seems incorrect to associate the waves with geostrophic adjustment. Other possibilities for the generation of the waves are (spontaneous) wave emission and instability of the underlying balanced dipole. We are exploring these possibilities using numerical simulations of the primitive equations linearized about the steadily propagating dipole solution of surface quasigeostrophy.