Mark D. Fruman
IFREMER, Brest, France
in collaboration with B. L. Hua and R. Schopp

Zonal Jet Formation and Equatorial Super-rotation in the Destabilization of Short Mixed Rossby-gravity Waves

The meridional and vertical structure of zonal jets formed in the destabilization of short mixed Rossby-gravity (MRG) waves is investigated numerically and analytically.
High resolution numerical simulations with a primitive equations model initialized with a short MRG wave in a zonally periodic channel show the emergence of a nearly zonally-symmetric quasi-steady state with a rich structure in depth and latitude that depends on the parameters of the initial wave. A striking feature of the simulation results is the emergence of low vertical mode eastward jets at the equator, notable as an example of local equatorial "superČrotation" and an exception to Hide's Theorem for steady zonally symmetric flow. The structure of these eastward jets is sensitive to the inclusion of the (non-traditional) Coriolis force terms due to the northward component of the planetary rotation vector.
The early time development of the instability is explained by a linear analysis showing that in the short wavelength limit, a MRG wave is unstable to zonally symmetric and effectively barotropic disturbances of meridional scale comparable to, but longer than, the zonal wavelength of the initial wave. The destabilization is thus a mechanism for an inverse energy cascade from shorter to longer horizontal scales.
The long time adjustment and equilibration of the flow is explained in terms of potential vorticity redistribution and secondary instabilities of the unstable linear modes at finite amplitude.