Barotropic and super-rotating jet formation in the evolution of very short mixed Rossby-gravity waves

The formation of zonal jets is studied in the context of the evolution of very short mixed Rossby-gravity waves in a zonally periodic equatorial channel. High resolution numerical initial-value simulations 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.

The barotropic component of the long-time quasi-steady state consists of strong westward flow at the equator and short meridional scale extra-equatorial zonal jets alternating in direction with latitude. It is partly explained by a linear theory for the destabilization of the initial wave adapted from that for the destabilization of midlatitude barotropic Rossby waves.

A striking feature not easily explained by linear theory is the emergence of low vertical mode eastward jets at the equator, notable as an example of local equatorial "super-rotation" and an exception to Hide's Theorem for steady zonally symmetric flow. An explanation for the super-rotating jets is sought in terms of finite amplitude effects and energy cascades.

The effect of the non-traditional Coriolis force terms, due to the northward component of the planetary rotation vector, is also considered, and is observed to lead to a breaking of the vertical mirror symmetry of the final state.