Abstract

Zonal jets are strong and persistent east-west flows that arise spontaneously in planetary atmospheres and oceans. They are ubiquitous, with key examples including mid-latitude jets in the troposphere, multiple jets in the Antarctic Circumpolar Current and flows on gaseous giant planets such as Jupiter and Saturn. Turbulent flows on a beta-plane lead to the spontaneous formation and equilibration of persistent zonal jets. However, the equilibrated jets are not steady and the nature of the time variability in the equilibrated phase is of interest both because of its relevance to the behaviour of naturally occurring jet systems and for the insights it provides into the dynamical mechanisms operating in these systems. I will discuss aspects of zonal jet variability using a framework of barotropic beta-plane models. Using a generalization of the quasilinear approximation, in which nonlinear interactions between eddies are neglected, I will explore using direct numerical simulations (DNS) a new type of variability in which jets systematically migrate either north or south, spontaneously breaking symmetry. Furthermore, I will discuss additional insights from direct statistical simulations (DSS) in which the flow statistics, truncated at second order in equal-time cumulants, are solved for directly.