Multiple Zonal Baroclinic Jets in a Laboratory Annulus

We observe multiple baroclinic zonal jets in the laboratory using a differentially heated, rotating annulus of fluid. A large-scale buoyancy gradient is imposed by differential heating across the annulus gap, and a topographic β-effect is created by a gradient in fluid depth with radius. At high rotation rates and low thermal forcing, the apparatus reaches regimes of low thermal Rossby number Ro_T where the baroclinic Rossby radius of deformation is much smaller than the tank scale and the flow becomes fully turbulent. In regimes where the baroclinic radius of deformation is of the same order as the Rhines length, the Rhines (1975) mechanism of zonation sets in, resulting in a series of meandering zonal jets. By only forcing at large (basin) scales, the baroclinic turbulence is allowed to evolve freely, and thereby set its own scales of motion. Because of this, the fully baroclinic situation observed is closely related to the type of circumpolar turbulence that would occur in a gas giant atmosphere or in certain parts of the Antarctic Circumpolar Current, and is thus a natural extension of traditional stochastic forcing scenarios for testing the barotropic Rhines mechanism. We further observe a proportionality of jet width to Rhines scale that is broadly consistent with similar laboratory and numerical studies. The long term (>1800 rotation periods) evolution of these structures is observed, and transients, possibly linked to the slow equilibration of the system, are described. 2D wavenumber spectra are calculated and show the classic Vallis and Maltrud (1993) "dumbbell" shape. For experiments that are similar in regime to Earth's atmosphere, we observe a transition between a low wavenumber range with a k^-3 spectral slope (indicative of a forward enstrophy cascade) to a k^-5/3 range at smaller scales with finite Rossby number: a spectral structure that is reminiscent of the enigmatic Nastrom-Gage (1985) spectrum.