Evolution of Jupiter‐Style Critical Latitudes: Initial Laboratory Altimetry Results

Abstract

AbstractThis is a laboratory zonal‐jet study using a rotating water tank. The bottom topography has a tent‐shaped radial cross section designed to generate two critical latitudes, that is, two positions whereβe, the radial gradient of the potential vorticity (PV), changes sign. This configuration is motivated by observations indicating that Jupiter and Saturn have not only multiple zonal jets, but multiple stable critical latitudes. It is known that “supersonic” critical latitudes (with respect to Rossby waves) are stable, whereas “subsonic” critical latitudes are posited to be unstable. Because Rossby waves are unidirectional, “supersonic” critical latitudes come in two varieties: Rossby Mach numberMR > 1 and < 0, where the latter holds when the waves are directed downstream. Experiments focus on: (a) how do zonal jets emerge from localized forcing in a system with alternating PV gradients? and (b) what differences are there between the evolution of various types of critical latitudes? The water is forced by mass injection along one radius. Laboratory altimetry provides accurate, unobtrusive records of the circulations that reveal the emergence of counter‐propagatingβ‐plumes (Rossby‐wave envelopes), which expand into tank‐encircling zonal jets. The tank's negativeβezone (annulus) is characterized byMR∼ 1. The weaker critical latitude adjusts its position by about 4% of the tank radius and maintainsMR∼ 1, similar to the state surmised for Jupiter and Saturn, whereas the stronger critical latitude vacillates while maintaining |MR| ≪ 1 and may be relevant to steep oceanic seamounts.