Reversal and amplification of zonal flows by boundary enforced thermal wind

Abstract

Zonal flows in rapidly-rotating celestial objects such as the Sun, gas or ice giants form in a variety of surface patterns and amplitudes. Whereas the differential rotation on the Sun, Jupiter and Saturn features a super-rotating equatorial region, the ice giants, Neptune and Uranus harbour an equatorial jet slower than the planetary rotation. Global numerical models covering the optically thick, deep-reaching and rapidly rotating convective envelopes of gas giants reproduce successfully the prograde jet at the equator. In such models, convective columns shaped by the dominant Coriolis force typically exhibit a consistent prograde tilt. Hence angular momentum is pumped away from the rotation axis via Reynolds stresses. Those models are found to be strongly geostrophic, hence a modulation of the zonal flow structure along the axis of rotation, e.g. introduced by persistent latitudinal temperature gradients, seems of minor importance. Within our study we stimulate these thermal gradients and the resulting ageostrophic flows by applying an axisymmetric and equatorially symmetric outer boundary heat flux anomaly (Y 20 ) with variable amplitude and sign. Such a forcing pattern mimics the thermal effect of intense solar or stellar irradiation. Our results suggest that the ageostrophic flows are linearly amplified with the forcing amplitude q leading to a more pronounced dimple of the equatorial jet (alike Jupiter). The geostrophic flow contributions, however, are suppressed for weak q , but inverted and re-amplified once q exceeds a critical value. The inverse geostrophic differential rotation is consistently maintained by now also inversely tilted columns and reminiscent of zonal flow profiles observed for the ice giants. Analysis of the main force balance and parameter studies further foster these results.

Asymptotic scaling relations for rotating spherical convection with strong zonal flows

“…Dietrich et al. 2017), and we also find this to be the case in our simulations. While this advection term may be important for transporting angular momentum (McIntyre 1998; Miesch & Hindman 2011), it cannot play a role in setting the amplitude of the time-averaged (steady) zonal flow.…”

“…Averaging (3.11) along the axial direction and in time leaves only the two advective terms and the viscous term. For a careful derivation of the balance between the averaged advection term and the viscous term in spherical coordinates, see Dietrich, Gastine & Wicht (2017). Previous studies have found that the mean-mean term is small (e.g.…”

High‐Latitude Westward Jets in the Earth's Outer Core Due to Small‐Scale Convection

“…Thus, the constant of integration is specified by requiring the z-average of U tw to be zero. Numerical simulations by Dietrich et al (2017) demonstrated that this is indeed the case (see their Figure 4).…”

Generation of shear flows and vortices in rotating anelastic convection

“…These studies have employed a variety of techniques including Direct Numerical Simulation (DNS) [9,10], turbulence closure models [11,12] or Direct Statistical Simulation (DSS) [13][14][15]. Investigations have been performed in both spherical geometry [9,[16][17][18][19][20][21][22][23][24] and in local Cartesian models [25][26][27][28][29][30][31]. There is also a class of intermediate local models that captures some of the effects of the vortex stretching engendered by the spherical geometry by use of an annulus geometry [32][33][34].…”