Downstream Self-Destruction of Storm Tracks

The Northern Hemisphere storm tracks have maximum intensity over the Pacific and Atlantic basins; their intensity is reduced over the continents downstream. Here, simulations with an idealized aquaplanet general circulation model are used to demonstrate that even without continents, storm tracks have a self-determined longitudinal length scale. Their length is controlled primarily by the planetary rotation rate and is similar to that of Earth’s storm tracks for Earth’s rotation rate. Downstream, storm tracks self-destruct: the downstream eddy kinetic energy is lower than it would be without the zonal asymmetries that cause localized storm tracks. Likely involved in the downstream self-destruction of storm tracks are the energy fluxes associated with them. The zonal asymmetries that cause localized storm tracks enhance the energy transport through the generation of stationary eddies, and this leads to a reduced baroclinicity that persists far downstream of the eddy kinetic energy maxima

Turbulent convection in an anelastic rotating sphere : a model for the circulation on the giant planets

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2008This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Different from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to significant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility effects. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, contributing to the observed flat meridional emission. We show the transition from weak convection cases with symmetric spiraling columnar modes similar to those found in previous analytic linear theory, to more turbulent cases which exhibit similar, though less regular and solely cyclonic, convection columns which manifest on the surface in the form of waves embedded within the superrotation. We develop a mechanical understanding of this system and scaling laws by studying simpler configurations and the dependence on physical properties such as the rotation period, bottom boundary location and forcing structure. These columnar cyclonic structures propagate eastward, driven by dynamics similar to that of a Rossby wave except that the restoring planetary vorticity gradient is in the opposite direction, due to the spherical geometry in the interior. We further study these interior dynamics using a simplified barotropic annulus model, which shows that the planetary vorticity radial variation causes the eddy angular momentum flux divergence, which drives the superrotating equatorial flow. In addition we study the interaction of the interior dynamics with a stable exterior weather layer, using a quasigeostrophic two layer channel model on a beta plane, where the columnar interior is therefore represented by a negative beta effect. We find that baroclinic instability of even a weak shear can drive strong, stable multiple zonal jets. For this model we find an analytic nonlinear solution, truncated to one growing mode, that exhibits a multiple jet meridional structure, driven by the nonlinear interaction between the eddies. Finally, given the density field from our 3D convection model we derive the high order gravitational spectra of Jupiter, which is a measurable quantity for the upcoming JUNO mission to Jupiter.Funding was provided by the MIT Presidential Fellowship, the Charney Fellowship, WHOI Academics Programs, NSF grants OPP-9910052, OCE-0137023, AST-0708106 and NASA grant NN-6066GC286

The westward drift of Jupiter's polar cyclones explained by a center-of-mass approach

The first orbits around Jupiter of the Juno spacecraft in 2016 revealed a symmetric structure of multiple cyclones that remained stable over the next five years. Trajectories of individual cyclones indicated a consistent westward circumpolar motion around both poles. In this paper, we propose an explanation for this tendency using the concept of beta-drift and a "center-of-mass" approach. We suggest that the motion of these cyclones as a group can be represented by an equivalent sole cyclone, which is continuously pushed by beta-drift poleward and westward, embodying the westward motion of the individual cyclones. We support our hypothesis with 2D model simulations and observational evidence, demonstrating this mechanism for the westward drift. This study joins consistently with previous studies that revealed how aspects of these cyclones result from vorticity-gradient forces, shedding light on the physical nature of Jupiter's polar cyclones.Comment: 10 pages, 4 figures and supplementary informatio

The Role of Stationary Eddies in Shaping Midlatitude Storm Tracks

Transient and stationary eddies shape the extratropical climate through their transport of heat, moisture, and momentum. In the zonal mean, the transports by transient eddies dominate over those by stationary eddies, but this is not necessarily the case locally. In particular, in storm-track entrance and exit regions during winter, stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport. Here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity. Simulations with an idealized aquaplanet GCM show that zonally localized surface heating alone (e.g., ocean heat flux convergence) gives rise to storm tracks, which have a well-defined length scale that is similar to that of Earth's storm tracks. The storm tracks terminate downstream of the surface heating even in the absence of continents, at a distance controlled by the stationary Rossby wavelength scale. Stationary eddies play a dual role: within about half a Rossby wavelength downstream of the heating region, stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track; farther downstream, enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability. Transports both of sensible and latent heat (water vapor) play important roles in determining where storm tracks terminate