Ice front blocking of ocean heat transport to an Antarctic ice shelf.

Mass loss from the Antarctic Ice Sheet to the ocean has increased in recent decades, largely because the thinning of its floating ice shelves has allowed the outflow of grounded ice to accelerate1,2. Enhanced basal melting of the ice shelves is thought to be the ultimate driver of change2,3, motivating a recent focus on the processes that control ocean heat transport onto and across the seabed of the Antarctic continental shelf towards the ice4-6. However, the shoreward heat flux typically far exceeds that required to match observed melt rates2,7,8, suggesting that other critical controls exist. Here we show that the depth-independent (barotropic) component of the heat flow towards an ice shelf is blocked by the marked step shape of the ice front, and that only the depth-varying (baroclinic) component, which is typically much smaller, can enter the sub-ice cavity. Our results arise from direct observations of the Getz Ice Shelf system and laboratory experiments on a rotating platform. A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf9. Representing the step topography of the ice front accurately in models is thus important for simulating ocean heat fluxes and induced melt rates.

Statistical mechanics of the shallow water system.

We extend the formalism of statistical mechanics developed for the two-dimensional (2D) Euler equation to the case of shallow water system. Relaxation equations towards the maximum entropy state are proposed, which provide a parametrization of subgrid-scale eddies in 2D compressible turbulence.

Effect of a vertical magnetic field on turbulent rayleigh-Benard convection

The effect of a vertical uniform magnetic field on Rayleigh-Benard convection is investigated experimentally. We confirm that the threshold of convection is in agreement with linear stability theory up to a Chandrasekhar number Q approximately 4x10(6), higher than in previous experiments. We characterize two convective regimes influenced by MHD effects. In the first one, the Nusselt number Nu proportional to the Rayleigh number Ra, which can be interpreted as a condition of marginal stability for the thermal boundary layer. For higher Ra, a second regime Nu approximately Ra0.43 is obtained.

Evidence for supersonic turbulence in the upper atmosphere of Jupiter.

Spectra of the hydrogen Lyman alpha (Ly-alpha) emission line profiles of the jovian dayglow, obtained by the Goddard High Resolution Spectrograph on the Hubble Space Telescope, appear complex and variable on time scales of a few minutes. Dramatic changes occur in the Ly-alpha bulge region at low latitudes, where the line profiles exhibit structures that correspond to supersonic velocities of the order of several to tens of kilometers per second. This behavior, unexpected in a planetary atmosphere, is evidence for the particularly stormy jovian upper atmosphere, not unlike a star's atmosphere.