No abstract

The latitudinal width of atmospheric eddy-driven jets and scales of macroturbulence are examined latitude by latitude over a wide range of rotation rates using a high-resolution idealized GCM. It is found that for each latitude, through all rotation rates, the jet spacing scales with the Rhines scale. These simulations show the presence of a “supercriticality latitude” within the baroclinic zone, where poleward (equatorward) of this latitude, the Rhines scale is larger (smaller) than the Rossby deformation radius. Poleward of this latitude, a classic geostrophic turbulence picture appears with a − spectral slope of inverse cascade from the deformation radius up to the Rhines scale. A shallower slope than the −3 slope of enstrophy cascade is found from the deformation radius down to the viscosity scale as a result of the broad input of baroclinic eddy kinetic energy. At these latitudes, eddy–eddy interactions transfer barotropic eddy kinetic energy from the input scales of baroclinic eddy kinetic energy up to the jet scale and down to smaller scales. For the Earth case, this latitude is outside the baroclinic zone and therefore an inverse cascade does not appear. Equatorward of the supercriticality latitude, the − slope of inverse cascade vanishes, eddy–mean flow interactions play an important role in the balance, and the spectrum follows a −3 slope from the Rhines scale down to smaller scales, similar to what is observed on Earth. Moreover, the length scale of the energy-containing zonal wavenumber is equal to (larger than) the jet scale poleward (equatorward) of the supercriticality latitude.

Future emissions of greenhouse gases into the atmosphere are projected to result in significant circulation changes. One of the most important changes is the widening of the tropical belt, which has great societal impacts. Several mechanisms (changes in surface temperature, eddy phase speed, tropopause height, and static stability) have been proposed to explain this widening. However, the coupling between these mechanisms has precluded elucidating their relative importance. Here, the abrupt quadrupled-CO 2 simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to examine the proposed mechanisms. The different time responses of the different mechanisms allow us to disentangle and evaluate them. As suggested by earlier studies, the Hadley cell edge is found to be linked to changes in subtropical baroclinicity. In particular, its poleward shift is accompanied by an increase in subtropical static stability (i.e., a decrease in temperature lapse rate) with increased CO 2 concentrations. These subtropical changes also affect the eddy momentum flux, which shifts poleward together with the Hadley cell edge. Transient changes in tropopause height, eddy phase speed, and surface temperature, however, were found not to accompany the poleward shift of the Hadley cell edge. The widening of the Hadley cell, together with the increase in moisture content, accounts for most of the expansion of the dry zone. Eddy moisture fluxes, on the other hand, are found to play a minor role in the expansion of the dry zone.