Double jets and storm tracks in a two-level primitive equation model of the atmosphere

Abstract

Although there is only a single zonally averaged tropospheric jet in observations of the Northern Hemisphere, two separated tropospheric jets are observed in the Southern Hemisphere winter. An attem pt is made in this thesis to understand, using a two-level primitive equation model, conditions under which the general circulation in the troposphere shows one or more zonally averaged jets. Comparisons sire made with previous results using this model and with more idealized quasi-geostrophic models. In a dry two-level primitive equation model, by changing the temperature difference from the equator to the pole and from the top level to the bottom level, the subtropical and polar front jets can be separated in zonally-averaged zonal wind at the upper level. Diagnostic calculations show that the subtropical jetstream is maintained by a balance between Coriolis torques, convergence of mean momentum fluxes, and divergence of eddy momentum fluxes, while for the polar front jet the balance is primarily between Coriolis torques and eddy momentum flux convergence. The transition between one jet and two jets is quite smooth in the normal regime. As horizontal forcing decreases or static stability increases, with other parameters fixed, the polar front jet weakens, and the subtropical jet begins to emerge at a more realistic latitude and eventually becomes the dominant one. Equatorial superrotation occurs suddenly either as the planetary radius is increased beyond a critical value, while keeping temperature gradient unchanged; or mid-latitude baroclinicity is substantially reduced. Instead of poleward eddy momentum transport as in the observed circulation, this regime has equatorward eddy momentum flux. Storm-track structures have rather different distributions about jet axes in the normal regime as compared to the superrotation regime. In a normal regime, a single storm-track is located near the peak of the polar front jet, as in observations of the Southern Hemisphere (Trenberth 1991). In simulations of the superrotation regime, storm tracks were observed to occur on the poleward flank of the equatorward jet, and on the equatorward flank of the poleward jet. These results are qualitatively consistent with those of a quasi-geostrophic model (Panetta 1993).