A spontaneously generated tropical atmospheric general circulation

A series of idealized atmospheric general circulation model (AGCM) experime nts are presented. These experiments examine whether and how atmospheric de ep moist convection, in the absence of meridional gradients in external for cing, interacts with the targe-scale flow, becoming spatially organized and yielding a coherent general circulation. In a control simulation, where th e SST and the incident solar flux are prescribed to be independent of latit ude, longitude, and time, a well-defined intertropical convergence zone (IT CZ) forms. This result suggests that the interaction between convection and the rotation of the earth causes convection and a corresponding general ci rculation to organize. The actual latitude that the ITCZ forms at, however, may be parameterization dependent. In this control simulation, the SST is not interactive and cannot respond to the spatial variations of the heat fl ux into the ocean that result from the organization of the circulation. In order to examine the circulation that arises without horizontal gradients i n the forcing in a physically consistent, energetically closed, model, the AGCM is coupled to a mixed layer ocean model. In this case, the ITCZ still forms at the equator even though a "reversed" pole-to-equator surface tempe rature gradient develops. The SST distribution and the tropospheric circulation are very different be tween these two experiments, but the surface zonal mean zonal wind is quite similar. In the Tropics, the surface zonal wind is easterly and in the sub tropics it is westerly, implying a net poleward transport of angular moment um in both simulations. Large-scale zonally asymmetric convective "events" apparently produce this momentum transport by the barotropic tilted trough mechanism. The role of three-dimensional zonally asymmetric motions in the momentum transport mechanism is tested in another experiment, where the AGC M is truncated to be zonally symmetric. In this case, the model enters a li mit cycle where the ITCZ transits between 20 degrees N and 20 degrees S wit h a 22-month period. The motions associated with this oscillatory behavior accomplish the same poleward transport of angular momentum that the convect ive events produced in the zonally asymmetric model, but by a drastically d ifferent mechanism, suggesting that there may be some undiscovered general principle governing the momentum transport. Finally, a simple argument is used to estimate the minimum modification to the uniform specified SST necessary to displace the ITCZ off the equator. A Last experiment verifies this argument.