A series of idealized atmospheric general circulation model (AGCM) experiments are presented. These experiments examine whether and how atmospheric deep moist convection, in the absence of meridional gradients in external forcing, interacts with the large scale flow, becoming spatially organized and yielding a coherent general circulation. In a control simulation, where the sea-surface temperature (SST) and the incident solar flux are prescribed to be independent of latitude, longitude and time, a well defined Inter-Tropical Convergence Zone (ITCZ) forms at the equator. This result suggests that the interaction between convection and the rotation of the Earth causes convection to prefer to organize at the equator. In this control simulation, the SST is not interactive, and cannot respond to the spatial variations of the heat flux into the ocean that result from the organization of the circulation. In order to examine the circulation that arises without horizontal gradient in 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 temperature gradient develops.
The SST distribution and the tropospheric circulation are very different between 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 subtropics it is westerly, implying a net poleward transport of angular momentum 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 mechansim is tested in another experiment, where the AGCM is truncated to be zonally symmetric. In this case, the model enters a limit cycle where the ITCZ transits between 20°N and 20°S with a 22 month period. The motions associated with this oscillatory behavior accomplish the same poleward transport of angular momentum that the convective events produced in the zonally asymmetric model, but by a drastically different 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 ITCZ off the equator. A last experiment verifies this argument.
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 16 April 1999
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