Two ensembles of simulations of the middle atmosphere are compared for early northern hemisphere winter. The two ensembles differ only in the position of the model lid (80 km or 96 km). Two sets of comparisons are carried out using a primitive equation mechanistic model; the first uses Rayleigh friction in the mesosphere to mimic the effect of gravity waves, and the second uses the Hines gravity wave parameterization. Simulations which use Rayleigh friction are found to show little climatic sensitivity to the position of the lid. Simulations which use the Hines parameterization are shown to be very sensitive to the position of the lid, and this sensitivity is seen directly up to six scale heights below the lid. Indirect effects may extend throughout the middle atmosphere. This sensitivity is believed to be due to real circulation changes and not any numerical artifacts resulting from reflections from the model lid. ¿ 1997 American Geophysical Union