The response of the general circulation to climate change as a vertical rescaling

It has been noted by a number of authors that many features of the atmospheric circulation tend to exhibit an upwards shift in simulations of greenhouse-gas induced warming. Motivated by these findings, we discuss a scaling to the vertical co-ordinate which allows for a vertical shift in the circulation while preserving the governing equations. In a dry atmosphere the scaling involves multiplying the pressure co-ordinate by a constant factor, and dividing the pressure velocity by the same factor. In a moist atmosphere, a solution can still be found, however, an adjustment to the temperature field is also required. Using an idealized aqua-planet general circulation model we investigate the ability of the scaling to predict the model response to a uniform increase in sea-surface temperature. We also examine simulations from the IPCC archive to determine how much of the circulation change due to greenhouse warming can be encapsulated by our simple vertical scaling. Despite the complexity of the modeled changes, a number of features are captured. The spatial structure of lapse rate changes and relative humidity changes are well captured in the middle and upper troposphere, as is the weakening of the overturning circulation that occurs in greenhouse simulations. Deviations from the scaling prediction are due primarily to the boundary conditions, which do not conform to the scaling, especially in the case of the zonal winds. Complications also arise because latitudinal shifts in the circulation occur that cannot be predicted by the scaling. We discuss the implications of these findings for our understanding of the changes in the atmospheric general circulation under greenhouse-gas warming.