A fundamental problem in climate research is that of explaining how the Earth's climate remains stable on very long time scales. Positive feedback mechanisms such as the ice-albedo feedback and the lower tropospheric water vapor/longwave radiative feedback are known to exist which could, in principle, drive the climate system far from its observed mean state even in the absence of any external forcings. There is at present no generally accepted explanation for the stability of the Earth's climate. A number of possible stabilizing mechanisms have been proposed, but all are controversial and their relative importance a matter of debate.
A new stabilizing mechanism has recently been proposed (Bates, 1998, submitted for publication). It is based on an empirically discovered linear relationship between the vertically integrated poleward transport of atmospheric angular momentum across 30 latitude and the difference between the mean heights of the 500hPa surface in the tropics and the extratropics. This empirical relationship, which has been found using observed seasonal mean atmospheric statistics, is incorporated into a simple two-zone atmosphere-ocean model. Assuming that the atmosphere is in a state of dynamic balance on climatic timescales, the torque about the Earth's axis exerted by the tropical surface easterlies balances that exerted by the extratropical surface westerlies, and both are determined by the angular momentum transport across 30 . Regarding the low level relative humidity and air-sea temperature difference as being fixed at their mean observed values, the angular momentum transport then determines the rates of evaporation in the tropical and extratropical zones.The solar radiation reaching the sea surface is prescribed and the net infrared radiative energy loss from the sea surface is parameterized in terms of the SST using results from a radiative model, which incorporates the effects of the positive lower tropospheric water vapour feedback. Using energy equations for the tropical and extratropical ocean basins, an equilibrium climate state for the model is determined and the stability of small perturbations about the equilibrium state is examined. The tendency towards instability resulting from the positive lower tropospheric water vapor/longwave radiative feedback on SST perturbations is overcome by a negative feedback resulting from the evaporative heat losses determined by the atmospheric angular momentum transport.
GCM experiments have been carried out to test the conclusions of the simple model. Surface boundary conditions corresponding to an aquaplanet with prescribed latitudinally varying SSTs have been used. An initial long integration is carried out with the observed zonal mean SSTs and with a surface albedo chosen such that the surface fluxes are globally in equilibrium. A perturbation consisting of a uniform increase of 2K is then added to the SST field and a second long integration is performed. It is found that the tendencies of the surface fluxes are such as to exert a negative feedback on the perturbed SSTs, in the way suggested by the simple model.
The basis of the mechanism as suggested by the simple model and the supporting results from the GCM experiments will be presented.