2002 Annual

Wednesday, 16 January 2002: 5:00 PM
On the mechanism of the enhanced greenhouse effect: a radiative and dynamical perspective
J. Ray Bates, University of Copenhagen, Copenhagen, Denmark
A view of the enhanced greenhouse effect resulting from anthropogenic increase in the well-mixed greenhouse gases is proposed in which emphasis is placed on the non-radiative forms of heat transport and in which dynamical interaction between the tropics and extratropics plays a central role. The proposed perspective is based on an analysis of a simple two-zone climate system model that uses radiative and dynamical parameterizations derived from observation. The stability of the model's sea surface temperature (SST)is maintained by a dynamically modulated balance between the variability in thickness of the lower tropospheric water vapor infrared absorbers, which is destabilizing, and the rate of short-circuiting of these absorbers by latent heat release consequent on evaporation, which is stabilizing. The rate of evaporation is a function of the surface winds, which are related to the rate of atmospheric angular momentum (AM) transport between the tropics and extratropics. The model shows the sensitivity of the SST to radiative forcing by enhanced anthropogenic greenhouse gases to be related in a specific way to the degree of stability of the system to free perturbations about the equilibrium climate. For a uniform forcing, an extratropical amplification of the warming is found. The amplification is dynamically induced, being related to the fact that in the tropics the Clausius-Clapeyron and wind factors in the evaporative stabilizer act together to oppose the forcing whereas in the extratropics they are of opposite sign. For a given global average forcing, the global average temperature response is greater if the forcing is weighted towards the extratropics. With the best estimates of the parameters determining the AM/evaporative feedback, and with values of the parameters determining the water vapor feedback that lie within current estimates, the model gives a global average warming for a CO2 doubling that lies within the range of that given by GCMs.

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