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A dynamical stabilizer in the climate system: a summary of progress to date and new results on the influence of the Earth's rotation
J. Ray Bates, University of Copenhagen, Copenhagen, Denmark; and V. A. Alexeev
A dynamical negative feedback mechanism of sufficient strength to maintain the stability of the observed annual mean climate against the destabilizing influence of the positive water vapor/infrared (WVIR) feedback on SST perturbations has recently been proposed (Bates, 1999). The mechanism, based on atmospheric angular momentum transport and its link to the surface winds and evaporation, was found from an analysis of a simple two-zone atmosphere-ocean model. Supporting evidence for the validity of the mechanism has been provided by the GCM experiments of Alexeev and Bates (1999). These experiments have consisted of two sets of multiyear integrations with aquaplanet boundary conditions, the first with the climate in equilibrium (surface fluxes integrating globally to zero), and the second with a uniform perturbation of 2K added to the equilibrium SST field. By examining the changes in the surface fluxes in the tropics and extratropics that develop on going from the equilibrium to the non-equilibrium state, the mechanisms by which the model attempts to return to equilibrium have been pinpointed. It has been found, as predicted by the simple model, that the angular momentum flux increases in response to the SST perturbation. A negative evaporative feedback then occurs, with wind and humidity factors contributing in approximately equal measure. The GCM experiments also confirm the existence of a positive WVIR feedback in both zones, and show that the dynamical feedback associated associated with the angular momentum transport is of sufficient strength to counteract it. Further evidence for the validity of the dynamical stabilizing mechanism has been found from an observational study (Alexeev and Bates, 2000) in which the underlying parameterizations of the simple model have been examined using 40 years of the NCEP/NCAR Reanalysis.
GCM experiments aimed at studying the sensitivity of the dynamical stabilizing mechanism to the Earth's rate of rotation have recently been carried out. The motivation is to see whether the fast rotation rate of the Earth is an important factor in maintaining the stability of its climate and preventing a Venus-like runaway greenhouse effect from occurring. The experiments indicate that the dynamical stabilizing mechanism would become so weak at low rotation rates that it would no longer be capable of countering the WVIR feedback, suggesting that a runaway greenhouse effect could occur under these circumstances.
References:
Alexeev, V.A. and J.R. Bates, 1999: GCM experiments to test a proposed dynamical stabilizing mechanism in the climate system. Tellus, 51A, 630-651.
Alexeev, V.A. and J.R. Bates, 2000: An observational study of the basic parameterizations underlying a proposed dynamical stabilizing mechanism in the climate system. (Submitted for publication).
Bates, J.R., 1999: A dynamical stabilizer in the climate system: a mechanism suggested by a simple model. Tellus, 51A, 349-372.
Session 1, Natural Climate Variability
Monday, 15 January 2001, 9:00 AM-12:00 PM
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