4.4 Mechanism of Fast Atmospheric Energetic Equilibration Following Radiative Forcing by CO2

Monday, 9 July 2018: 4:15 PM
Regency E/F (Hyatt Regency Vancouver)
Tra Dinh, University of Auckland, Auckland, New Zealand; and S. Fueglistaler

In energetic equilibrium, the atmosphere’s net radiative divergence (R) is balanced by sensible (S) and latent (L) heat fluxes, i.e. R=S+L. Radiative forcing from increasing CO2 reduces R, and the surface warming following an increase in CO2 is largely due to the reduction in atmospheric energy demand in S and L, with only a smaller surface radiative budget perturbation. With an idealized General Circulation Model, we show that the fast atmospheric adjustment at fixed surface temperature produces the required decrease in the sum of S and L through changes in the near-surface temperature and specific humidity. In layers near the surface, the reduced radiative cooling forces a temperature increase that leads to a negative Planck radiative feedback and, because of the reduced surface-atmosphere temperature difference, also to a reduction in sensible heat flux. In the free troposphere, the reduced radiative cooling leads to a weakening of the tropospheric circulation. Consequently, there is a decrease in the water flux exported from the layers near the surface, and as such in precipitation. By mass conservation, the near-surface specific humidity increases and surface evaporation decreases until it balances the reduced export flux. Other processes can amplify or dampen the responses in S and L and change the partitioning between these two fluxes, but by themselves do not ensure R=S+L.
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