Mechanism of Fast Atmospheric Energetic Equilibration Following Radiative Forcing by CO2

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Abstract

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=0. 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+L+S=0.

Original languageEnglish (US)
Pages (from-to)2468-2482
Number of pages15
JournalJournal of Advances in Modeling Earth Systems
Volume9
Issue number7
DOIs
StatePublished - Nov 2017

All Science Journal Classification (ASJC) codes

  • Global and Planetary Change
  • Environmental Chemistry
  • Earth and Planetary Sciences(all)

Keywords

  • CO forcing
  • atmospheric circulation
  • fast atmospheric adjustment
  • hydrological cycle

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