Response of idealized Walker circulations and zonal surface temperature gradients to changes in longwave radiation

We examine variations in zonally-asymmetric tropical overturning circulations (Walker circulations) and zonal surface temperature gradients over a wide range of climates simulated with an idealized atmospheric general circulation model. The asymmetry in the tropical climate is generated by an imposed ocean energy flux, which does not vary with climate. The range of climates is simulated by modifying the optical thickness of an idealized longwave absorber (representing greenhouse gases).

As in comprehensive climate simulations, the Walker circulation weakens as climate warms in the idealized simulations. The change in the zonally-asymmetric part of the tropical upward mass flux is larger than in the zonally-symmetric net upward mass flux (forming the ascending branch of the Hadley cell). The wide range of climates allows a systematic test of thermodynamic and energetic arguments that have been proposed to account for these changes in the tropical circulation.

The zonal surface temperature gradient in low latitudes generally decreases with warming in the idealized GCM simulations. We show that a scaling relationship based on a two-term balance in the surface energy budget accounts for the changes in the zonally-asymmetric component of the GCM-simulated surface temperature gradients.