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Title : The influence of cloud-radiative effects on the location of the inter-tropical convergence zone
Name of the speaker : Aiko Voigt
Affiliation : Max Planck institute
Laboratory organizer : LMD
Date and time : 21-01-2013 14h30
Location : LMD - UPMC - 4 place Jussieu - Paris 5e - Salle de réunion (T45-55, 3e étage)
Summary :

The intertropical convergence zone (ITCZ) is a region of abundant rainfall and clouds and a central element of Earth's climate system. This talk reports on recent simulations with the atmosphere general circulation model ECHAM6 that study the influence of cloud-radiative effects (CRE) on the location of the ITCZ.

Surprisingly, and despite a long history in ITCZ research, this issue has received relatively little  attention so far. The talk first presents simulations that investigate meridional ITCZ shifts in response to hemispherically-asymmetric surface albedo perturbations.

The ITCZ experiences a temperature-driven shift into the dark-surface hemisphere. It is found that the ITCZ shift is amplied by clouds through their effect on the outgoing longwave radiation. The associated feedback explains, at least partly, the sensitivity of the magnitude of the ITCZ shift to the convection scheme and the surface heat  capacity. The second part of the talk demonstrates that clouds in ECHAM6 work such as to make the climate symmetric with respect to the equator. To this purpose, ECHAM6 is run in aquaplanet mode with perfectly symmetric boundary conditions. For enabled cloud-radiative effects the model produces a symmetric climate with a single ITCZ at the equator. However, when clouds are made transparent to radiation the climate becomes highly asymmetric and the ITCZ moves into one of the hemispheres. This suggests that cloud-radaitive effects overwhelm the wind-evaporation feedback in these ECHAM6 aquaplanet simulations. The latter is often involved to explain why the East Pacific ITCZ is located north of the equator.

Contact :

MP Lefebvre 0144272799