The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.
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The authors would like to thank Aurore Voldoire, Sophie Tytecas and Antoinette Alias for their help on the CNRM-CM5 model. We also would like to thank Florent Brient, Laurent Fairhead and Musat Ionela for their help on the IPSL-CM5A model. Thanks are extended to Sandrine Bony, Laurent Li, Hervé Douville, Jean-Luc Redelsperger for helpful discussions throughout the course of this work. Thanks are also due to the editor and reviewers for their helpful comments.
This paper is a contribution to the special issue on the IPSL and CNRM global climate and earth system models, both developed in France and contributing to the 5th coupled model intercomparison project.
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Oueslati, B., Bellon, G. Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models. Clim Dyn 40, 2345–2358 (2013). https://doi.org/10.1007/s00382-012-1344-x
- Tropical precipitation regimes
- Double ITCZ
- Atmospheric dynamics and feedbacks