Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models
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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.
KeywordsTropical precipitation regimes Double ITCZ Atmospheric dynamics and feedbacks
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.
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