The double-ITCZ syndrome in coupled general circulation models: the role of large-scale vertical circulation regimes

The double-intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCM) is examined in the multi-model Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. Aim of this study is to quantify the DI error on precipitation in the tropical Pacific, with a specific focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analysed using a regime sorting approach for the vertical circulation regimes. This methodology allows to partition the precipitation into deep and shallow convective components. Also, it makes possible to decouple the error on the magnitude of precipitation associated with individual convective events, from the error affecting the frequency of occurrence of convective regimes. It is shown that, despite the existing large intra-model differences, CGCMs can be ultimately grouped into a few homegenous clusters, each featuring a well defined rainfall-vertical circulation pattern in the DI region. A critical parameter controlling the strength of the DI is identified in the SST threshold leading to the onset of deep convection (THR), combined with the mean SST in the south-eastern Pacific. The models featuring a THR which is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern ITCZ.