The relationship between tropical precipitation biases and the Saharan heat low bias in CMIP5 models
This study focuses on the relationship in global climate models between three features: the Saharan Heat Low (SHL), Sahel precipitation, and the Atlantic Intertropical Convergence Zone (ITCZ). Previous work showed that both coupled (CMIP) and uncoupled (AMIP) ocean/atmosphere models that place the SHL farther to the north are associated with increased precipitation across the Sahel. Further, the northward SHL placement is also associated with a northward shift in the Atlantic ITCZ in coupled CMIP models, but an eastward shift in uncoupled AMIP models. We perform three experiments with the Community Earth System Model to better understand relationships between these features. We find that when a northward-shifted Atlantic ITCZ is locally forced, there is no coherent response in the SHL and Sahel precipitation. However, when a northward-shifted Atlantic ITCZ is forced by altering the cross equatorial energy transport, the SHL shifts northward and Sahel precipitation increases, consistent with model biases. Finally, when the SHL strength is forced directly, there is a weak but robust increase in Sahel precipitation and a northward shift in the Atlantic ITCZ. The results of these experiments emphasize the important role of global scale energy biases on the simulation of West African climate, and show a possible feedback from West African climate onto the Atlantic ITCZ.
KeywordsWest Africa monsoon Atlantic ITCZ Saharan heat low Climate modeling
This research was supported by NSF Grants ATM-0849689 and 1463970 and the University of Wisconsin Climate, People, and Environment Program. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modeling groups for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. Many thanks to Erin Thomas for helping with altering the CESM code to perform the fixed tropical SST runs and to Larissa Back for helpful discussions and encouragement concerning ITCZ dynamics. We thank an anonymous reviewer whose comments helped clarify and strengthen the paper. We also acknowledge and thank the high-performance computing support from Yellowstone provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation (Computational and Information Systems Laboratory 2012). EUMETSAT and the Earth Observation Portal (EOP) are acknowledged for the provision of the license under which the SEVIRI image was obtained.
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