The competitive impacts of global SST warming and CO2 increase on Sahelian rainfall: results from CMIP5 idealized simulations

Abstract : the West african Monsoon (WaM) is affected by large climate variability at different timescales, from interannual to multidecadal, with strong environmental and socioeconomic impacts associated to climate-related rainfall variability, especially in sahelian countries. state-of- the-art coupled climate models still show poor ability in correctly simulating the WaM historical variability and also a large spread is observed in future climate projections. in this work, climate simulations from a set of 10 CMip5 atmospheric global climate models are used to study the July-to-september WaM variability in the period 1979- 2008. the individual roles of global sst warming and CO2 concentration increase are investigated through idealized experiments simulating a 4K warmer sst and a 4x CO2 concentration, respectively. results show a dry response in sahel to 4K sst warming, with dryer conditions over western sahel. On the contrary, wet conditions are observed when CO2 is 4x increased, with the strongest response over central-eastern sahel. the precipitation changes are associated to coherent modifications in the regional atmospheric circulation: dry (wet) conditions are associated with reduced (increased) convergence in the lower troposphere, a southward (northward) shift of the african easterly Jet (aeJ), a weaker (stronger) tropical easterly Jet (teJ). the analysis of the changes in the regional and global connections of WaM dynamics reveals that in the 4K experiment the observed relationships with the global sst and the regional dynamics patterns are weaker, while in the 4xCO2 simulation the connections are more robust. Specifically, a strong coupling between WAM precipitation and Equatorial Pacific and atlantic sst is observed, along with a reinforcement of the regional connections with the saharan heat low, the aeJ and the teJ. the above results suggest a competitive action of global sst warming and CO2 increase on the WaM climate variability, with opposite effects on precipitation. the global sst warming affects sahelian precipitation by weakening the global tropical convection, while the CO2 increase results in a strengthening of the regional dynamics features associated to wet conditions in the sahel. it is argued that the large spread in CMip5 future rainfall projections may be related to the weight given to sst warming and direct CO2 effect by individual models.
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Submitted on : Thursday, August 6, 2015 - 3:22:53 PM
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Marco Gaetani, Cyrille Flamant, Frédéric Hourdin, Sophie Bastin, P. Braconnot, et al.. The competitive impacts of global SST warming and CO2 increase on Sahelian rainfall: results from CMIP5 idealized simulations. Our Common Future Under Climate Change, International Scientific Coference, Jul 2015, Paris, France. pp.P-3330-29, 2015, ⟨http://www.commonfuture-paris2015.org⟩. ⟨insu-01183130⟩

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