The present and future of the West African monsoon: a process-oriented assessment of CMIP5 simulations along the AMMA transect

Abstract:

The present comprehensive assessment of the West African monsoon in the models of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) indicates little evolution since CMIP3 in terms of both biases in present-day climate and climate projections.

The outlook for Sahel precipitation in twenty-first-century coupled simulations remains uncertain, as models still disagree on the sign of the trends. This contrasts with the relatively robust spring and summer warming of the Sahel, larger by 10 to 50% compared to the global warming.

CMIP5 coupled models underestimate the monsoon decadal variability, although simulations with imposed sea surface temperatures (SSTs) succeed in capturing the recent partial recovery of monsoon rainfall. Coupled models still display major SST biases in the equatorial Atlantic, inducing a systematic southward shift of the monsoon. Because of these strong biases, further evaluation of the African monsoon is performed in SST-imposed CMIP5 simulations along the AMMA meridional transect, across a range of timescales ranging from seasonal cycle, intraseasonal fluctuations and diurnal cycle.

Our emphasis is on the comprehensive set of observational data now available to evaluate in depth the monsoon representation across those scales and on the promising usefulness of high-frequency outputs provided by some CMIP5 models at selected sites along the AMMA transect. Most of CMIP5 models capture many features of the African monsoon with varying degrees of accuracy. In particular, the top-of-atmosphere and surface energy balances, in relation with the cloud cover, and the intermittence and diurnal cycle of precipitation, demand further work from modelling centres to achieve a reasonable realism.