Sub-Saharan Northern African climate at the end of the twenty-first century: forcing factors and climate change processes

Abstract

A regional climate model, the Weather Research and Forecasting (WRF) Model, is forced with increased atmospheric CO 2 and anomalous SSTs and lateral boundary conditions derived from nine coupled atmosphere-ocean general circulation models to produce an ensemble set of nine future climate simulations for northern Africa at the end of the twenty-first century. A well validated control simulation, agreement among ensemble members, and a physical understanding of the future climate change enhance confidence in the predictions. The regional model ensembles produce consistent precipitation projections over much of northern tropical Africa. A moisture budget analysis is used to identify the circulation changes that support future precipitation anomalies. The projected midsummer drought over the Guinean Coast region is related partly to weakened monsoon flow. Since the rainfall maximum demonstrates a southward bias in the control simulation in July-August, this may be indicative of future summer drying over the Sahel. Wetter conditions in late summer over the Sahel are associated with enhanced moisture transport by the West African westerly jet, a strengthening of the jet itself, and moisture transport from the Mediterranean. Severe drought in East Africa during August and September is accompanied by a weakened Indian monsoon and Somali jet. Simulations with projected and idealized SST forcing suggest that overall SST warming in part supports this regional model ensemble agreement, although changes in SST gradients are important over West Africa in spring and fall. Simulations which isolate the role of individual climate forcings suggest that the spatial distribution of the rainfall predictions is controlled by the anomalous SST and lateral boundary conditions, while CO 2 forcing within the regional model domain plays an important secondary role and generally produces wetter conditions.

Using an AGCM to Diagnose Historical Effective Radiative Forcing and Mechanisms of Recent Decadal Climate Change

“…Similar experimental frameworks (i.e., using AGCMs forced with SST changes and various forcing agent combinations) have been used to investigate mechanisms of atmospheric circulation trends (Deser and Phillips 2009;King et al 2010), regional African climate change (e.g., Patricola and Cook 2011;Skinner et al 2012), changes in the diurnal temperature range (Caminade and Terray 2006), perturbations to the hydrological cycle (e.g., Bosilovich et al 2005;Bichet et al 2011;Allan et al 2013) variability of the Indian monsoon rainfall (Kucharski et al 2009), and decreasing land wind speeds (e.g., Bichet et al 2012). Most of these studies, in various ways, point to the importance of including time-varying forcing agents-not just SST changes-in simulating certain climate trends and hence motivates further examination of separating adjustments from those aspects of climate change that are mediated through large-scale changes in SST.…”

Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

“…Greenhouse gas (GHG) increases affect Sahel rainfall not only via the warming of global SST (slow response), but also via its direct effect on the temperature and energy fluxes of the land-atmosphere system (fast response; Patricola and Cook 2011;Giannini 2010). Using idealized CMIP5 simulations, Biasutti (2013) found that throughout the rainy season the slow response leads to less rain across the Sahel while the fast response forces wet anomalies that span the entire Sahel, although they are stronger in the east.…”

Warm Season Response over North America to a Shutdown of the Atlantic Meridional Overturning Circulation and CO2 Increases

“…This approach is not used here because it is important to have an accurate present day simulation, and using boundary conditions directly from an AOGCM has been shown to degrade the results in previous applications (e.g., Cook et al 2008;Cook 2010, 2011a). Additionally, AOGCM output for all of the needed fields is not available at the 6-hourly interval.…”

“…The vertical coordinate in the WRF model is a terrain-following hydrostatic-pressure vertical coordinate that varies from 1 at the surface to 0 at the top of the model. Our previous studies indicate that at this spatial resolution the model is able to realistically simulate the warm season climate over northern Africa (e.g., Cook 2010, 2011a) and North America Cook 2011b, manuscript submitted to Climate Dyn., 2012). The physical parameterizations chosen include the Mellor-Yamada-Janjic planetary boundary layer (Mellor and Yamada 1982;Janjic 1990Janjic , 1996Janjic , 2002, the Monin-Obukhov-Janjic surface layer (Monin and Obukhov 1954;Janjic 1994Janjic , 1996Janjic , 2002, new Kain-Fritsch cumulus convection (Kain andFritsch 1990, 1993;Kain 2004), Purdue Lin microphysics (Lin et al 1983;Rutledge and Hobbs 1984;Tao et al 1989;Chen and Sun 2002), Rapid Radiative Transfer Model (RRTM) longwave radiation (Mlawer et al 1997), Dudhia shortwave radiation (Dudhia 1989), and the unified Noah land surface model (LSM) (Chen and Dudhia 2001).…”