11.2.3.2 Mean Precipitation
Figure 11.2 and Table 11.1 illustrate some of the robust aspects of the precipitation response over Africa in the MMD-A1B projections. The fractional changes in annual mean precipitation in each of the 21 models are provided in Supplementary Material Figure S11.13. With respect to the most robust features (drying in the Mediterranean and much of southern Africa, and increases in rainfall in East Africa), there is qualitative agreement with the results in Hulme et al. (2001) and Ruosteenoja et al. (2003), which summarise results from the models available at the time of the TAR.
The large-scale picture is one of drying in much of the subtropics and an increase (or little change) in precipitation in the tropics, increasing the rainfall gradients. This is a plausible hydrological response to a warmer atmosphere, a consequence of the increase in water vapour and the resulting increase in vapour transport in the atmosphere from regions of moisture divergence to regions of moisture convergence (see Chapter 9 and Section 11.2.1).
The drying along Africa’s Mediterranean coast is a component of a larger-scale drying pattern surrounding the Mediterranean and is discussed further in the section on Europe (Section 11.3). A 20% drying in the annual mean is typical along the African Mediterranean coast in A1B by the end of the 21st century. Drying is seen throughout the year and is generated by nearly every MMD model. The drying signal in this composite extends into the northern Sahara, and down the West Coast as far as 15°N. The processes involved include increased moisture divergence and a systematic poleward shift of the storm tracks affecting the winter rains, with positive feedback from decreasing soil moisture in summer (see Section 11.3).
In southern Africa, a similar set of processes produces drying that is especially robust in the extreme southwest in winter, a manifestation of a much broader-scale poleward shift in the circulation across the South Atlantic and Indian Oceans. However, the drying is subject to the caveat that strong orographic forcing may result in locally different changes (as discussed in Section 11.4.3.2, Box 11.3). With the exception of the winter rainfall region in the southwest, the robust drying in winter corresponds to the dry season over most of the sub-continent and does not contribute to the bulk of the annual mean drying. More than half of the annual mean reduction occurs in the spring and is mirrored in some RCM simulations for this region (see below). To an extent, this can be thought of as a delay in the onset of the rainy season. This spring drying suppresses evaporation, contributing to the spring maximum in the temperature response.
The increase in rainfall in East Africa, extending into the Horn of Africa, is also robust across the ensemble of models, with 18 of 21 models projecting an increase in the core of this region, east of the Great Lakes. This East African increase is also evident in Hulme et al. (2001) and Ruosteenoja et al. (2003). The Guinean coastal rain belts and the Sahel do not show as robust a response. A straight average across the ensemble results in modest moistening in the Sahel with little change on the Guinean coast. The composite MMD simulations have a weak drying trend in the Sahel in the 20th century that does not continue in the future projections (Biasutti and Giannini, 2006; Hoerling, et al., 2006), implying that the weak 20th-century drying trend in the composite 20th-century simulations is unlikely to be forced by greenhouse gases, but is more likely forced by aerosols, as in Rotstayn and Lohmann (2002), or a result of low-frequency internal variability of the climate.
Individual models generate large, but disparate, responses in the Sahel. Two outliers are GFDL/CM2.1, which projects very strong drying in the Sahel and throughout the Sahara, and MIROC3.2_midres, which shows a very strong trend towards increased rainfall in the same region (see Supplementary Figure S11.13; and see Table 8.1 for model descriptions). Cook and Vizy (2006) find moderately realistic interannual variability in the Gulf of Guinea and Sahel in both models. While the drying in the GFDL model is extreme within the ensemble, it generates a plausible simulation of 20th-century Sahel rainfall trends (Held et al., 2005; Hoerling et al., 2006) and an empirical downscaling from AOGCMs (Hewitson and Crane, 2006) shows a similar response (see below). More research is needed to understand the variety of modelled precipitation responses in the Sahel and elsewhere in the tropics. Progress is being made in developing new methodologies for this purpose (e.g., Chou and Neelin, 2004; Lintner and Chiang, 2005; Chou et al., 2007), leading to better appreciation of the sources of model differences. Haarsma et al. (2005) describe a plausible mechanism associated with increasing land-ocean temperature contrast and decreasing surface pressures over the Sahara, which contributes to the increase in Sahel precipitation with warming in some models.
It has been argued (e.g., Paethe and Hense, 2004) that the partial amelioration of the Sahel drought since the 1990s may be a sign of a greenhouse-gas driven increase in rainfall, providing support for those models that moisten the Sahel into the 21st century (e.g., Maynard et al., 2002; Haarsma et al., 2005; Kamga et al., 2005). However, it is premature to take this partial amelioration as evidence of a global warming signature, given the likely influence of internal variability on the inter-hemispheric SST gradients that influence Sahel rainfall, as well as the influence of aerosol variations.
Table 11.1 provides information on the spread of model-projected precipitation change in the four African sub-regions. The regions and seasons for which the central half (25 to 75%) of the projections are uniformly of one sign are: EAF where there is an increase in DJF, March, April and May (MAM), SON and in the annual mean; SAF where there is a decrease in austral winter and spring; and SAH where there is a decrease in boreal winter and spring. The Tebaldi et al. (2004a,b) Bayesian estimates (Supplementary Material Table S11.2) do not change this distinction between robust and non-robust regions and seasons. The time required for emergence of a clearly discernible signal in these robust regions and seasons is typically 50 to 100 years, except in the Sahara where even longer times are required.
Land use change is a potential contributor to climate change in the 21st century (see also Section 11.7, Box 11.4). C.M. Taylor et al. (2002) project drying over the Sahel of 4% from 1996 to 2015 due to changing land use, but suggest that the potential exists for this contribution to grow substantially further into the century. Maynard and Royer (2004a) suggest that estimated land use change scenarios for the mid-21st century would have only a modest compensating effect on the greenhouse-gas induced moistening in their model. Neither of these studies includes a dynamic vegetation model.
Several climate change projections based on RCM simulations are available for southern Africa but are much scarcer for other regions. Tadross et al. (2005a) examine two RCMs, Providing Regional Impacts for Climate Studies (PRECIS) and Mesoscale Model version 5 (MM5), nested for southern Africa in a time-slice AGCM based in turn on lower-resolution Hadley Centre Coupled Model (HadCM3) coupled simulations for the Special Report on Emission Scenarios (SRES) A2 scenario. During the early summer season, October to December, both models predict drying over the tropical western side of the continent, responding to the increase in high-pressure systems entering from the west, with MM5 indicating that the drying extends further south and PRECIS further east. The drying in the west continues into late summer, but there are increases in total rainfall towards the east in January and February, a feature barely present in the ensemble mean of the MMD models. Results obtained by downscaling one global model must be assessed in the context of the variety of responses in southern Africa among the MMD models (Supplementary Material Figure S11.13).
Hewitson and Crane (2006) use empirical downscaling to provide projections for daily precipitation as a function of six GCM simulations. The degree of convergence in the downscaled results for the SRES A2 scenario near the end of the 21st century suggests more commonality in GCM-projected changes in daily circulation, on which the downscaling is based, than in the GCM precipitation responses. Figure 11.3 shows the response of mean JJA monthly total precipitation for station locations across Africa. The ensemble mean of these downscaling results shows increased precipitation in east Africa extending into southern Africa, especially in JJA, strong drying in the core Sahel in JJA with some coastal wetting, and moderate wetting in DJF. There is also drying along the Mediterranean coast, and, in most models, drying in the western portion of southern Africa. The downscaling also shows marked local-scale variation in the projected changes, for example, the contrasting changes in the west and east of Madagascar, and on the coastal and inland borders of the Sahel.
While this result is generally consistent with the underlying GCMs and the composite MMD projections, there is a tendency for greater Sahel drying than in the underlying GCMs, providing further rationale (alongside the large spread in model responses and poor coupled model performance in simulating droughts of the magnitude observed in the 20th century) for viewing with caution the projection for a modest increase in Sahel rainfall in the ensemble mean of the MMD models.