9.5.4.3 Regional Precipitation Changes
Observed trends in annual precipitation during the period 1901 to 2003 are shown in Figure 3.13 for regions in which data is available. Responses to external forcing in regional precipitation trends are expected to exhibit low signal-to-noise ratios and are likely to exhibit strong spatial variations because of the dependence of precipitation on atmospheric circulation and on geographic factors such as orography. There have been some suggestions, for specific regions, of a possible anthropogenic influence on precipitation, which are discussed below.
9.5.4.3.1 Sahel drought
Rainfall decreased substantially across the Sahel from the 1950s until at least the late 1980s (Dai et al., 2004; Figure 9.19, see also Figure 3.37). There has been a partial recovery since about 1990, although rainfall has not returned to levels typical of the period 1920 to 1965. Zeng (2003) note that two main hypotheses have been proposed as a cause of the extended drought: overgrazing and conversion of woodland to agriculture increasing surface albedo and reducing moisture supply to the atmosphere, and large-scale atmospheric circulation changes related to decadal global SST changes that could be of anthropogenic or natural origin (Nicholson, 2001). Black carbon has also been suggested as a contributor (Menon et al., 2002b). Taylor et al. (2002) examine the impact of land use change with an atmospheric GCM forced only by estimates of Sahelian land use change since 1961. They simulate a small decrease in Sahel rainfall (around 5% by 1996) and conclude that the impacts of recent land use changes are not large enough to have been the principal cause of the drought.
Several recent studies have demonstrated that simulations with a range of atmospheric models using prescribed observed SSTs are able to reproduce observed decadal variations in Sahel rainfall (Bader and Latif, 2003; Giannini et al., 2003; Rowell, 2003; Haarsma et al., 2005; Held et al., 2005; Lu and Delworth, 2005; see also Figure 9.19; Hoerling et al., 2006), consistent with earlier findings (Folland, 1986; Rowell, 1996). Hoerling et al. (2006) show that AGCMs with observed SST changes typically underestimate the magnitude of the observed precipitation changes, although the models and observations are not inconsistent. These studies differ somewhat in terms of which ocean SSTs they find to be most important: Giannini et al. (2003) and Bader and Latif (2003) emphasize the role of tropical Indian Ocean warming, Hoerling et al. (2006) attribute the drying trend to a progressive warming of the South Atlantic relative to the North Atlantic, and Rowell (2003) finds that Mediterranean SSTs are an additional important contributor to decadal variations in Sahel rainfall. Based on a multi-model ensemble of coupled model simulations Hoerling et al. (2006) conclude that the observed drying trend in the Sahel is not consistent with simulated internal variability alone.
Thus, recent research indicates that changes in SSTs are probably the dominant influence on rainfall in the Sahel, although land use changes possibly also contribute (Taylor et al., 2002). But what has caused the differential SST changes? Rotstayn and Lohmann (2002) propose that spatially varying, anthropogenic sulphate aerosol forcing (both direct and indirect) can alter low-latitude atmospheric circulation leading to a decline in Sahel rainfall. They find a southward shift of tropical rainfall due to a hemispheric asymmetry in the SST response to changes in cloud albedo and lifetime in a climate simulation forced with recent anthropogenic changes in sulphate aerosol. Williams et al. (2001) also find a southward shift of tropical rainfall as a response to the indirect effect of sulphate aerosol. These results suggest that sulphate aerosol changes may have led to reduced warming of the northern tropical oceans, which in turn led to the decrease in Sahel rainfall, possibly enhanced through land-atmosphere interaction, although a full attribution analysis has yet to be conducted. Held et al. (2005) show that historical climate simulations with the both the GFDL-CM2.0 and CM2.1 models (see Table 8.1 for details) exhibit drying trends over the Sahel in the second half of the 20th century, which they ascribe to a combination of greenhouse gas and sulphate aerosol changes. The spatial pattern of the trends in simulated rainfall also shows some agreement with observations. However, Hoerling et al. (2006) find that eight other coupled climate models with prescribed anthropogenic forcing do not simulate significant trends in Sahel rainfall over the 1950 to 1999 period.
9.5.4.3.2 Southwest Australian drought
Early winter (May–July) rainfall in the far southwest of Australia declined by about 15% in the mid-1970s (IOCI, 2002) and remained low subsequently. The rainfall decrease was accompanied by a change in large-scale atmospheric circulation in the surrounding region (Timbal, 2004). The circulation and precipitation changes are somewhat consistent with, but larger than, those simulated by climate models in response to greenhouse gas increases. The Indian Ocean Climate Initiative (IOCI, 2005) concludes that land cover change could not be the primary cause of the rainfall decrease because of the link between the rainfall decline and changes in large-scale atmospheric circulation, and re-affirms the conclusion of IOCI (2002) that both natural variability and greenhouse forcing likely contributed. Timbal et al. (2005) demonstrate that climate change signals downscaled from the PCM show some similarity to observed trends, although the significance of this finding is uncertain.
Some authors (e.g., Karoly, 2003) have suggested that the decrease in rainfall is related to anthropogenic changes in the SAM (see Section 9.5.3.3). However, the influence of changes in circulation on southwest Australian drought remains unclear as the largest SAM trend has occurred during the SH summer (December–March; Thompson et al., 2000; Marshall et al., 2004), while the largest rainfall decrease has occurred in early winter (May–July).
9.5.4.3.3 Monsoon precipitation
Decreasing trends in precipitation over the Indonesian Maritime Continent, equatorial western and central Africa, Central America, Southeast Asia and eastern Australia have been observed over the period 1948 to 2003, while increasing trends were found over the USA and north-western Australia (Section 3.7). The TAR (IPCC, 2001, pp. 568) concluded that an increase in Southeast Asian summer monsoon precipitation is simulated in response to greenhouse gas increases in climate models, but that this effect is reduced by an increase in sulphate aerosols, which tend to decrease monsoon precipitation. Since then, additional modelling studies have come to conflicting conclusions regarding changes in monsoon precipitation (Lal and Singh, 2001; Douville et al., 2002; Maynard et al., 2002; May, 2004; Wardle and Smith, 2004; see also Section 9.5.3.5). Ramanathan et al. (2005) were able to simulate realistic changes in Indian monsoon rainfall, particularly a decrease that occurred between 1950 and 1970, by including the effects of black carbon aerosol. In both the observations and model, these changes were associated with a decreased SST gradient over the Indian Ocean and an increase in tropospheric stability, and they were not reproduced in simulations with greenhouse gas and sulphate aerosol changes only.