6.6.5.3 Asian Monsoon Variability

Fifteen severe (three years or longer) droughts have occurred in a region of China dominated by the East Asian Monsoon over the last 1 kyr (Zhang, 2005). These palaeodroughts were generally more severe than droughts in the same region within the last 50 years. In contrast, the South Asian (Indian) monsoon, in the drier areas of its influence, has recently reversed its millennia-long orbitally driven low-frequency trend towards less rainfall. This recent reversal in monsoon rainfall also appears to coincide with a synchronous increase in inferred monsoon winds over the western Arabian Sea (Anderson et al., 2002), a change that could be related to increased summer heating over and around the Tibetan Plateau (Braüning and Mantwill, 2004; Morrill et al., 2006).

6.6.5.4 Northern and Eastern Africa Hydrologic Variability

Lake sediment and historical documentary evidence indicate that northern Africa and the Sahel region have for a long time experienced substantial droughts lasting from decades to centuries (Kadomura, 1992; Verschuren, 2001; Russell et al., 2003; Stager et al., 2003; Nguetsop et al., 2004; Brooks et al., 2005; Stager et al., 2005). Although there have been attempts to link these dry periods to solar variations, the evidence is not conclusive (Stager et al., 2005), particularly given that the relationship between hypothesised solar proxies and variation in total solar irradiance remains unclear (see Section 6.6.3). The palaeoclimate record indicates that persistent droughts have been a common feature of climate in northern and eastern Africa. However, it has not been demonstrated that these droughts can be simulated with coupled ocean-atmosphere models.