11.4.1 Key Processes
As monsoons are the dominant phenomena over much of Asia, the factors that influence the monsoonal flow and precipitation are of central importance for understanding climate change in this region. Precipitation is affected both by the strength of the monsoonal flows and the amount of water vapour transported. Monsoonal flows and the tropical large-scale circulation often weaken in global warming simulations (e.g., Knutson and Manabe, 1995). This arises out of an increase in dry static stability associated with the tropical warming in these models, and the reduction in adiabatic warming/cooling needed to balance a given amount of radiative cooling/condensational heating (e.g., Betts, 1998). But there is an emerging consensus that the effect of enhanced moisture convergence in a warmer, moister atmosphere dominates over any such weakening of the circulation, resulting in increased monsoonal precipitation (e.g., Douville et al., 2000; Giorgi et al., 2001a,b; Stephenson et al., 2001; Dairaku and Emori, 2006; Ueda et al., 2006).
There is an association of the phase of ENSO with the strength of the summer monsoons (Pant and Rupa Kumar, 1997), so changes in ENSO will have an impact on these monsoons. However, such an impact can be compounded by a change in the ENSO-South Asian monsoon connection under greenhouse gas warming (Krishna Kumar et al., 1999; Ashrit et al., 2001; Sarkar et al., 2004; see Section 3.7). Moreover, there is a link between Eurasian snow cover and the strength of the monsoon (see also Section 3.7), with the monsoon strengthening if snow cover retreats. Aerosols, particularly absorbing aerosols, further modify monsoonal precipitation (e.g., Ramanathan et al., 2005 for South Asia), as do modifications of vegetation cover (e.g., Chen et al., 2004 for East Asia). However, most emission scenarios suggest that future changes in regional climate are still likely to be dominated by increasing greenhouse gas forcing rather than changes in sulphate and absorbing aerosols, at least over the South Asian region.
For South Asia, the monsoon depressions and tropical cyclones generated over the Indian seas modulate the monsoon anomalies. For East Asia, the monsoonal circulations are strengthened by extratropical cyclones energised in the lee of the Tibetan Plateau and by the strong temperature gradient along the East Coast. The influence of ENSO on the position and strength of the subtropical high in the North Pacific influences both typhoons and other damaging heavy rainfall events, and has been implicated in observed inter-decadal variations in typhoon tracks (Ho et al., 2004). This suggests that the spatial structure of warming in the Pacific will be relevant for changes in these features. The dynamics of the Meiyu-Changma-Baiu rains in the early summer, which derive from baroclinic disturbances strongly modified by latent heat release, remain poorly understood. While an increase in rainfall in the absence of circulation shifts is expected, relatively modest shifts or changes in timing can significantly affect East Chinese, Korean and Japanese climates.
Over central and Southeast Asia and the maritime continent, interannual rainfall variability is significantly affected by ENSO (e.g., McBride et al., 2003), particularly the June to November rainfall in southern and eastern parts of the Indonesian Archipelago, which is reduced in El Niño years (Aldrian and Susanto, 2003). Consequently, the pattern of ocean temperature change across the Pacific is of central importance to climate change in this region.
In central Asia, including the Tibetan Plateau, the temperature response is strongly influenced by changes in winter and spring snow cover, the isolation from maritime influences, and the spread of the larger winter arctic warming into the region. With regard to precipitation, a key issue is related to the moisture transport from the northwest by westerlies and polar fronts. How far the projected drying of the neighbouring Mediterranean penetrates into these regions is likely to be strongly dependent on accurate simulation of these moisture transport processes. The dynamics of climate change in the Tibetan Plateau are further complicated by the high altitude of this region and its complex topography with large elevation differences.