3.7 Changes in the Tropics and Subtropics, and in the Monsoons
The global monsoon system embraces an overturning circulation that is intimately associated with the seasonal variation of monsoon precipitation over all major continents and adjacent oceans (Trenberth et al., 2000). It involves the Hadley Circulation, the zonal mean meridional overturning mass flow between the tropics and subtropics entailing the Inter-Tropical Convergence Zone (ITCZ), and the Walker Circulation, which is the zonal east-west overturning. The South Pacific Convergence Zone (SPCZ) is a semi-permanent cloud band extending from around the Coral Sea southeastward towards the extratropical South Pacific, while the South Atlantic Convergence Zone (SACZ) is a more transient feature over and southeast of Brazil that transports moisture originating over the Amazon into the South Atlantic (Liebmann et al., 1999).
Tropical SSTs determine where the upward branch of the Hadley Circulation is located over the oceans, and the dominant variations in the energy transports by the Hadley cell, reflecting its strength, relate to ENSO (Trenberth et al., 2002a; Trenberth and Stepaniak, 2003a). During El Niño, elevated SST causes an increase in convection and relocation of the ITCZ and SPCZ to near the equator over the central and eastern tropical Pacific, with a tendency for drought conditions over Indonesia. There follows a weakening of the Walker Circulation and a strengthening of the Hadley Circulation (Oort and Yienger, 1996; Trenberth and Stepaniak, 2003a), leading to drier conditions over many subtropical regions during El Niño, especially over the Pacific sector. As discussed in Section 3.4.4.1, increased divergence of energy out of the tropics in the 1990s relative to the 1980s (Trenberth and Stepaniak, 2003a) is associated with more frequent El Niño events and especially the major 1997–1998 El Niño event, so these conditions play a role in inter-decadal variability (Gong and Ho, 2002; Mu et al., 2002; Deser et al., 2004). Examination of the Hadley Circulation in several data sets (Mitas and Clement, 2005) suggests some strengthening, although discrepancies among reanalysis data sets and known deficiencies raise questions about the robustness of this strengthening, especially prior to the satellite era (1979).
Monsoons are generally referred to as tropical and subtropical seasonal reversals in both the surface winds and associated precipitation. The strongest monsoons occur over the tropics of southern and eastern Asia and northern Australia, and parts of western and central Africa. Rainfall is the most important monsoon variable because the associated latent heat release drives atmospheric circulations, and because of its critical role in the global hydrological cycle and its vital socioeconomic impacts. Thus, other regions that have an annual reversal in precipitation with an intense rainy summer and a dry winter have been recently recognised as monsoon regions, even though these regions have no explicit seasonal reversal of the surface winds (Wang, 1994; Webster et al., 1998). The latter regions include Mexico and the southwest USA, and parts of South America and South Africa. Owing to the lack of sufficiently reliable and long-term oceanic observations, analyses of observed long-term changes have mainly relied on land-based rain gauge data.
Because the variability of regional monsoons is often the result of interacting circulations from other regions, simple indices of monsoonal strength in adjacent regions may give contradictory indications of strength (Webster and Yang, 1992; Wang and Fan, 1999). Decreasing trends in precipitation over the Indonesian Maritime Continent, equatorial parts of western and central Africa, Central America, Southeast Asia and eastern Australia have been found for 1948 to 2003 (Chen et al., 2004; see Figure 3.13), while increasing trends were evident over the USA and northwestern Australia (see also Section 3.3.2.2 and Figure 3.14), consistent with Dai et al. (1997). Using NRA, Chase et al. (2003) found diminished monsoonal circulations since 1950 and no trends since 1979, but results based on NRA suffer severely from artefacts arising from changes in the observing system (Kinter et al., 2004).
Two precipitation data sets (Chen et al., 2002; GHCN, see Section 3.3) yield very similar patterns of change in the seasonal precipitation contrasts between 1976 to 2003 and 1948 to 1975 (Figure 3.34), despite some differences in details and discrepancies in northwest India. Significant decreases in the annual range (wet minus dry season) were observed over the NH tropical monsoon regions (e.g., Southeast Asia and Central America). Over the East Asian monsoon region, the change over these periods involves increased rainfall in the Yangtze River valley and Korea but decreased rainfall over the lower reaches of the Yellow River and northeast China. In the Indonesian-Australian monsoon region, the change between the two periods is characterised by an increase in northwest Australia and Java but a decrease in northeast Australia and a northeastward movement in the SPCZ (Figure 3.34). However, the average monsoonal rainfall in East Asia, Indonesia-Australia and South America in summer mostly shows no long-term trend but significant interannual and inter-decadal variations. In the South African monsoon region there is a slight decrease in the annual range of rainfall (Figure 3.34), and a decreasing trend in area-averaged precipitation (Figure 3.14).
Monsoon variability depends on many factors, from regional air-sea interaction and land processes (e.g., snow cover fluctuations) to teleconnection influences (e.g., ENSO, NAO/NAM, PDO, IOD). New evidence, relevant to climate change, indicates that increased aerosol loading in the atmosphere may have strong impacts on monsoon evolution (Menon et al., 2002) through changes in local heating of the atmosphere and land surface (see also Box 3.2 and Chapter 2).