Global monsoon: Interannual-to-centennial variability and future changes

The global monsoon (GM) is an evolving concept that has not yet been fully recognized, especially whether its interannual-multidecadal climate variability can be viewed as one of the major modes of the global climate system. In this chapter we focus on reviewing the common variability and linkages among all regional monsoons. We begin with a brief review of the concept of the GM and elaborate what the essence of the GM is and why it is a defining feature of the Earth's climatology. The GM variability and change are driven by two sets of factors. One is external forcing, both natural (solar orbital change, volcanic aerosols) and man-made (greenhouse gases concentration, anthropogenic aerosols, land use, etc.). The other is internal feedback processes within the coupled Earth's climate system. Evidence is reviewed to show that on the interannual-to-multidecadal time scale, the monsoon variations are not merely of isolated regional characters; rather, the monsoon can be driven by common internal feedback processes on global scale such as El Nino-Southern Oscillation (ENSO), mega-ENSO, and Atlantic Multidecadal Oscillation (AMO). The changes in solar radiation flux and volcanic forcing is shown to generate a coherent variability of all regional monsoons; thus the GM represents a leading mode of variability on centennial or longer time scales in response to changing solar forcing. In addition, we review how anthropogenic forcing affects future change of GM precipitation. It is important to understand the difference in GM responses to anthropogenic and natural forcings. In summary, it is argued that the GM is not only a defining feature of the Earth's climatology but also a major mode of Earth's climate variability across time scales ranging from interannual to centennial.