The present paper addresses driving mechanisms of global monsoon (GM) variability and outstanding issues in GM science. This is the second synthesis of the PAGES GM Working Group following the first synthesis “The Global Monsoon across Time Scales: coherent variability of regional monsoons” published in 2014 (Climate of the Past, 10, 2007–2052). Here we introduce the GM as a planetary scale circulation system and give a brief accounting of why it exhibits regional structure. The primary driver of the GM is solar insolation, and the specific features in the underlying surface, such as land-sea distribution, topography, and oceanic circulations, are mainly responsible for the differences among regional monsoon systems. We then analyze the monsoon formation mechanisms, together with the major processes that drive monsoon variations at various timescales, including external forcings and internal feedbacks. On long time scales, external forcings often induce variability on a global scale, whereas short-term variability in regional monsoon systems is usually caused by internal feedbacks within the climate system. Finally, a number of debatable issues are discussed, with an emphasis on time scales beyond the instrumental record. These include the dual nature of the monsoon as wind and rain, the meaning of oxygen isotope in hydrological cycle, in particular of speleothem δ18O, the role of ice-sheet in monsoon variations, etc. In general, the GM as a system comprises a hierarchy of regional and local monsoons with various degrees of similarity, though all show coherent variability driven by a common solar forcing. The goal of the GM concept, therefore, is by no means to replace or diminish research on the regional monsoons, but to help dissect the mechanisms and controlling factors of monsoon variability at various temporal-spatial scales.
Bibliographical noteFunding Information:
This work was supported by the PAGES project and written on the basis of two PAGES symposia on global monsoon. We thank all participants of the symposia whose contributions form the basis of the present paper. We also thank anonymous reviewers for their constructive comments that significantly improved the manuscript. Jun Tian is deeply acknowledged for his assistance throughout the Working Group's activities, as well as in the manuscript preparation. P. X. Wang is thankful for the support of NNSFC grant 91128000 . B. Wang acknowledges supports from NNSFC Grant 41371209 , 41420104002 and NRF of Korea through Korean Ministry of Education, Science and Technology (MEST, # 2011-0021927 ). Image 1
- Climate variability
- Hydrological cycle
- Monsoon mechanism
- Solar insolation