Monsoons, ITCZs, and the Concept of the Global Monsoon
Earth's tropical and subtropical rainbands, such as Intertropical Convergence Zones (ITCZs) and monsoons, are complex systems, governed by both large‐scale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as regional large‐scale sea breeze circulations, driven by land‐sea contrast. More recently, a perspective has emerged of a global monsoon, a global‐scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation. This results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone. Regional subsystems are embedded in this global monsoon, localized by surface boundary conditions. Parallel with this, much theoretical progress has been made on the fundamental dynamics of the seasonal Hadley cells and convergence zones via the use of hierarchical modeling approaches, including aquaplanets. Here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings. After summarizing the dynamical and energetic balances that distinguish an ITCZ from a monsoon, we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the large‐scale tropical circulation.
Additional Information© 2020 American Geophysical Union. Issue Online: 01 December 2020; Version of Record online: 01 December 2020; Accepted manuscript online: 30 October 2020; Manuscript accepted: 14 October 2020; Manuscript revised: 21 August 2020; Manuscript received: 22 February 2020. R. G. was supported by the UK‐China Research and Innovation Partnership Fund, through the Met Office Climate Science for Service Partnership (CSSP) China, as part of the Newton Fund. S. B. and K. L. H. acknowledge support from the Caltech Terrestrial Hazard Observation and Reporting (THOR) center and the Caltech GPS Discovery Fund. D. S. B. was funded by the Tamaki Foundation. We thank Dennis Hartmann, John Chiang, Mike Wallace, Fabio Bellacanzone, Peter Molnar and three anonymous reviewers for their thoughtful and helpful comments on the manuscript. Data Availability Statement: Data sets for this research are available in these in‐text data citation references: Japan Meteorological Agency/Japan (2013); Mesoscale Atmospheric Processes Branch/Laboratory for Atmospheres/Earth Sciences Division/Science and Exploration Directorate/Goddard Space Flight Center/NASA (2018); Huffman et al. (2016); Bordoni (2020).
Published - 2020RG000700.pdf
Submitted - essoar.10502409.2.pdf