Abstract
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.