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.This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordMet Offic

Rainfall hotspots over the southern tropical Andes: Spatial distribution, rainfall intensity, and relations with largescaleatmospheric circulation

International audienceThe Andes/Amazon transition is among the rainiest regions of the world and the interactionsbetween large-scale circulation and the topography that determine its complex rainfall distribution remainpoorly known. This work provides an in-depth analysis of the spatial distribution, variability, and intensity ofrainfall in the southern Andes/Amazon transition, at seasonal and intraseasonal time scales. The analysis isbased on comprehensive daily rainfall data sets from meteorological stations in Peru and Bolivia. We com-pare our results with high-resolution rainfall TRMM-PR 2A25 estimations. Hotspot regions are identified atlow elevations in the Andean foothills (400–700 masl) and in windward conditions at Quincemil and Chipir-iri, where more than 4000 mm rainfall per year are recorded. Orographic effects and exposure to easterlywinds produce a strong annual rainfall gradient between the lowlands and the Andes that can reach190 mm/km. Although TRMM-PR reproduces the spatial distribution satisfactorily, it underestimates rainfallby 35% in the hotspot regions. In the Peruvian hotspot, exceptional rainfall occurs during the austral dryseason (around 1000 mm in June–July–August; JJA), but not in the Bolivian hotspot. The direction of thelow-level winds over the Andean foothills partly explains this difference in the seasonal rainfall cycle. Atintraseasonal scales in JJA, we found that, during northerly wind regimes, positive rainfall anomalies pre-dominate over the lowland and the eastern flank of the Andes, whereas less rain falls at higher altitudes. Onthe other hand, during southerly regimes, rainfall anomalies are negative in the hotspot regions. The influ-ence of cross-equatorial winds is particularly clear below 2000 masl