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Monsoons as eddy-mediated regime transitions of the tropical overturning circulation


Monsoons are generally viewed as planetary-scale sea-breeze circulations, caused by contrasts in the thermal properties between oceans and land surfaces that lead to thermal contrasts upon radiative heating1,2. But the radiative heating evolves gradually with the seasons, whereas the onset of monsoon precipitation, and the associated circulation changes such as reversal of surface winds, occur rapidly3,4. Here we use reanalysis data to show that the onset of the Asian monsoon marks a transition between two circulation regimes that are distinct in the degree to which eddy momentum fluxes control the strength of the tropical overturning circulation. Rapid transitions of the circulation between the two regimes can occur as a result of feedbacks between large-scale extratropical eddies and the tropical circulation5. Using simulations with an aquaplanet general circulation model, we demonstrate that rapid, eddy-mediated monsoon transitions occur even in the absence of surface inhomogeneities, provided the planet surface has sufficiently low thermal inertia. On the basis of these results, we propose a view of monsoons in which feedbacks between large-scale extratropical eddies and the tropical circulation are essential for the development of monsoons, whereas surface inhomogeneities such as land-sea contrasts are not.

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Figure 1: Rapid shifts of precipitation zones.
Figure 2: Observed monsoon onset over Asia.
Figure 3: Rapid changes in near-surface zonal wind at 15 N.
Figure 4: Simulated monsoon onset.

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This work was supported by the Davidow Discovery Fund, a David and Lucile Packard Fellowship, a Moore Postdoctoral Fellowship and the National Science Foundation (grant no. ATM-0450059). The simulations were carried out on Caltech’s Geological and Planetary Science Dell Cluster, and the reanalysis data were provided by the National Center for Atmospheric Research (which is sponsored by the National Science Foundation). Part of the research was carried while S.B. was with the Department of Atmospheric and Oceanic Sciences at UCLA (supported by a UCLA Dissertation Year Fellowship). We thank B. Stevens for comments on drafts of this paper.

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Correspondence to Simona Bordoni.

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Bordoni, S., Schneider, T. Monsoons as eddy-mediated regime transitions of the tropical overturning circulation. Nature Geosci 1, 515–519 (2008).

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