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 heating(1,2). 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 rapidly(3,4). 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 circulation(5). 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.