Monsoons are generally viewed as planetary-scale sea-breeze circulations, caused by contrasts in thermal properties between oceans and land-surfaces that lead to thermal contrasts upon radiative heating. But while the radiative heating evolves gradually with seasons, the onset of the monsoon precipitation is rapid and accompanied by rapid circulation changes. In this talk, we discuss how monsoon-like transitions can occur in idealized numerical experiments even in the absence of land-sea contrasts in the lower-boundary, provided the surface has sufficiently low thermal inertia. In these experiments, monsoon transitions are mainly driven, and are rendered rapid, by dynamical feedbacks involving large-scale midlatitude eddies and the tropical circulation. Implications for present and future monsoon systems are discussed.
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