Monsoons as eddy-mediated regime transitions of the tropical overturning circulation: I. Simulations with an aquaplanet GCM

Since Halley (1686), monsoons are generally viewed as planetary-scale sea-breeze circulations, caused by contrasts in thermal properties between low-latitude ocean and land surfaces that lead to thermal contrasts through radiative heating of the surfaces. But while the radiative heating of the surfaces evolves gradually with season, the onset and end of monsoon precipitation is rapid and accompanied by rapid circulation changes such as reversal of surface winds. Here we show that simulations with an aquaplanet GCM reproduce transitions of the tropical overturning circulation that resemble the onset and end of Earth's large-scale monsoons such as the Asian monsoon, even in the absence of land masses, provided the heat capacity (mixed layer depth) of the underlying ocean is sufficiently low. A transition in early summer is associated with a rapid shift into the subtropics and intensification of the major precipitation zone, an abrupt strengthening and broadening of the cross-equatorial Hadley cell, and wind reversals at both upper and lower levels. A reverse transition occurs in late summer.

The rapid rearrangements of the meridional overturning circulation mark shifts between an equinox and summer regime, in which momentum fluxes associated with the energy-containing midlatitude eddies strongly influence the circulation strength, and a monsoon regime, in which eddy momentum fluxes weakly influence the circulation strength. (In Part II, we use reanalysis data to show that transitions between the two circulation regimes mark the onset and end of the Asian-Australian monsoon system). Feedbacks between the large-scale eddies and the overturning circulation render the transition rapid and lead to rapid circulation changes, even in the absence of surface inhomogeneities, provided the ocean surface has sufficiently low thermal inertia.

Based on these results, we propose a new view of monsoons as eddy-mediated regime transitions of the tropical overturning circulation. According to this view surface inhomogeneities such as land-sea contrasts in thermal properties are not necessary but feedbacks between extratropical eddies and the tropical overturning circulation are essential for monsoons.