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Aquaplanet monsoons
Figure 1: Seasonal cycle of zonal- and pentad-mean precipitation (color contours) and sea-level air temperature (SAT) (grey contours) from observations over the Asian monsoon sector (a) and from aquaplanet simulations with ocean mixed-layer thickness 1 m (b) and 10 m (c). SAT is evaluated at 1000hPa in the observations and at the lowest model level in the simulations.
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 monsoonal precipitation is rapid and accompanied by rapid circulation changes. Using reanalysis data, we show that the onset of the Asian monsoon marks a transition between two circulation regimes that can be distinguished according to the degree to which eddy momentum fluxes control the strenth of the circulation. Rapid transitions of the circulation between the two regimes can occur as a results of feedbacks between large-scale eddies of extratropical origin and the tropical circulations (Schneider and Bordoni, 2008). Using simlations with an aquaplanet GCM, 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. Based on 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.
Further details can be found in Bordoni and Schneider (2008) and in the poster presentation for the 28th AMS Conference on Hurricanes and Tropical Meteorology. Please, also see the Caltech press release.
Figure 2: Zonal- and temporal-mean circulation in the Asian monsoon sector at two 20-day periods before and after monsoon onset. (a,b) Streamfunction of meridional overturning circulation (black contours), angular momentum (grey contours) and transient eddy momentum flux divergence (color contours). (c,d) Zonal wind (black contours) and eddy momentum flux divergence (color contours) as in (a,b). (e,f) Precipitation P (blue) and near-surface (850-hPa) moist static energy h (red).
Figure 3: Zonal- and temporal-mean circulation at two 20-day periods before and after monsoon onset in a simulation with a 1-m ocean mixed layer. Quantities shown are as in Fig.2.
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