Understanding the Tropical Overturning Circulation: The Importance of Eddy Momentum Fluxes and Eddy Heat Fluxes

We study an idealized tropical overturning circulation forced by seasonal insolation variations on a zonally symmetric beta-plane domain. By altering the width of the domain, large-scale eddies may be either permitted or suppressed, allowing for an evaluation of the effect of such eddies on the mean circulation. We further investigate the relative effects of eddy momentum fluxes and eddy heat fluxes by applying artificial sources of momentum and energy diagnosed from eddy-permitting simulations to simulations in which large-scale eddies are not present. For the equinoctial case without a seasonal cycle, eddy momentum fluxes play an important role in determining the strength of the resultant circulation. When a seasonal cycle is included, both eddy momentum fluxes and eddy heat fluxes are important for the development of the seasonally varying overturning circulation. In particular, the inclusion of either eddy heat fluxes or eddy momentum fluxes produces a rapid strengthening of the Solsticial circulation, similar in some respects to the observed onset of the Asian monsoon. The results are interpreted using a theoretical framework based on the assumption of convective quasi equilibrium, and it is found that the low-level entropy distribution is a key determinant of the behavior of the overturning circulation in our simulations. Possible implications for Earth's monsoons in the present and future climate are discussed.