The response of tropical precipitation to Earth's precession: the role of energy fluxes and vertical stability

The changes in Earth's precession have an impact on the tropical precipitation. This has been attributed to the changes in seasonal solar radiation at the top of the atmosphere. The primary mechanism that has been proposed is the change in thermal gradient between the two hemispheres. This may be adequate to understand the zonal mean changes, but cannot explain the variations between land and oceans. We have used a simple model of the intertropical convergence zone (ITCZ) to unravel how precipitation changes with precession. Our model attributes the changes in precipitation to the changes in energy fluxes and vertical stability. We include the horizontal advection terms in this model, which were neglected in the earlier studies. The final response of the land and oceans is a result of complex feedbacks triggered by the initial changes in the insolation. We find that the changes in precipitation over the land are mainly driven by changes in insolation, but over the oceans, precipitation changes on account of changes in surface fluxes and vertical stability. Hence insolation can be a trigger for changes in precipitation on orbital timescales, but surface energy and vertical stability play an important role too. The African monsoon intensifies during a precession minimum (higher summer insolation). This intensification is mainly due to the changes in vertical stability. The precipitation over the Bay of Bengal decreases for minimum precession. This is on account of a remote response to the enhanced convective heating to the west of the Bay of Bengal. This weakens the surface winds and thus leads to a decrease in the surface latent heat fluxes and hence the precipitation.