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

Abstract

Abstract. 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.

Different precipitation response over land and ocean to orbital and greenhouse gas forcing

“…All the variables have been described in Table 1 . A detailed derivation can be found in previous studies 8 , 27 – 29 .…”

“…To calculate the contribution of and GMS in mm , to a change in between two climates, we use the following equation, which was derived earlier 8 : Q and G are short-forms used in place of and GMS , respectively. represents the perturbation of the climate of interest with respect to a reference climate (we have taken the climate at 22 ka as a reference in Figs.…”

“…The oceans import the excess energy by changing the vertical structure of the atmosphere. Though this explains the mean tropical picture, the mechanisms are more complex for regional monsoons 8 . Over the BoB, the surface latent heat fluxes play an important role in driving orbital scale changes in precipitation.…”

“…This has been attributed to the latitudinal shift in the zonal mean intertropical convergence zone (ITCZ), which is driven by the differential warming of the two hemispheres 2 – 4 . Recent modeling studies have, however, pointed out that the precipitation over land and ocean respond differently to orbital forcings 5 – 8 . This is further supported by a 4,000-year speleothem record from the Baratang cave in the Andaman Islands located in the Bay of Bengal (BoB) 9 .…”

“…The mechanism for this differential response has been explained with the moist static energy budget 7 and energetics of monsoon and ITCZ 5 , 8 , 11 , 12 . The atmosphere over land responds quickly to a change in insolation forcing and acts to export the excess energy out of the column 7 .…”