Water and energy variation associated with the wet season onset over the Amazon
Wenhong
Li, Georgia Institute of Technology, wenhong@eas.gatech.edu
Rong
Fu, Georgia Institute of Technology, fu@eas.gatech.edu
Kátia
Fernandes, Georgia Institute of Technology, kfernandes@eas.gatech.edu
(Presenting)
Using fifteen-year instantaneous European Center for Medium-range Weather Forecasts Reanalysis data (ERA), we have examined the changes of surface energy, water fluxes and the energetics of the atmospheric circulation during the transition from dry to wet seasons over the Southern Amazon region (5–15S, 45–75W). Our composite results suggest that the increase of local land surface fluxes, especially latent heat flux, initiates the transition three months prior to the onset of the wet season, by increasing the available potential energy of the lower troposphere. At this stage, the cross-equatorial flow and upper tropospheric circulation remain unchanged from those of the dry season. Starting from about one and one-half months before the onset of the wet season, local convection begins to increase. The stretching of the atmospheric column converts the available potential energy to divergent kinetic energy. The latter further converts into rotational kinetic energy in the upper troposphere. This energy conversion accelerates the transition of the large-scale atmospheric circulation, leading to the reversal of the low-level cross-equatorial flow in the western Amazon, and increase geopotential height and anticyclonic circulation in the upper tropospheric. These processes lead to the onset of the wet season. After onset, the lower tropospheric potential energy reaches equilibrium, but the conversion from divergent to rotational kinetic energy continues to spin up the upper troposphere anticyclonic circulation associated with the Bolivian High.
Our analysis implies that a slower increase of land surface energy and water fluxes, or increase the Bowen ratio of the surface energy and water fluxes could delay or prolong the initiation of the transition. The transition could also be delayed by a late or weaker reversal of the cross-equatorial flow, which consequently delays/weakens the moisture transport to the Amazonian basin. Abnormally strong upper troposphere westerly wind and subsidence could also slow the energy conversion in the upper troposphere and consequently the transition of the upper tropospheric circulation.