The influence of land surface on the wet season onset over the Amazonia
Wenhong
Li, Georgia Institute of Technology, wenhong@eas.gatech.edu
Rong
Fu, Georgia Institute of Technology, fu@eas.gatech.edu
(Presenting)
The influence of Amazon ecosystems change on the large variation of wet season onset has been analyzed using the fifteen years of European Centre for Medium Range Weather Forecasts reanalysis (ERA). Our results suggested that the transition from dry to wet season in Southern Amazonia is initially driven by increases of surface latent heat flux. These fluxes rapidly reduce Convective Inhibition Energy (CINE) and increase Convective Available Potential Energy (CAPE), consequently providing favorable conditions for increased rainfall even before the large-scale circulation has changed. The increase of rainfall presumably initiates the reversal of the cross-equatorial flow, leading to large-scale net moisture convergence over Southern Amazonia. An analysis of early and late wet season onsets on an interannual scale shows that a longer dry season with lower rainfall reduces surface latent heat flux in the dry and earlier transition periods compared to that of a normal wet season onset. These conditions result in a higher CINE and a lower CAPE, causing a delay in the increase of local rainfall in the initiating phase of the transition and consequently in the wet season onset. Conversely, a wetter dry season leads to a higher surface latent heat flux and weaker CINE, providing a necessary condition for an earlier increase of local rainfall and an earlier wet season onset. Our results support the notion that if land use change in Amazonia reduces rainfall during dry and transition seasons, it could significantly delay the wet season onset and prolong the dry season as pointed in Nobre et al. (1991).