The influence of pre-seasonal land surface condition on South American Monsoon Onset and Its Interannual Variability

We have examined the influence of land surface fluxes on the South American Monsoon onset and its interannual variations using the European Centre for Medium Range Weather Forecasts reanalysis (ERA), raingauges and satellite observations of rainfall, flux tower and soil moisture measurements. Our results suggest that the transition from dry to wet season 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 prior to the reversal of the large-scale circulation. This initial increase of rainfall provides elevated atmospheric heating, and consequently initiates the reversal of the cross-equatorial flow and large-scale moisture to the South America. On interannual scale, a longer dry season with lower rainfall reduces surface latent heat flux in the earlier transition periods compared to that of a normal wet season onset. This delays and reduces the initial increase of rainfall during early phase of the transition, and consequently delays 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 rainfall and an earlier wet season onset. These results suggest that, although the large-scale moisture transport is the primary supplier of water vapor for monsoon rainfall. Its seasonal reversal from dry to wet season is initiated by an increase of land surface latent flux during early transition season. The latter is controlled by pre-seasonal rainfall and soil moisture. Thus, the land surface flux plays a more important role than their direct contribution to the water budget in determining the climate variability of the South American monsoon onset.