![]() It is therefore necessary to examine the dry-to-wet season transition by using observable quantities. The potential influences of aerosols on the dry-to-wet season transition are an additional source of uncertainty ( 29), because the aerosol climatologies used by most reanalyses neglect or underestimate seasonal and interannual variations in aerosol loading in this region ( 30, 31). In situ observations indicate that maximum ET leads the late dry season increase in rainfall ( 26– 28) however, it has been unclear whether modest increases in ET can contribute sufficient moisture above the ABL at regional scales. Enhanced rainfall and associated heating in the atmosphere directly affect reanalysis estimates of ET and MFC, potentially confounding moisture source attributions based on reanalysis products. For example, the increase in rainfall over the southern Amazon during the dry-to-wet season transition occurs ∼2–3 wk earlier in the European Center for Medium-Range Weather Forecasting Interim Reanalysis (ERA-Interim) than in observations ( SI Text). Inadequate treatments of surface hydrology, vegetation, and turbulent mixing near the top of the atmospheric boundary layer (ABL) lead to large uncertainties in reanalysis estimates of ET, rainfall, and moisture flux convergence (MFC) ( 25). These products are heavily influenced by the behavior of the underlying model in data-poor regions like Amazonia. Previous studies on this topic have been overwhelmingly based on reanalysis products that combine available observations with numerical model simulations. To clarify the mechanisms involved in activating the DCMP, the first question that must be answered is whether the late dry season increase in lower tropospheric humidity primarily derives from rainforest transpiration or advection from the ocean. We refer to this transition mechanism as the deep convective moisture pump (DCMP). Large-scale moisture transport reinforces the conditions that favor deep convection, ultimately leading to wet season onset. Lifting of this humid near-surface air by cold fronts moving northward from midlatitude South America ( 20) could cause large-scale increases in deep convection and upper-level heating ( 21), thereby initiating moisture transport from the tropical Atlantic. An alternative hypothesis holds that late dry season increases in rainforest transpiration may increase surface air humidity and buoyancy ( 18, 19). Conventional mechanisms therefore cannot explain wet season onset over the southern Amazon. However, wet season onset over the southern Amazon precedes the southward migration of the Atlantic ITCZ by ∼2–3 mo ( 16) and occurs without a reversal in the land–ocean surface temperature gradient ( 17, 18). Wet season onset in the tropics is generally associated with either monsoon reversals in the land–ocean temperature gradient or north–south migration of the Intertropical Convergence Zone (ITCZ), both of which are driven by seasonal changes in the distribution of solar radiation. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought. Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2–3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall.
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