Mechanisms for precipitation variability of the eastern Brazil/SACZ convective margin

The present study examines the mechanisms for connection between the variability of precipitation along the eastern Brazil and the South Atlantic convergence zone convective margin (eastern Brazil/SACZ convective margin), and of the low-level inflow on interannual time scales. The methodology is based on the analysis of observational datasets and simulations by the UCLA atmospheric general circulation model (AGCM). It is demonstrated that the inflow variability is associated with the leading mode of wind variability over sub-tropical South America, and the connection with precipitation is established through the mechanism for convective margin shifts. Over the eastern Brazil/SACZ convective margin, the stronger (weaker) convection tends to occur with weaker (stronger) low-level inflows in reference to the mean easterly trades. By changing the “ventilation” effect, anomalous inflows with high (low) moist static energy from central Amazonia (Atlantic Ocean) enhance (suppress) convection. The causal relationship is verified by AGCM mechanism-testing experiments performed in perpetual-February mode, in which low-level, nondivergent, and time-independent wind perturbations are imposed in a region overlapping eastern Brazil and the western Atlantic Ocean. With solely the imposed-wind perturbations acting on the moisture advection in the model equation, the AGCM can reproduce the precipitation variability in the eastern Brazil/SACZ convective margin. This AGCM capability suggests that the mechanism can be applied to other regions of convective margins or to other time scales.