Impacts of the Pacific Meridional Mode on June–August precipitation in the Amazon River Basin

This study examines the impacts of the Pacific Meridional Mode (PMM) on Amazon precipitation during June–August using observations and several experiments with the National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory (NOAA/GFDL) Forecast-oriented Low Ocean Resolution version of CM2.5 (FLOR). We find that the positive (negative) PMM can lead to precipitation surplus (deficit) using both observations and climate simulations with FLOR. The impacts of PMM on Amazon precipitation during June–August are induced by the forcing of sea-surface temperature (SST) anomalies associated with PMM. Positive PMM can force the baroclinic Gill responses to the heat source in the Pacific with two low-level cyclones (anticyclones) located west (east) of the heating source. The anomalous low-level anticyclone and high-level cyclone located in the Amazon region are associated with low-level moisture transport from the Atlantic. There is significant positive correlation between the PMM index and moisture flux convergence in most parts of the Amazon basin, with negative correlation in its northwestern part. Such physical mechanisms underlying the linkage between PMM and the Amazon precipitation are supported by both the 500 years control experiment and a suite of perturbation experiments with FLOR.