ENSO Modulation of the Amazonian Low-Level Jet: More Moisture, Less Rain, and the Role of Land Surface Reception

Ali Bin Shahid 

The Amazonian Low-Level Jet (ALLJ) carries moisture from the tropical Atlantic coast into the basin interior, sustaining wet-season convection. Here we analyze 45 years of ERA5 reanalysis (1979–2023), 44 years of CHIRPS precipitation (1981–2024), GRACE terrestrial water storage (2002–2025), and SMAP root-zone soil moisture (2015–2024) to determine how ENSO modulates this transport pathway and its downstream consequences. We find that the ALLJ delivers 15–25% more moisture during El Niño than during La Niña, yet basin-wide rainfall drops by more than 10%. This paradox identifies a convective triggering failure, not a supply failure: the atmosphere contains more moisture but cannot convert it to precipitation because Walker-circulation subsidence suppresses CAPE and shallows the boundary layer. The subsidence anomaly is modest (less than 5% of background ascent), indicating that it is necessary but insufficient to explain the full rainfall deficit. Independent satellite observations reveal that deforested land in the arc of deforestation dries approximately 50% faster than intact forest following wetting events (p < 0.001, SMAP 2015–2024), reducing moisture persistence, evapotranspiration, recycling, and the mesoscale soil moisture heterogeneity that promotes convective initiation. We propose a three-lever framework in which Amazon drought is governed by (i) atmospheric moisture supply, (ii) convective triggering, and (iii) land surface water retention. During El Niño, lever (ii) fails while lever (i) strengthens, and lever (iii) compounds the deficit where the land surface is degraded. The ALLJ matters most during El Niño, precisely when the Amazon is most vulnerable.