Intensified low-level jet and increased humidity drove half of the large wetting trend in southeastern South America from 1949-2020

Austral summer precipitation over Southeastern South America (SESA; southern Brazil, Paraguay, Uruguay, and northern Argentina) increased by 27% from 1902-2020, one of the largest observed precipitation trends across the globe. This precipitation increase is qualitatively in line with projections due to anthropogenic forcing, but is much stronger than models suggest should have occurred thus far. We assess the influence of the South American low-level jet (SALLJ) on SESA precipitation variability and trends by analyzing the low-level moisture flux through the jet region as a metric for predicting SESA precipitation. We analyze these connections during austral summer and decompose the SALLJ into its wind- and humidity-driven components. The SALLJ index calculated from the total low-level moisture flux accounts for 52% of the observed precipitation increase in SESA from 1949-2020. Of this 52%, the wind-driven component of the SALLJ accounts for 21%, while the humidity-driven component accounts for 23%. The remaining 8% is accounted for by the Q*V interaction term. These components are further investigated for their control on SESA precipitation by exploring the correlations with global sea surface temperature and sea level pressure. Our results suggest that increases in windspeed through the low-level jet, which are likely associated with natural climate variability, led to increases in moisture transports into the SESA region. Increases in humidity in SESA associated with the SALLJ are likely associated with increases in global humidity due to warming of sea surface temperatures driven by anthropogenic forcing. We, therefore, conclude that 52% of the increasing precipitation trend in SESA from 1949-2020 is attributable to an intensified SALLJ, which arises from a combination of natural climate variability and increased humidity trends tied to anthropogenic forcing.