The South American Low‐Level Jet: A New Climatology, Variability, and Changes

The South American low‐level jet (SALLJ) east of the Andes plays an important role in regional weather and hydrology by transporting large amounts of moisture from the Amazon to the subtropics. A new methodology is introduced for detecting SALLJ events based on seasonal‐percentile thresholds of wind speed and wind shear. Direct comparisons are made between new and conventional fixed‐threshold methods. Identification of SALLJ events is compared between five different reanalysis products and validated against available radiosonde observations. A new climatology of the SALLJ is presented for one particular reanalysis during a 38‐year period (1979–2016). Based on this new definition, the present study analyzes the spatiotemporal variability of the SALLJ on seasonal and interannual timescales, as well as trends in the jet over recent decades. Results show that the interannual variability of the jet's strength and frequency is significantly modulated by the El Niño Southern Oscillation, especially during spring. Trends in the vertically integrated moisture transport during SALLJ days reveal significant increases in the northwesterly moisture flux associated with the jet toward southern Brazil in spring, summer, and fall. These changes likely contribute to the increase in precipitation and extreme precipitation events observed over southeastern South America in recent decades. Key Points A new definition and climatology of the South American Low‐Level Jet (SALLJ) substantially decreases seasonal biases present in previous studies of the SALLJ A comparison of five reanalyses shows that modern reanalyses generally agree in representing the structure and intensity of the SALLJ The SALLJ is stronger and more frequent during El Niño episodes; La Niña episodes are associated with weaker and less frequent SALLJ days; the frequency and intensity of the SALLJ has exhibited significant trends over the past few decades